The common notion is that history begins with

monuments and literary records—that is, when

civilization is already far advanced … Properly speaking,

however, no time should be called prehistoric, if, by any

means, we can ascertain the general character of the

events that took place.

John McLennan 1896

1 Introduction

In this essay, we outline an expanding program of methods, principles, and applications that we fashioned together in response to the challenges of early socio-biologists (Wilson 1975, 1978; Sloan and Wilson 2007) and evolutionary psychologists (Barkow et al. 1992). However, with refinements over the years in these perspectives (and in our work as well), we now see our research program as potentially complementary to recent socio-biology and evolutionary psychology. Indeed, we believe it is time to build alliances with other evolutionary approaches because we are linked by a shared objective—to bring evolutionary thinking into mainstream sociology by showing that it can be useful if not essential for confronting the problems central to present-day sociology. At the same time, evolutionary sociology has the capacity to develop an entirely new approach—based on new types of selection of socio-cultural phenotypes—to explain evolution in the socio-cultural realm.

We came to this conclusion because we believe that biologists and psychologists assume that too much of the socio-cultural universe is somehow primarily “programmed” by genic properties, thus making evolutionary reasoning less appealing to most social scientists. As sociologists and anthropologists, we have always operated under the presumption that three distinct “universes” are relevant to understanding the human condition: the physical, biotic, and socio-cultural. Each has, we believe, unit-properties and dynamics that can be theorized. The physical universe has the most effects on the biotic universe, which was built from the physical in the creation of life, whereas the biotic has the most influence on the socio-cultural because this universe was initially built and is sustained by human agency. This does not mean, however, that the laws of biology explain the laws of sociology because the socio-cultural universe, like the physical and biotic universes, is also an emergent domain that operates by its own laws driving human organization. Once in place, moreover, these emergent properties have large effects on human behavior and, to a lesser but sometimes a significant degree, on the structure of humans as biotic organisms.

For example, this duality is evident when hominins (like Homo erectus) learned to control fire. With this new knowledge, they could keep warm (without fur), keep predators at bay, migrate to colder regions (promoting gene flow), and cook foods. Cooking (a technology) meant less chewing of vegetable and meat fibres, which led to smaller molars in size and shape (as teeth reflect dietary preferences), a reduced jawbone and other body traits related to diet. Richard Wrangham et al. (1999; Wrangham 2009) hypothesized that this sociocultural leap also resulted in the consumption of more calories that, in turn, abetted the growth of the hominin brain.

We also wish to demonstrate the value of taking an evolutionary lens to commonplace misconceptions, such as the belief that humans have deeply rooted needs for many strong ties and extended kinship. This belief is so entrenched in the humanities and social sciences that critics of modernity posit a future world of rootless and socially starved human beings. Yet, most who worry about the atomizing effects of a new world order know nothing about the biology of human sociality.

We lack a Webb telescope but by employing the methods of network theory, cladistics, and neuro-anatomy, we designed a research program that can serve as a distant mirror to carry out a systematic study of primate and hominin heritage in the light of evolution. What we discovered is the pivotal importance of the interplay between biology and the environment, with the evolutionary record pointing not to collectivism but to a hominoid (i.e., ape and human) heritage of individualism, with predispositions for weak ties and with sociability an evolved product of a still mystifying ancestral legacy. Thus, rather than take familiar sociological phenomena for granted with untested suppositions about innate human sociality, an evolutionary perspective makes possible a means to determine to what degree sociality and social structure stem from an inherited biology or to an emergent socio-cultural legacy and, if so, what makes it possible in the first place?

Our initial inquiry into human evolution exposed the twists and turns of a riveting set of events, culminating in the survival of one hominin lineage—of various ancestral forms—that eventually led to Homo sapiens. As our statistics and facts are mostly drawn from interdisciplinary sources that include biology, genetics, neurobiology, primatology, geology, archaeology, anthropology, and paleontology (such that the discovery of a new fossil can realign, change, or restore a hypothesis), these data are continually updated with new findings and entailments. Thus, this essay is both a synopsis of our research model on human’s ultimate history and up to date with the latest research findings.

To easily follow the logic of this essay, we start with a summary of two of our methods, cladistic analysis and network analysis. These procedures, along with a third methodology, comparative neuro-anatomy, are what we regard as cutting-edge tools for an evolutionary sociology because they can be used to isolate and set apart the biology of humans from the socio-cultural universe that humankind created.

2 Cladistic Analyses and Network Analysis

2.1 Crossing Species Lines: The Social Network Approach

Humans (Homo sapiens) belong to a tiny lineage—the hominoids, which includes fossil (and near) ancestors and apes. Based on anatomical and DNA comparisons, humans and apes constitute a clade, which is composed of Homo sapiens, little gibbons, and siamangs (Hylobatidae), and the great apes, orangutans (Pongo), gorillas (Gorilla), common chimpanzees, and bonobos (Pan). The other 250+ primate species are New and Old World monkeys, and descendants of early prosimian primates (i.e., lemurs, lorises, galagos, aye-ayes (Strepsirrhini), and tarsiers (Haplorrhini).

Our shared DNA with chimpanzees is nearly 99% (with orangutans and gorillas a tad less), which brought to light the notion that at the genome level, “the chimpanzee–human difference is far smaller than that between species within a genus of mice, frogs or flies” (King and Wilson 1978, p. 91). Apes and humans, then, are cognates and recently (in geological times) shared a stem hominoid ancestor (Waterson et al. 2005; Andrews 2020). As Daniel Gebo (2014, p. 81) remarked: “Living primates represent a diverse group of mammals … As a group they represent the ancestral foundation for all human biology.”

Most primates live in year-round societies, which, like those of humans, are composed of adult males and females, infants, juveniles, and adolescents. Monkeys and apes are highly intelligent, mature slowly, and single births are the norm, as is a long socialization for the young and a long life span (free-ranging great apes can live 50 years, longer in captivity). Years ago, primatologists realized that “each species brings a different phylogenetic heritage into a particular ecological scene, and that a species’ present state of organization mirrors a compromise between current ecological conditions and conservative ancestral traits” (Struhsaker 1969, pp. 13–15). Hence, diverse species in similar habitats often converge in response to environmental constraints such as the food supply, sleeping arrangements, response to predation, and group size, but resist changes in mating habits and social relationships. It was also discovered that closely related species in different habitats shared identical traits (Chalmers 1973). This signaled the operation of phylogenetic inertia for some traits, notably the kinds and frequencies of interactions between primate sex and age classes. That is, “who likes to be with whom” (Hinde 1976, 1983; Kummer 1971a, p. 191).

So, a first step in our effort to isolate out human social proclivities was to cross species lines and compare attachment patterns among our closest living relations, the great apes. A social network approach (rather than a behavioral approach)Footnote 1 is useful here because it is centered on social structure or the network linkages among two or more individuals produced and reproduced by some of their social behaviors. As Peter Blau (1982, p. 275) phrased it, a focus on social structure “assumes that social life, including its cultural manifestation, is rooted in the structure of social positions and relations and must be explained by analyzing these patterns and these networks or rates of relations in groups and societies.”

Hence, by focusing on relations rather than behaviors, an underlying social structure can be uncovered. Of course, much is ignored with this analysis but it has the virtue of uncovering the underlying relational structure of a population, a configuration that is often difficult to detect when the focus is only on the attributes and social behaviors of individuals. In our original analysis the coding protocol was to access all the relational data on great apes (and even little gibbons and siamangs), but for illustrative purposes we need to access only one crucial property of great ape social structure, the strength of social bonds and the effects of migration or dispersal on these bonds (for discussions of network analysis, see Hinde 1983; Borgatti et al. 2013).

