The Hybrid Origin of “Modern” Humans

Abstract

Recent genomic research has shown that hybridization between substantially diverged lineages is the rule, not the exception, in human evolution. However, the importance of hybridization in shaping the genotype and phenotype of Homo sapiens remains debated. Here we argue that current evidence for hybridization in human evolution suggests not only that it was important, but that it was an essential creative force in the emergence of our variable, adaptable species. We then extend this argument to a reappraisal of the archaeological record, proposing that the exchange of cultural information between divergent groups may have facilitated the emergence of cultural innovation. We discuss the implications of this Divergence and Hybridization Model for considering the taxonomy of our lineage.

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Acknowledgments

We would like to thank Charles Roseman and Dietmar Zinner for their comments that greatly improved this manuscript. RRA hybrid research supported by Grants from the National Research Foundation of South Africa and the DST/NRF Centre of Excellence in Palaeosciences (COE-Pal).

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The authors declare that they have no conflict of interest.

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Correspondence to Rebecca Rogers Ackermann.

Appendices

Appendix 1: Traditional Views (and Our View) of Modernity

Homo sapiens is the only hominin species alive today; we consider all humans living today modern. Traditionally, archaeologists and palaeoanthropologists have defined modernity in both cultural and biological terms (Fig. 1). The evidence for biological modernity has come from the fossil record, and refers to hominins that look (essentially) like us in terms of their skeletons (e.g. large brains, gracile postcrania). Fossils considered to be early modern humans are by no means homogenous, and indeed even the expression of modern features varies across these specimens (McBrearty and Brooks 2000). The evidence for cultural modernity derives from archaeological materials that signal aspects of modern intellectual, symbolic, linguistic and technological capabilities; interpretation of specific artefacts as modern is not straightforward and often controversial (e.g. Botha 2008; Klein 2013).

Fig. 1
figure1

Representative fossil and archaeological evidence traditionally used to indicate biological and cultural modernity. From left to right A modern human mandible from Klasies River, South Africa; perforated shell beads and incised ochre from Blombos, South Africa; bone harpoon from Katanda, Democratic Republic of Congo [images not to scale] (Illustration: TA Sumner)

There are multiple views on when and how modern humanness arose. One view is that while the earliest modern humans in Africa showed derived morphological traits that put them on the path to modernity circa 200 ka, true modernity only arose sometime around 50 ka when an adaptively beneficial neurological change prompted behavioural innovation, providing Africans with a fitness advantage over other archaic peoples (e.g. Klein 1995). Others view the emergence of modernity as more cumulative (e.g. McBrearty and Brooks 2000), occurring over the course of hundreds of thousands of years or more, again primarily in Africa. Both of these models are directional. Regardless, most researchers agree that signatures of cognitive modernity (such as figurative art) only become commonplace after 50 ka, suggesting that complete cultural and biological modernity are relatively recent phenomena.

Our view of the emergence of modernity differs in key respects to those depicted above. We view the emergence of our lineage as a continuing dynamic (process) rather than an outcome (product); there is no clear starting point, or ending point, but rather an ongoing, repeating process of divergence and hybridization at multiple points in its evolutionary history. It is the dynamics of this repeated lineage divergence and remerger that has produced the variation observable in our genome (and phenome) today. We would not expect the directional accumulation of modernity in such a scenario, but rather a sporadic, flickering signal (see also d’Errico and Stringer 2011 for a similar argument on the early archaeological record of the human lineage). In this scenario, references to “modernity” or “archaicness” become problematic (see “Appendix 2”); we would suggest abandoning the terms in the context of the origins and evolution of H. sapiens.

Appendix 2: Questions of Taxonomy

Currently the term modern humans, or modern Homo sapiens, is used almost exclusively in the palaeoanthropological and archaeological literature to refer to people emerging from Africa—i.e. people of African origin. However, we now know that Neanderthals and Denisovans (and potentially other archaic lineages) are part of many peoples’ ancestry today, raising the question of whether it is valid to exclude them from our species. Considered another way, if you send your cheek swab into learn your ancestry and find out you are 10 % Neanderthal, it would be nonsense to say that you are 90 % modern human. Additionally, given the increasing genetic and morphological evidence for hybridization emerging from the fossil record, it is likely that the assignment of certain individuals to current taxonomic categories is impossible. It is almost certain that individuals previously defined as modern humans or Neanderthals (or something else) are actually hybrids, as recently argued on the basis of both genetics (Fu et al. 2015) and morphology (Ackermann 2010).

Defining Homo sapiens has always been problematic (see discussion in Wood 2011). Relying on shared derived characteristics such as big brains and language abilities does not incontrovertibly exclude groups like Neanderthals who also had large brains and may have had comparable cognitive abilities (Vanhaeren et al. 2006; Bouzouggar et al. 2007; Peresani et al. 2011, 2013; Rodríguez-Vidal et al. 2014). Using a common approach for determining affinities of fossils—i.e. a phenotype within or close to the range of humans living today—is also problematic given that today’s range includes the effects of hybridization, as discussed here. More pointedly, even traits acquired through hybridization are ‘modern’ in the sense that they contribute to the range of variation seen in people living today. We suggest at this time, and until more is known about phenotypic and genomic variation in Pleistocene groups, that considering Homo sapiens as a single complex lineage, with significant divergence and anastomosis among sub-groups, is the most inclusive and accurate approach. We would like to see the elimination of the term ‘modern humans’ in exchange for simply calling our taxon H. sapiens, with people alive today referred to as living (extant) humans. We would recommend that researchers consider everyone prior to living people who contributed directly to the variation in our lineage as human ancestors with regional population names like Denisovans and Europeans, rather than giving them species-level distinctions.. We recommend this last measure because referring to e.g. Neanderthals versus ‘modern humans’ gives the incorrect impression that certain human groups living today are less modern than others.