2.2 Cladistic Analysis

Cladistics is a powerful discovery tool that starts by identifying closely related entities who once shared a last common ancestor (LCA) at a point in time. As this procedure follows the principle of parsimony, it is straightforward but rigorous in scope. After this identification, it then isolates and compares their similarities or “held in common” characters, with the assumption that the shared traits or “derived characteristics” stem from an inherited legacy from their LCA. In this analysis, the entities are great-ape taxa, which can be good proxies for the behavioral and organizational characteristics of human’s early hominin ancestors (Andrews 2015, 2020; Forey et al. 1994; Maryanski and Turner 1992; Maryanski 1992, 1995; McGrew 2010).Footnote 2

A second assumption is that modifications from the reconstructed ancestral form to descendent forms were not randomly acquired but show a clear systematic bias that connects descendant species to a common ancestor and, at the same time, differentiates them from a closely related “sister” lineage. As Old-World monkeys are the closest lineage to orangutans, gorillas, chimpanzees, and humans, they are our “control group” or “out-group population” for our cladistic comparison. Cladistics also builds in two testable hypotheses: a (1) related hypothesis and (2) a regularity hypothesis to be discussed (Jeffers and Lehiste 1979).

It is important to note that the Old-World monkey lineage and the ape/hominin lineage shared a  LCA circa 28 million years ago. Both lineages then branched away from the mother population to become independent lineages. As a result, present-day monkeys, apes, and humans still share an inherited legacy from this early anthropoid ancestor. For example, Old-World monkeys and apes/humans share a dental formula (i.e., the number of teeth), sophisticated sensory modalities, the same finely tuned prehensile hands, and many cognitive traits. For this reason, Old-World monkeys are our control group, for this is the only way we can precisely and accurately identify the derived traits or “evolutionary novelties” that are “held in common” only by present-day great apes and humans—bequeathed by a stem hominoid ancestor who lived about 15 million years ago.

Now if all we knew about Old-World monkeys and great apes were behaviors, these “sister” lineages would naturally appear similar. However, things look dramatically different when we compare their respective patterns of migration and social relationships. This is why cladistics can uncover what a behaviorist approach might miss. A more sociological approach to biologically driven organizational patterns also gives us a blueprint for understanding the forces that promote or constrain not just great ape organization but human organization as well.

2.3 The Dynamics of Social Relations as Opposed to Behavior

In this network/cladistic analysis, only one crucial relational property is scored—the strength of ties and the consequences of dispersal at puberty (or migration) on these tie patterns. To synopsize this procedure for this essay, only sets of core social relations are depicted: Adult-to-Adult Ties; Adult-to-Adult Kinship ties; Adult-to-Child ties. As these tie patterns were documented for over a century, using a variety of methods, a simple sliding scale of tie strength is the only way to represent these field data.

Non-existent ties (e.g., father-to-son) or those who seldom or never interact = 0, or very weak or null ties; those who socialize on some occasions, or those who affiliate closely for a time but with little emotion = O/+, or weak to moderate ties; and those who enjoy high rates of interaction, long-term contact, and very positive emotions = +, or strong ties. We should mention that assessing tie strength for primates is a simple procedure as it is easy to see “who likes whom.”Footnote 3

Table 1 compares the strength of social network ties for core age and sex categories for great apes. The LCA reconstruction based upon these tie patterns is listed in the right column of the table—as is the convention in cladistic analysis. The striking feature is the large number of weak and non-existent ties, signified by the “O” in the relevant columns. What is clear is that great apes have only a few strong and mostly weak ties. Of consequence, is that all ape ties are “checkmate” in that dyads cannot serve to create an expansion network of ties (as Old-World monkeys do in Table 2). There are no strong ties between adult males and females because great apes are sexually promiscuous (although gorillas somewhat less so); thus, they do not form stable mating ties or create families beyond the mother–infant bond typical of all mammals. Moreover, all females leave their natal unit after puberty—never to return. Hence, relations between a mother and her adult daughter are severed, while fathers are unknown in promiscuous mating systems. Male orangutans and gorillas also leave their mother and natal community after puberty. Male chimpanzees are the exception here as they remain in their mother’s community and have lifetime ties with her and their male siblings (Pusey and Packer 1987).

Table 1 Strengths of ties among extant species of Great Apes
Full size table
Table 2 Strength of ties among a sample of well-studied monkeys
Full size table

Great ape networks nicely correspond to loose-knit “fission–fusion” communities. Orangutans are tree living and nearly solitary, but social on occasion. About 50 or more locals inhabit a solid block of tropical rainforest about three miles square. Orangutans typically avoid contact with conspecifics, but now and then mingle in brief encounters that average about 1.8 individuals. Adolescent males and females are more socially inclined and occasionally congregate for brief “parties.” A sexually receptive female and adult male will keep company and even remain together for a week or more. Yet, despite their loner lifestyle, the consensus view is that orangutans live in a widely dispersed community organization, with shared ownership in a locality, some cultural traditions, and a keen awareness of who belongs to the community. At puberty, both sexes leave their natal community (Knott and Kahlenberg 2011; Galdikas 1995; Van Schaik et al. 2003).

Gorillas are ground living, spending most days on the forest floor. Unlike reclusive orangutans, gorillas live in loose-knit bands composed of adult females with dependents, a leader silverback male who protects the band against predation, and up to four adult males, with an average group size of 20 gorillas. Four or more gorilla bands are co-residents of a shared locality. Although receptive females typically favor the leader silverback male, they are free to mate with any male, move to another band, or join a lone adult male. The leader silverback and a resident female with offspring typically form a moderate social tie, but it is linked to the silverback’s ability to help to care for her dependent offspring. Should he fail this responsibility, the moderate tie is broken and resorts to a weak tie, or a female transfers to another band or a lone male. Adult female residents are tolerant of each other, but interactions are minimal. Adult males in a band interact occasionally, even friendships sometimes exist.

Gorilla bands are somewhat stable over time but, overall, reflect a shifting collection of individuals who come and go as they please. After puberty, both males and females leave their natal band. Band stability is always conditional because it is anchored by the leader silverback male. Once he dies, all band members disperse, so gorilla bands lack intergenerational continuity. The easy relocation of band members, the evidence that four or more bands share a locality, and at times feed, mingle, and even bed down occasionally, implies that gorilla bands are not discrete units, but segments of a “big band” community organization (Bradley et al. 2004; Stewart and Harcourt 1987; Goodall and Groves 1977; Harcourt and Stewart 2007; Schaller 1962).Footnote 4

Chimpanzees are both tree and ground living. In their rainforest habitat, up to 150 chimpanzees are co-residents of a well-documented community organization, which can vary in territory from 15 square miles in rainforest to 70 square miles in a mixed woodland habitat. Chimpanzees (especially males) are far more social than orangutans or gorillas. Indeed, co-residents often socialize in mixed, ever-changing gatherings that can last a few minutes, an hour, or occasionally a day. Rarely, if ever, does the entire community gather together.

At puberty, female chimpanzees leave their mothers and transfer to another community, whereas males are lifetime locals. After puberty, however, males become fancy-free nomads (although they visit their mother frequently) and socialize with preferred male friends, join momentary parties, or wander about alone. Adult females joining a community are tolerant of other females (what Jane Goodall calls a “neutral relationship”) but have few ties with either adult males or females, seemingly because they are migrants (much like females in gorilla bands). Mothers with dependents avoid social interactions but will occasionally join “nursery groups” so that their offspring can play. Yet, despite mostly weak ties, co-residents share long-established “cultural” traditions, passed on from generation to generation (Luncz and Boesch 2014, p. 656; Goodall 1986; Watts 2002; Boesch and Boesch-Achermann 2000; Stumpf et al. 2009).

By contrast, Old-World monkey networks correspond to bounded groups with strong-tie stability. By just glancing at Table 2, it may appear similar to Table 1, but monkey relational patterns are almost the exact opposite of ape relational ties. Monkeys organize at the group level and, with rare exceptions, have either a one male–multiple female grouping, or a multi-male–multi-female grouping. Old-World monkey groups are called female-bonded societies because mother–daughter bonds are the cornerstone for creating matrilines, which can expand to four generations of blood-tied cliques of adult females and dependents. As monkey females are lifetime locals and mother–daughter ties endure for a lifetime, this results in self-recruiting stable groups with intergenerational continuity. In a notable difference, male monkey leave after puberty, and move to neighboring groups to compete in hierarchies for status positions or to form one-male groups (Rhine and Maryanski 1996; Mitani et al. 2012; Cords 2012).

Hence, in contrast to great apes, who have few kinship ties without any really permanent groups, monkeys have two layers of integration at the group level: (1) male hierarchies and (2) kinship-based matrilines. Thus, monkeys live in a strongly-tied social world, and apes live in a weakly-tied social world. Such striking differences tell us that Old-World monkeys and great apes, after branching away from a LCA about 28 million years ago, adapted to poles-apart habitats at some point in time. Given that apes share social ties that are “evolutionary novelties” and rare in the primate (and mammal) world, they were likely present in the LCA social structure, which has enormous consequences for understanding humans and their societies.

2.4 The Relatedness and Regularity Hypotheses

Great ape ties and field studies document that our closest living relatives organize into “fission–fusion” societies at the community level. And, despite wide variations in organization, all females leave their maternal parent after puberty. Orangutan and gorilla males also leave after puberty. In contrast, chimpanzee males are lifetime locals, but strong ties are limited to a few friends, male siblings, and mothers. Kinship networks then play only a small integrative role in chimpanzee society. Indeed, adult males often spend more time with non-related peers than with male siblings because mothers give birth to a single offspring only once every 5 years (9 years for orangutans). And this sibling age difference is extended when interrupted by baby sisters and infant mortality.

As a result, apes have organizational arrangements where kinship relations are not essential for integration, the nuclear family does not exist, and an expanded kinship system does not existFootnote 5, leaving residents free to socialize with whom they wish on a voluntary basis. Great ape communities are seemingly integrated mostly on a “sense of belonging” as the criterion for membership, an emergent or cognitive perception of reality that there is something greater than the self.Footnote 6 Ape societies also counter the widespread belief that the nuclear family of parents and offspring is the basal human group; indeed, it is probably not even a natural or genetically driven formation, for, given the rules of cladistic analysis, its non-existence in the ancestral social structure or in great ape societies should also apply to humans. Instead, groups such as the nuclear family are more likely an adaptive social construction created by socio-cultural selection, which helps to explain why groups and social ties among humans are often unstable.

Is the reconstructed social structure of the last LCA credible? Do the shared traits of great apes stem from this ancestor? To find out, we need to follow the procedures of cladistics by applying the (1) relatedness and (2) regularity hypotheses. In line with the relatedness hypothesis, we must first consider the null hypothesis that great ape traits were all acquired independently, or by chance after each ape lineage branched away from the LCA. The null hypothesis cannot be entirely ruled out, but great ape phylogeny, the striking differences between ape and monkey ties, the findings that primate social relations are conservative traits that typically reflect phylogeny, the discovery of a mutated uricase enzyme exclusive to great apes and humans (to be discussed), as well as support for the regularity hypothesis below greatly minimize this probability. Hence, we can logically assume that the shared tie patterns reflect a monophyletic status, with tie variations a function of environmental pressures after each lineage branched away from the mother population.

In line with the regularity hypothesis, we need to show that the organizational changes from the LCA to present-day apes did not occur randomly but evidence a systemic bias or “founder’s effect” to connect great apes to each other and their LCA. As living apes and humans are end products of their own long evolutionary history (as the LCA lived about 15 million years ago), identifying a systemic bias may be challenging. Fortunately, as social ties are generally averse to change, species usually keep the social structure they inherit.

Great apes have similar social structures but, unlike monkeys, live in idiosyncratic “fission–fusion” societies that differ from monkeys and each other. However, they all share a rare transfer pattern for primates and social mammals: female migration. Now if we look at Table 1, gorilla and chimpanzee societies added a few social ties compared with the reconstructed LCA social structure in the far-right column. What spiked this enhanced sociality is unknown, because fossils linked directly to the ancestral lineages of gorillas and chimpanzees have yet to be found to reconstruct their ancestral environments. Still, we can assume that conditions for natural selection to operate were met.Footnote 7 Surprisingly, instead of tinkering with female dispersal by strengthening the adult mother–daughter bond (opening the door for extended matriliny), selection acted to strengthen very different alternative ties with an adaptive advantage.

For example, gorilla social structure, which superficially resembles monkey one-male–matriline groups, is centered around a leader silverback male and mothers with dependent offspring. Yet, this moderate tie depends upon the silverback protecting and caring for dependent offspring. Chimpanzee social structure is centered around adult male relationships and mother–adult son relationships. Only orangutans have a social structure that still mirrors the LCA social structure.

But, above all, it is the decidedly quirky ties of bonobo chimpanzees (Pan paniscus) that win the prize for originality. Bonobos live where forest resources are highly concentrated. So, to promote sociality between unrelated females, selection acted to link sociality to sexual arousal or what the literature calls GG rubbing or sociosexual interactions. Thus, when two females are in propinquity, they greet by rubbing their genitals together, which sparks sexual excitement, and apparently an orgasm (Yokoyama and Furuichi 2023). This greeting ritual is not an affect tie but, following White (1989, p. 162) a means “to reduce tension among unrelated females.” Bonobos were excluded from this cladistic analysis because as Rice and Moloney (2005, p. 205) put it, “This kind of sexuality was not found in the LCA either.” Thus, when selection acted to enhance ape social ties, it sidestepped female migration by favoring unconventional ties in the primate world and truly unique ones in the case of bonobos.

Humans also have idiosyncratic organizational arrangements and unconventional ties for a primate. If the regularity hypothesis can be supported in ape societies, it should also hold for human societies. And, with female migration, there is tangible evidence that it has a long evolutionary history. Among early hominins, a chemical analysis of Australopithecine dentition dated between 2.0 and 1.8 million years ago revealed differences in mineral deposits between male and female teeth, indicating that males grew up and stayed in one locality, whereas females grew up in one locality and later moved to another locality (Copeland et al. 2011 and see Foley and Gamble 2009).