Were these ancestral groups distinct species? Most evolutionary biologists would agree that species should be so-called if they retain morphological, behavioural and genetic differences even in the face of gene flow (e.g. Coyne and Orr 2004), and certainly Africans and Neanderthals (and potentially others) likely remained divided over a (relatively) geologically short evolutionary time period due to geographic barriers, becoming distinct lineages in isolation (or near isolation). Humans living today represent a genomically coherent species. Whether and which ancient Homo taxa (including Pleistocene H. sapiens) can be considered monophyletic, genomically coherent species will have to await sequences of numerous genomes from across the range of their distribution(s). However, this description of what keeps species distinct also implies that the ability to coexist geographically (sympatry) without the fusion of lineages is a result of being separate species (Mallet 2008). It is less clear whether this was the case. Indeed, post-contact gene flow and the subsequent disappearance of all but a single lineage suggests that these archaic lineages may not have been able to coexist without the fusion of lineages.

Glossary
Accretion Model A model of modern human origins that proposes an African origin for biologically modern humans, but includes a small but significant amount of admixture (~10 %) between these groups and local populations upon leaving Africa and spreading across Eurasia (Smith 2010).
Acheulean A technological industry associated with the production of large cutting tools, such as handaxes and cleavers. The Acheulean has a vast temporal and spatial spread, persisting from ~1.75 Ma to <500 ka (Herries 2011), and extending from Africa through Europe to west Asia. The presence of an Acheulean in east Asia is the subject of ongoing debate.
Admixture The production of new genetic combinations in hybridizing groups.
Afro-European sapiens Hypothesis A version of modern human origins that favours a gradual, mosaic-like process of anatomical modernization in Africa over approximately a half million years. Accepts possibility of very minimal evidence for interbreeding outside of Africa (e.g. with Neanderthals) as African people migrated to the rest of the world (Bräuer 2008).
Denisovans A recently discovered group of genetically distinct hominins known from a handful of bones and teeth, who lived 41 kya in Siberia. DNA analyses have shown that they share a common ancestor with Neanderthals, and contributed to the genetic make-up of some Oceanic populations (Krause et al. 2010; Reich et al. 2011).
Hominin The colloquial term for members of the tribe Hominini. This tribe includes humans, as well as all of our extinct ancestors that evolved after the split from the tribe Panini (chimpanzees and bonobos).
Gene Flow The transfer of genes (DNA) from one population to another.
Hybridization (Natural Hybridization) Successful matings (in nature) between individuals from genetically differentiated lineages.
Introgressive Hybridization, or Introgression The transfer of DNA between individuals from two genetically differentiated lineages via hybridization, followed by repeated backcrossing between hybrid and parental individuals and the subsequent movement of genes back into parental population(s). This can be unidirectional or bidirectional.
Later Stone Age The period following the Middle Stone Age in Africa, and often seen to parallel the Upper Palaeolithic in Europe, the Later Stone Age is viewed by many researchers as reflecting the appearance of fully modern behaviour in the archaeological record. Art, symbolism and ornamentation become entrenched in the Later Stone Age after episodic expression in the Middle Stone Age. The advent of the Later Stone Age is spatially and temporally complex, with earliest dates ranging from >46 ka in east Africa to <22 ka in parts of southern Africa (Opperman and Heydenrych 1990; Ambrose 1998).
Levant An area of south west Asia that lies between Africa and Europe, the Levant records the fluctuating range extensions of Eurasian and African fauna—including Neanderthals and African-derived modern humans—through the Middle and Late Pleistocene (Tchernov 1994).
Mousterian A technological industry originally associated with Neanderthals in Europe and featuring a diversity of forms linked by use of the Levallois core reduction concept (Boëda 1995). Middle Stone Age industries in North Africa are sometimes also referred to as ‘Mousterian’, though clear Neanderthal remains are absent in the region.
Neanderthals A taxon of hominins that lived in Eurasia from approximately ~250–40 ka. Neanderthals are known to differ both morphologically and culturally from their African counterparts, being physically robust and having a well-established stone tool tradition, the Mousterian.
Pleistocene A geological epoch dated from approximately 2.588 million to 11,700 years before present (BP). Humans evolved into their present form during this time.
Transgressive Phenotypes Phenotypes in hybrids that exceed the range of phenotypes that are observed in the parental taxa (Seehausen et al. 2014).

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Ackermann, R.R., Mackay, A. & Arnold, M.L. The Hybrid Origin of “Modern” Humans. Evol Biol 43, 1–11 (2016). https://doi.org/10.1007/s11692-015-9348-1

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Keywords

  • Cultural and biological modernity
  • Hybridization
  • Frontiers
  • Neanderthals
  • Denisovans