Research on European Neanderthal populations also supports a female dispersal pattern. By using DNA methodologies to reconstruct male and female genetics, researchers concluded that “Neanderthal communities were predominantly linked by female migration.” Similarly, mtDNA lineages suggest that Neanderthals practiced “patrilocal mating behavior” (Skov et al. 2022, p. 524; Lalueza-Fox et al. 2010, p. 252). That female migration is documented in great apes and Australopithecines, and Neanderthals seemingly practiced patrilocal residence, adds fuel to a longstanding debate over whether Paleolithic hunter-gatherers practiced (a) bio-locality or residential flexibility, with male or female exogamy; (b) matri-locality, with male exogamy, or (c) patrilocality with female exogamy. Cross-cultural studies overwhelmingly support the patrilocal residence model, but some kinship theorists now support ancestral bio-locality, arguing (without evidence) that patrilocality is unlikely to be relevant in human evolution (for discussion on this debate see Ember and Ember 1983; Radcliffe-Brown 1931; Service 1962; Lee and DeVore 1968; Maryanski 2021). To be sure, studied human societies will vary in residential rules (see footnote 8 for percentages of each residential pattern), but what of ancestral hunters and gatherers?

Hunting and gathering societies remained the dominant societal mode until 10,000 years ago, with ethnographic studies supporting patrilocality. If apes, Australopithecines, and Neanderthals also practiced female dispersal after puberty, this is strong evidence that humans also share a proclivity for female migration at puberty. Although a predisposition is only a tendency to behave in a certain way, this finding offers clues into the origin of the human family and, without question, the social structure of early human societies.Footnote 8

This begs the question of why the LCA of apes/humans evolved a pattern of low sociality, weak ties, and female migration. In brief, some eye-catching surprises. Turning to the molecular clock and the fossil record, proto-monkey and proto-ape lineages branched away from an African stem ancestor about 28 million years ago. For the ape lineage, it was a “Golden Age” with lots of species and peak reproduction, whereas for the monkey lineage, it was a challenging age with few species and modest reproduction.

By the later Miocene, however, the ape “Golden Age” became a “Dark Age” as fossil beds began filling up with dead apes. In contrast, the once rare monkeys began to proliferate in species and numbers. Multiple factors (e.g., sweeping climatic change, declining resources) account for this ape decline, but of relevance here is a longstanding hypothesis that one reason why monkeys began to multiply and apes decline is that monkeys evolved a digestive specialization for detoxifying unripe fruits for consumption. If so, this constituted a major dietary advantage for monkeys. In rainforests, the primary food source for apes is ripe fruits because of their high nutrients and sugary energy. Raw fruits would be toxic and difficult to digest as they are for humans and apes today. In the end, the fossil record documents that nearly all 100+ species of apes had died out by the late Miocene.

How did the LCA of apes and humans survive this catastrophic event? The fossil record tells us that when apes first evolved in the early Miocene, they had, strangely enough, monkey-like skeletal structures. This means they had a quadrupedal gait and walked on the tops of sturdy tree branches like present-day monkeys. Yet, a few rare ape species living on the sidelines were endowed with an enduring gift: skeletal structures trending in an upright posture, vertical climbing, and a capacity for hanging below-branches for suspensory activity (Andrews 2015, 2020, p. 133)—similar to the abilities of present-day apes and humans.

If monkeys evolved a specialized digestive tract for detoxifying unripe fruits for ingestion (and Miocene monkey fossils support this hypothesis), the quadrupedal apes surely faced a crisis because they could only forage for food on stable branches in the central forest canopy. And, they had to wait for fruits to ripen. However, the rare suspensory apes were not so food challenged because their joints and carriage enabled below-branch foraging to reach soft ripe fruits at the ends of thin, swaying branches where neither a quadrupedal ape nor any monkey could step. Hence, locomotion mode seemingly played a role in which apes lived and died (for discussions see Conroy and Pontzer 2012; Andrews 2015; Maclatchy 2004; Gebo et al. 1997).

How did the few surviving apes organize? A suspensory body plan points to a borderline niche with slim resources. As foraging strategies interface with ecological, organizational, and social patterns in complex interdependencies, environmental pressures would likely favor a “hang-loose” (pun intended) community structure with weak ties so that individuals could forage independently for dispersed and seasonal fruits.

Data from fossil beds suggest that early monkeys organized into matrilines with male dispersal (like Old-World monkeys today). And, given that monkeys and apes once shared a LCA (as discussed), early ape organization was probably also anchored by matrifocal units. If so, selection later acted on suspensory apes in marginal niches to thin out the population by dispersing both sexes after puberty. And it had far-reaching consequences. For once the quadruped apes were swept away, it was the suspensory apes that drove the future course of hominin evolution.

We recently added an intriguing piece of data that seemingly contributed to the survival of the LCA and its descendants. About 15 million years ago, the LCA inherited (or acquired) mutations that elevate uric acid levels in blood. Primates and nearly all mammals have low uric acid levels because they possess an active enzyme called uricase (urate oxidase), which breaks down uric acid for removal in urine. A mutated inactive uricase blocks this breakdown process. Only great apes and humans (and a few dogs from artificial selection) have elevated uric acid levels (Bannasch et al. 2008), although this presents a problem only when refined sugars and organ meats are consumed in excess (which can lead to obesity, diabetes, gout, and cardiovascular diseases).

The uricase mutation was likely fitness enhancing for the LCA because ripe fruits have lots of fructose (a fruit sugar). In its presence, it increases production of uric acid and higher levels of uric acid assist in the accumulation and storage of triglycerides (a type of body fat). As fruits are seasonal, a greater capacity to store fat more easily in lean times would have survival value (Andrews 2015, p. 135; Álvarez-Lario and Macarró-Vicente 2010). All the same, the ape lineage survived but never recovered from the great Miocene die-off; for most extant primates are monkeys, with apes and humans comprising only 5% of primate species.

3 Critical Adaptations for the Evolution of Hominins/Humans

Natural selection has nothing to do with the progress or perfection of a species. It is all about adaptation. Yet, a successful adaptation to a new or changing environment depends on what traits are at hand. Selection can remove extreme traits, or greatly enhance or modify characteristics, but some traits resist change. This led Hans Kummer, the legendary ethnologist, to conclude that “new behavioral adaptations are possible only if the necessary dispositions are within the scope of the behavioral heritage” (Kummer 1971b, p. 129).

We lack a crystal ball but we do have the methods and data to sketch a route that natural selection took to make hominins on the line to humans more group oriented and socially inclined. Oddly enough, the road that led to Homo sapiens started about 60 million years ago when selection acted to convert a ground-living mammal to tree-living with two foundational adaptations: (a) grasping hands and feet (which later became prehensile), and (b) upgrading the visual modality for three-dimensional vision. Over time, the continued enhancement of vision (e.g., a three-cone color system) led to a superiority for locating objects in space and processing most social information. It also led to bigger brains with sophisticated sensory and neo-cortical features. A large and complex brain (compared with most mammals) is the true hallmark of primate evolution.

This reworking of the brain, which is discussed in the next section, is a comparative study, resulting in not only vision becoming the dominant sense modality but also facilitating later selection for new association cortices such as Wernicke’s area (see Fig. 1), an adaptation of enormous importance because it pre-wired great ape and hominin neurology for future linguistic capacities (Geschwind 60,61,a, b; Damasio and Geschwind 1984).

Fig. 1
figure 1

Key areas of the brain

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It is not possible in an article format to go into more detail, except by short summaries in Tables 3 and 4 of the important outcomes of selection working on hominoid neuro-anatomy.Footnote 9 All of these represent the biological basis for hominin behaviors and patterns of social organization. These biologically based traits are pre-adaptations, behaviors, and organizational propensities that selection acted on during the five to six million years of hominin evolution.

Table 3 Preadaptations of consequence to hominin/human evolution
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Table 4 Behavioral capacities of Great Apes. Inherited by hominoids and then humans
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In Tables 3 and 4, these inherited traits have been broken down into one table on adaptations that would be consequential to human evolution, and a second table on what are to a high degree genetically driven behavioral propensities that could be selected immediately if they could help hominins to adapt to new ecological niches. Together, these consequential adaptations and genetically driven behavioral propensities would allow hominins to evolve slowly into Homo sapiens and create societies, not so much guided by bio-programmers, but by biologically based, generalized capacities for enhanced emotions, greater intelligence, speech, and symbolic culture. Still, what is unique about humans originates in traits inherited from the LCA of present-day great apes and humans—thus making humans not as unique as is often thought.

Tables 3 and 4 provide a sense of the biological traits that present-day great apes possess that, under the power of selection, would set the stage for the evolution of humans. Hence the biological nature of humans is, in essence, the biological nature of great apes, but subjected to intense selection to make humans more emotional, intelligent, verbal, and cultural.

Studying the biology of great apes and also hominin evolution is thus critical to understanding not only humans as biological entities, but humans as organisms with capacities for agency in societies now organized by social structures regulated by symbolic culture as much, and perhaps even more, than inherited bio-programmers. An evolutionary sociology needs to use what we can learn from primatology and biology to understand humans as a species and, most significantly, to understand the dynamics of societal evolution that were created and sustained in ways that are different from all other mammals on earth.

4 Comparative Neuro-Anatomy

4.1 Evolution of the Hominin Brain

Comparative neuro-anatomy, a third cutting-edge approach, compares the structures and functions of great ape and human brains. Great apes never left the forest zone, and so niche theory would predict mostly neurobiological statis as the forest zones remained relatively stable (Vandermeer 1972). The presumption here is that the brains of present-day great apes are much like early hominin brains on the human line of evolution. And we already know the results here: (1) a larger neo-cortex where cognitions and memories are stored and where decisions are made, (2) a larger sub-cortex where emotions are generated, (3) a capacity for voluntary control over vocalizations, essential for an oral language and, as a result, creating (4) a symbolic culture mediating thoughts, actions, social relations, and social structures. Although a few neurological structures did emerge during hominin evolution, selection otherwise took its normal conservative path, in this case by increasing the size of existing brain structures, while increasing the complexity of connections among brain structures.

If the hominin neo-cortex was to grow beyond the size of a common chimpanzee (at around 350 to 400 cm3), selection initially focused on the sub-cortical emotion centers. For one of the great discoveries of the second half of the twentieth century was that the number and complexity of cognitions in the neo-cortex can only increase with the complexity, range, and nuance of emotions attached to cognitions (see Damasio and Geschwind 1984 and LeDoux 1996 for summaries). Indeed, without a prior enhancement and growth in sub-cortical structures, growing the neo-cortex would not be fitness enhancing because there would be an insufficient variety of emotional valences with which to tag complex sets of cognitions. The neo-cortex without a wide range of emotions would simply be an empty warehouse consuming a significant proportion of the protein and calories needed by an organism, thus decreasing fitness. Thus, a larger neo-cortex was dependent upon a prior enlargement of sub-cortical areas of the brain where emotions in mammals are generated.

In Table 5, we list key hominins at various points in time, with estimates of their brain volume. What is evident is a near stasis in brain volume during the first three to four million years of hominin evolution. Australopithecus afarensis at 3.2 million years ago (Mya) had a brain in the chimpanzee range. With Australopithecus africanus at 3.0 to 2.5 Mya, the brain had increased about 100 cm3, with other species like Paranthropus aethiopicus and boisei showing incremental growth as well, even at 1.7 Mya. A big jump occurred only with early Homo erectus at 1.7 Mya, with at least a doubling of brain volume from mid-400 cm3 to mid-900 cm3. Thus, at best we can hypothesize that the initial growth of 100 cm3 may have been the result of growth in sub-cortical areas of the brain pushing out a still relatively small neo-cortex, thereby increasing the cranium of fossils.

Table 5 Brain volume of selected hominins and humans
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What was blocking the growth of the neo-cortex? One roadblock would have been the insufficient growth of the sub-cortex for a larger brain to be fitness enhancing. And it appears that this barrier was only removed by 2.0 to 1.6 Mya because that is when the brain began to expand dramatically.

Figure 1 and Table 6 outline key structures in the brain. The middle panel in Fig. 1 emphasizes some critical sub-cortical structures generating emotions that tag cognitions with affect. These structures tagging cognitions with emotions are needed for memory formation and decision making, because without them the neo-cortex would have not been fitness enhancing. Measurements of sub-cortical areas of great ape brains and human brains in Table 6 can confirm that the sub-cortical areas of the brain in humans are, on average, about twice as large in humans as in great apes, controlling for body size (which correlates with brain size), whereas the neo-cortex of humans is three times larger than that of great apes.

Table 6 Relative size of brain components of apes and humans, compared with Tenrecinae
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Thus, in our view, it took several million years of selection working on sub-cortical centers of the hominin brain for emotions to reach some threshold-point where the increase in the diversity and variations in emotions could by directional selection make expanding the neocortex adaptive (Ardesch et al. 2019; Holloway 2015a, 2015b; Passingham 1973, 1975). Thus, in this sense, emotions initially drove hominin evolution.

4.2 Emotions as a Driving Force in Hominin Evolution

The data on sub-cortical areas in Table 6 have a partial control of body size, which correlate with brain size. Part of this control of body size involved measuring the brain size of great apes and humans by how many times larger a structure is in humans and great apes than the equivalent structure in a small, rat-like mammal (Tenrecinae), which probably resembled the original mammals. As noted earlier, the sub-cortical structures are, on average, about twice as large for humans as for great apes, using Tenrecinae as our “measuring stick.” Other, more recent studies (e.g., Barger et al. 2007, 2012, 2014) using different methods (allometric measurements) reveal that the sub-cortical structures of humans are 50% larger than would be predicted by body size—thus again confirming that sub-cortical areas are larger than would be the case for primate species with the same body size.

We note again that growth in these sub-cortical areas had to have begun before the neo-cortex could grow. Then, once a certain threshold was reached, the enhanced emotionality would lead to fitness-enhancing neocortical growth that, somewhat surprisingly, occurred late in hominin evolution (see Turner 2007, 2000, 2021). Referencing the measurements in Table 6, most of the growth in size of the amygdala (the ancient area in all mammals for anger and fear), comes from the lateral nucleus (which was incorrectly conflated with the basal nucleiFootnote 10 in the earlier measures reported in Table 6). The lateral nucleus represents a new structure in humans compared with apes, or a “module” in terms of evolutionary psychology. This module is devoted to assessing social context and expectations for behavior, including emotional behavior (see Barger et al. 2007, 2012, 2014).Footnote 11 Two pathological conditions in damaged lateral nuclei in the amygdala confirm this conclusion. In persons with Williams’ syndrome there is the tendency to be overly empathic (e.g., children with Williams Syndrome will run to a stranger that they think is in distress and hug them), whereas just the opposite of Williams Syndrome can also occur and produce autism in which the inability to read emotions in others in social contexts is compromised. Thus, selection pressures were clearly working in hominins to enhance emotions in order to increase interpersonal attunement beyond the levels of the LCA, an increase in social bonds and solidarity that would eventually generate more stable group formations.

The location of this nucleus in the ancient sub-cortical structure generating fear and anger is interesting because it allows this nucleus to increase the emotional attunement of individuals to social relations and social structures. In so doing, it may have also worked to mitigate high-intensity anger and fear that inevitably disrupt social relations and group formations. Thus, greater emotional attunement leading to increased solidarity and, perhaps, increasing propensities to form groups held together by emotions, was the path that evolution initially took.

The septum and adjacent nuclei around this area of the brain generate the emotional pleasure associated with sex. Since great apes enjoy sex indiscriminately and often (especially chimpanzees), why would further enlargement of this area be needed among evolving hominins and then humans? The answer, we believe, is that additional emotions were being added to the septum. These new emotions had less to do with sexual pleasure and were more centered around love, attachment, and the bonding of sexual partners, something that great apes do not practice.Footnote 12 As there are no bio-programmers for the “nuclear family” in chimpanzees and, hence, their common ancestor with early hominins, this growth may signal one of the mechanisms by which selection was working to enhance emotional attachments sufficient to form a more permanent conjugal pair, which was critical to the formation of nuclear families. For, without the nuclear family as the foundational group structure from which hunter–gatherer bands were constructed, survival efforts to adapt to open-country habitats would be highly problematicFootnote 13. So, again, it is an emotional center that is being selected to increase interpersonal attachments and group structures in order to make weakly tied hominins and eventually humans fitter in open habitats.

The hippocampus is the sub-cortical area of the brain where emotions are attached to cognitions to create memories (Damasio 1994); and so, if more emotions are being generated and used to tag experiences that will be remembered, the potential for increasing the capacity to create and store greater numbers and varieties of cognitions would ensue. Without an enhanced hippocampus and the transition cortices holding cognitions in consciousness (to be tagged with emotions by the hippocampus), hominin intelligence could not increase. Moreover, complex cognitions can only emerge with complex and nuanced emotional states, again generating pressures to enhance the emotions of evolving hominins. Thus, our reading of these data in Table 6 and other studies (Barger et al. 2007, 2012) indicate that selection was enhancing the emotional capacities of early hominins, probably to increase social bonds and solidarities.

4.3 The Movement to an Open Country Adaptive Zone

The Pliocene (5.3–2.6 Mya) was an epoch of dramatic climatic cycles, as the earth cooled with glaciers in the forecast during the Pleistocene epoch (2.6 to 0.012 Mya). A habitat reconstruction using fossilized plants and animals associated with early hominin fossils (e.g., A. afarensis) indicate that by the Pliocene, hominins were foraging not just in the shrinking forests but exploiting woodlands, bushlands, and open terrain (Larson 2022, p. 300). Large predators inhabited these environments, making hominins highly vulnerable to predation.Footnote 14 But with prolonged cooling, they were gradually forced to migrate from protective African forests to predator-ridden ecologies.

4.4 Interaction Effects Among Emotions, Cognitions, Speech, Culture, and the Sociological Basis of Human Neurological Evolution

As early hominins migrated to terrestrial zones, they took along physical and neurological traits of a forest adaptive zone. Anatomically, this consisted of novel skeletal features. As discussed, apes and humans share a similar upper body, but early in hominin evolution lower anatomical structures (e.g., pelvis, hips, feet, thighs, knees) had undergone modifications for a bipedal gait with pre-australopithecines (circa 6 Mya) and later, australopithecines (4.2 Mya). Habitual bipedalism took time, but compared with neurological modifications it was relatively easy because apes can stand erect and walk upright for a short time.

Neurologically, this forest-living heritage consisted of (a) visual dominance for object recognition; (b) a weakly tied community organization; (c) proto-Wernicke’s association cortices for symbolic communication and the precursor to linguistic facilities; (d) cortical control of visual and touch (haptic) modalities for rational and intended responses; (e) cortical control of the auditory cortex for comprehending incoming sounds; and (f) subcortical (limbic) control–that is a largely fixed species- specific call system for most oral communication in the production and transmission of outgoing sounds.

Although all are well-suited for survival in forests, most are poorly suited for survival in predator-ridden terrains. For example, smell is superior to vision in open country because it is automatically alerting, with long-distance receptors for detecting prey and avoiding predators by lingering chemical cues. Vision is inferior because it is slow alerting, requires active attention, is inadequate in low visibility, and useless after dark. A weakly tied community organization is also a recipe for extinction in open country because it leaves females and young ripe for predators. Early hominin males and females were also gracile, lacked projecting canines; indeed, any built-in-defense weapons—even claws (selected out long ago). Thus, the only option was “strength in organization.” But following the results of cladistic analysis, direct bio-programmers for stable cohesive groups were not in the biological repertoire of the LCA or present-day apes and, by inference, early hominins.

Yet, one character that placed hominins in a less tenuous position was, like great apes and humans today, a proclivity for communal sentiments in a sphere of voluntary relations. By acting on the hominoid legacy of a cognitively perceived “sense of community,” selection could deepen this emotional connection which, in turn, would increase the level of interpersonal attunement and sense of obligation beyond narrow self-interest. And by acting on the social synergy that serves to link individuals to the collective whole, selection could set in motion a generative force for enlarging the neo-cortex that, in turn, would continue during hominin evolution to increase interpersonal skills and capacities.

This string of events—with selection initially intensifying the emotions that attach individuals to a community of weak ties, could also serve as a catalyst for overcoming additional problems. A primary one was the creation of the nuclear family by virtue of (1) increasing through emotions the strength of social ties and bonds among mating pairs and (2) developing symbolic culture and normative expectations on mating pairs forming nuclear families. In particular, with symbolic culture, the evolution of such emotions as guilt and shame would add yet another force for keeping mating pairs bonded to protect offspring. These forces combined operated to reduce the “free-rider” problem especially in hunting and gathering bands where threat of social sanctions operated as yet another force to sustain something very “unnatural” to a great ape: stable groups and the nuclear family. For example, take the problem of infidelity, whether viewed as a “free-rider” problem and, in the case of sociobiology, a fitness drive to pass on more male genes by sexual relations with multiple females. Such has, of course, been the case with great apes without nuclear families, but once the nuclear family evolved there were cultural, emotional, and monitoring forces working to sustain nuclear families. As long as adult males and females with dependent offspring could be held together, the fitness of hominins increased as they were forced onto open-country habitats where group organization was critical, and in the case of later hominins, where the nuclear family was the reproductive and economic unit as well as the anchor and building block for extended kinship relations in hunting and gathering bands.

Social control was probably never complete but adequate for sustaining hunting and gathering bands. Indeed, subsequent societal formations, such as horticulture, used a much-enhanced set of kinship rules and sanctions to sustain larger societies while, at the same time, often allowing hidden infidelities, as long as the infants of sexual infidelity were treated as offspring of the mated pair. Indeed, the embedding of nuclear families in dense and highly restrictive kin units (lineages, clans, and moieties), where monitoring by relatives was constant, coupled with sanctions generated by powerful emotions, were enough to sustain the nuclear family. If such had not been the case, humans and human societies could never have evolved.

The adaptive pressures on hominins were thus sociological in nature, revolving around fostering emotionality without waiting for random mutations to create new brain structures or modules.Footnote 15 In fact, major alterations to the hominin genotype after 25 million years of hominoid evolution would have had a destabilizing effect on the genome with its batteries of genes, and programmed sequences and relationships. For as Stebbins (1969, p. 105) relates, “Once a unit of action has been assembled at a lower level of the hierarchy of organization and performs an essential function in the development of organization at higher levels, mutations that might interfere with the activity of this unit are so strongly disadvantageous that they are rejected at the cellular level and never appear in the adult individual in which they occur.”

Thus, a conservative strategy of enlarging existing neurological structures to enhance emotions led the way to more group stability. At the same time, the neurological growth in sub-cortical emotion centers would eventually allow the neo-cortex to grow and make hominins (e.g., Homo erectus) highly intelligent, if other roadblocks to a successful adaptation could be overcome. The fact that major changes did not occur for millions of years suggest that a certain threshold had to be reached in emotional capacities before growth in the neo-cortex. It also suggests that other changes had to be overcome, especially ones with significance for communication. Indeed, a volitional oral language involved stages of development before it could reach a certain threshold and, in fact, likely blocked further enlargement of the neo-cortex for most of hominin evolution.

4.5 Limiting Factors and Roadblocks

Great apes are unable to articulate finely attuned sounds because they lack the necessary physiological structures—proper positioning and flexibility in the lips, tongue, larynx, phalanx, and related muscular structures—to speak as humans do. Seemingly, such was the case for at least four million years of hominin evolution. Yet, great apes do rely on their visual and haptic modalities for most communication and to convey information. As Menzel (1971, p. 220) related, “One chimpanzee can convey to others, who have no other source of information, the presence, direction, quality, and relative quantity or preference value of distant hidden objects that he himself has not seen for several minutes.” Great apes also carry the basic neurology for some key linguistic neurological structures (see Table 3). For example, young apes, when socialized in a language environment, spontaneously link a phonological code with sounds and images. These latent linguistic skills also enable them to understand spoken words (much like young humans) and even sentences, at about the level of 3‑year-old children (Segerdahl et al. 2005; Lyn et al. 2011).

Apes also communicate with conspecifics when taught human sign language or use computers with pictograms they can array in “sentences” and type out (Benson and Greaves 2005). Yet, it is one thing to understand spoken words (via a proto-Wernicke’s area, discussed earlier) and another to articulate words. One reason why apes cannot speak is that their vocal channel is largely under a fixed species-specific call system and their vocal anatomy, as discussed above, is poorly equipped for linguistically mediated sounds. Early hominins surely started out with the same potentials and limitations.

In addition, apes do not have a fully developed Broca’s area, which in humans is between the frontal and temporal lobes (see top panel of Fig. 2). In humans, Broca’s area allows for the downloading into sequences of words the brain’s way of thinking (which is probably in gestalts). Here, selection acted on what is denoted as Broca’s hump, again via directional selection, to increase its size, which is what allows humans to download brain thinking into sequences of oral sounds carrying common meanings.Footnote 16

Fig. 2
figure 2

Selection effects among subcortical growth, neocortical growth, speech, and culture. (Source: Adapted from J. H. Turner 2021)

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Long ago in the ancestral forest zone, visual and haptic modalities for hominoids were already under volitional control for rational, voluntary responses to environmental stimuli. The auditory modality was also under volitional control for incoming sounds, whereas outgoing sounds using the vocal–auditory channel remained under subcortical control. So, at some point in hominin evolution, selection took the next step of shifting the vocal–auditory channel from a limbic to a cortical zone. And, once vocalizations were under cortical control, voluntary oral responses became possible. However, linguistic articulations required major alterations in brain anatomy and in the vocal apparatus (Turner and Maryanski 2021).

How did hominins foster interactions to promote group affiliation before these linguistic skills evolved? One route, discussed above, was symbolic gesturing and signals, laced with emotions that can operate very much as a kind of quasi-language, as we acknowledge when we talk about “body” language, where nonverbal gestures are emitted, both simultaneously as a gestalt and as a sequence of gestures, to create common meanings. As the linguist Edward Sapir (1949, p. 556) aptly characterized it:

“(W)e respond to gestures with an extreme alertness, and, one might say, in accordance with an elaborate and secret code that is written nowhere, known by none, and understood by all.”

A “language of emotions” with gestures to communicate ideas and intentions could be in place early in hominin evolution, building on the subtle and deft social exchanges of chimpanzees who have a learned, socially conditioned, and rich gestural stock of nonverbal communication in their communities.Footnote 17 Moreover, as listed in Table 4 on hard-wired behavioral capacities, great apes have complex interpersonal skills, honed to allow them to sustain weak ties by episodic interactions that reinforce their “sense of community.” They also engage periodically in collective, emotion-arousing assemblies that resemble collective behaviors among humans in festivals such as Mardi Gras in New Orleans, or rock festivals just about anywhere. As Allen (1998, p. 158) remarked, “effervescent assemblies” are not unique to humans because chimpanzees also enjoy such high-spirited gatherings. These social rituals, following Vernon Reynolds (1965, p. 157) seem to be episodic “celebrations of community” involving loud and synchronized vocalizations, beating sticks on logs like a drum, and highly animated emotional outbursts. Here, chimpanzees are using emotions as a kind of “language” to communicate their solidarity, even though at their core, chimpanzees are strongly independent and self-reliant. They can also use their eyes and body language to coordinate more sinister activities such as the killing of a lone monkey walking through the forest (Menzel 1971).

Figure 2 diagrams these dynamics moving from left to right as enhancements of emotions allowed for growth of the neo-cortex, which then would make streams of speech fitness enhancing if the problems of re-structuring for finely tuned speech could be overcome. When speech could be fed back into the cycles outlined in the figure, then these cycles would accelerate, especially with increased symbolism, as was clearly the case in the last million years of evolution to the first early modern humans about 300,000 years ago.

Although selection operated to refine capacities for an oral language, the counterpart to Broca’s area, a precursor to what is today called Wernicke’s area (see Fig. 2) involves speech comprehension, which was much more developed than Broca’s hump in great apes and early hominins. Hence, uploading sensory information for comprehension in the brain’s mode of thinking was much faster than downloading the brain’s thinking for producing vocalizations. The existence of an analog in hominins that, eventually, would evolve into Wernicke’s area provided the path, via visually read gestures for communication. This is why present-day great apes can understand speech but cannot “speak” because they lack a full Broca’s area. But with half of the neurology for understanding speech in place, and with a brain pre-wired for symbolic communication (see Table 3), selection pressures for articulate vocalizations would increase.

And so, we believe that the long delay in the growth of the hominin neo-cortex was a function of overcoming these constraints. The brain could then evolve rapidly, which it did in the run-up to the first early humans. We know that natural selection was acting on capacities for articulate speech because there is evidence for a series of proteins labeled FOX. Of significance for the evolution of speech and, hence, auditory language is FOXP1 and FOXP2 genes. FOXP1 and FOXP2 clearly target genes that regulate the anatomical structures necessary for articulated speech (Crespi et al. 2017; Schulze et al. 2018). In particular, FOXP2 proteins were involved in developing and coordinating the vocal tract and the muscles affecting mouth and lips for speech, because damage to these areas causes speech impairments. Seemingly, FOXP2 existed among Homo erectus as early as 1.8 million years ago, thereby making FOXP2 a consequential adaptation for spoken language in a timeframe that allows for more than a million years of extra time to full speech production (Enard 2016). With the appearance of FOXP2 or FOXP1 2.0 to 1.8 Mya, the biggest roadblock to neurological evolution along the hominin line and the further development of the hominin brain to human proportions was removed.

Finally, without speech and the capacity to develop sophisticated symbolic culture, it would not be possible to fully moralize social relations and social structures. Great apes evidence little shame or guilt, which are critical emotions for the moral social control of relations and social structures at all levels of human organization (see Boehm 2012; see also Turner 2000, 2010, Abrutyn and Turner 2022). Guilt and shame require speech and articulated moral codes attached to social relations and social structures, so they gradually emerged after the roadblocks to intricate speech production had been removed.

Thus, if 1.8 million years is the point at which this process was underway, as the data on gene enhancers of FOXP1 and more significantly FOXP2 suggest, articulated speech likely emerged in the last 1.5 to 1.2 million years of hominin evolution. And, with speech moral codes could become part of late hominin symbolic culture. Therefore, with guilt and shame as the capstone of the elaboration of emotions during hominin evolution, late hominins and early humans could develop moral beliefs and rules that could be used to forge stronger bonds and group solidarities. For without human emotions like shame and guilt, the moralizing of social relations could not occur.Footnote 18

It would be a misconception to assume that all human traits are beneficial or adaptive, but natural selection is not random or by chance. It is directive by acting on traits with adaptive value at a point in time. Many of the hominoid and hominin traits sketched in this essay likely stem from adaptive trade-offs—that is, not perfect but good enough for survival and reproductive success. What is not appreciated is how this wonderful process indirectly led to a new socio-cultural universe with its own independent dynamics and emergent properties. Thus, the socio-cultural universe emerged on a small scale, but with a rich potential because of agency and the generalized and flexible propensities of humans rather than powerful bio-programmers.

5 Concluding Remarks

Many sociologists regard evolutionary thinking as passé and, for some, even a dangerous field of inquiry. To be sure, nineteenth-century evolutionary theory was riddled with flawed premises, and sullied ideological agendas that fueled the eugenics movement and Western imperialism. Indeed, it is difficult to fathom that sociology became a discipline with this kind of intellectual orientation. Yet, evolutionary theory keeps gaining momentum because, as Kingsley Davis ([1936] 1980, p. 18) put it, “a misused method is not necessarily an erroneous one. Intelligently employed the method could presumably give accurate results.” Indeed, a growing number of scholars believe that modern evolutionary theory has something useful to offer sociology; as well as other disciplines such as the history of religion (see for example, Peterson et al. 2019; Niedenzu et al. 2008; Hopcroft 2018).

Moreover, if we examine the evolutionary logic of early sociologists and anthropologists, we can see what they saw: that biological evolution is limited because the socio-cultural universe is an emergent property where selection works—groups, communities, organizations, institutional domains, societies, and inter-societal systems. But this type of selection is constructed by agency, intelligence, and emotions. What our research program emphasizes is that evolutionary theorizing in biology and psychology can explain some but really not much of the socio-cultural universe.

Our goal in developing our evolutionary program is straightforward: Use methods like cladistic analysis, network theory, and comparative neuro-anatomy, as well as the underlying predispositions and selection pressures we have sketched in this essay to explain the biology of human behavior and, perhaps further, its effects on the emergence and elaboration of early hominin organization. Knowing this background, we can develop sociological theories of evolution revolving around selection on socio-cultural units of organization that are fundamentally different than those driving the biological evolution of hominins/humans. Part of developing evolutionary sociology is to understand what has a biological basis in both behavior and social organization of late hominins and early humans. For one part of “an evolutionary sociology” sketched in this essay, the biological basis of human behavior and organization is inherited from primate evolution in general and the last common ancestor of hominins and great apes. We learn from this strategy mostly about biologically based behaviors and very little about human organization because the nuclear family and hunting and gathering bands are mostly socio-cultural constructions created by later hominins like Homo erectus and Homo ergaster. Here, we can go back to some of the founders of sociology: Spencer, Marx (not a sociologist but we have adopted him), and Durkheim. These founders of sociology recognized that human societies evolved, and they implied types of sociological selection, which is selection on social phenotypes so to speak, but of a very different kind because it was driven by a primate with capacities for agency.

Thus, at a theoretical level evolutionary sociology is Darwinian or biological in understanding how emotions, intelligence, language, and culture could create socio-cultural formations that, over time, became incredibly complex. And, yet, the evolutionary dynamics of selection and evolution still operate but on different phenotypes—socio-cultural in nature—and honed by selection processes that are not Darwinian but rather teleological and driven by human capacities for thinking and the agency of a collective. The first masters developed a variety of types of selection in the socio-cultural universe and these are the beginnings of evolutionary sociology (see Turner and Abrutyn 2017; and Abrutyn and Turner 2021). Of course, nineteenth-century sociologists were not aware of what we know now about biological evolution, but they had the right sense that the socio-cultural universe operates under different forms of evolution and different phenotypes.

Our goal is ultimately to create a powerful evolutionary sociology that can (1) correct for the excesses in much analysis of humans and human society by sociobiology and evolutionary psychology (while incorporating their insights), (2) add methods that are surprisingly rarely used in the social sciences (e.g., cladistics and, even more surprisingly, comparative neuro-anatomy), and (3) follow the lead of the first masters of sociology in developing a purely sociological analysis of socio-cultural evolution revolving around selection of various types of emergent phenotypes (e.g., groups, organizations, communities, institutional domains, stratification systems, societies, and inter-societal systems). Thus, what we have outlined in this paper is only the first step in bringing back theoretically informed evolutionary analysis in sociology, a  subfield that can incorporate both evolutionary theorizing in biology and theorizing of evolutionary dynamics in sociology.

Our analysis has hopefully highlighted how new insights into human behavior and organization can be gleaned by crossing species lines. This begs the question, of course, of whether our research program has relevance for conventional sociology. For openers, the literature on domestic relations and the family is stockpiled with clashing theories over whether the nuclear family is a facet of human nature or a social construct. The primate data and cladistic analysis strongly point to the nuclear family as a human-invented social formation. These findings also strongly point to a communal type of organization for both apes and hominins. A community structure likely preceded all other forms of human coalescence and, as the earliest social formation, it surely forms part of our inherited neurobiology. Although a community is a universal social formation for apes and humans, it is otherwise a rare form of organization for mammals. One might assume that “the quest for community” rests on an inherent human need for intimacy and strong ties. Yet, it is the fluid weak ties that account for why the concept gets generalized to all sorts of associations, even “virtual communities” without real face-to-face relations or even spatial boundaries.

And we should emphasize that although enhanced emotions drove most of hominin evolution, it is a precarious mode of adaptation. Highly emotional animals like humans without strong bio-programmers for group solidarities and with reliance on interpersonal skills and cultural codes are vulnerable to being over-emotional, a fact too often evident in the history of human societies, particularly over the last 10,000 years.

Finally, we need to reconsider the taken-for-granted assumption that humans are naturally highly social with collective needs for lots of strong ties and high-solidarity relations. To be sure, humans are social beings. Yet, human sociality is not derived in the same way as it is for most social mammals who inherited a legacy of evolved built-in bio-programmers for sociality (which in the case of young monkeys is automatically activated simply with exposure to a social environment). Instead, as documented in this article, human high sociality is an evolutionary product of the coordination of slowly evolved mechanisms that include enhanced vocal track elaborations involving changes to the larynx, pharynx, and related structures, an expanded Broca’s area essential for the planning and production of articulate speech, enhanced emotionally and other enrichments to human neuro-anatomy.

Still, if we think over the legacy of our LCA, and as members of the hominoid clade, humans at their core are still individualists and, given a choice, prefer some strong ties but mostly moderate or weak ties. We are after all, evolved apes and not collectivist monkeys. It is time, therefore, when philosophizing about “human nature,” to consider that humans may have retained many of the predispositions for hominoid-style individualism, with a penchant for self-reliance and fluid and flexible social structures.