Darwin’s “Extreme” Imperfection?
KeywordsMissing link Transitional features Transitional forms Fossil record Teaching
Darwin used the words “extreme imperfection” to refer not to any personal character flaw but to the gappy nature of the fossil record (Darwin 1859). The vast majority of organisms that have lived on Earth never fossilized. Jellyfish and worms? Too squishy to fossilize very often. Butterflies? Too delicate, for the most part. Anything that lived on mountain slopes or in fast-moving rivers? Unlikely to be covered by sediment and preserved. Darwin saw such haphazard preservation as a serious problem for the theory of evolution. After all, he was proposing that different modern species share common ancestors—that organisms as different as lobsters and butterflies have the same great-great-great-great…great-grandparent species. Direct fossil evidence of all the intermediate forms connecting an ancestor to its modern descendents would have provided undeniable evidence in favor of his theory. In Darwin’s view, however, the fossil record provided no such support. Much of On the Origin of Species is taken up with marshalling other forms of evidence to support his ideas about common ancestry and natural selection.
In an article included in this issue (Charles Darwin and Human Evolution), Ian Tattersall (2009) proposes that Darwin’s disparaging view of the fossil record, along with other factors, might help explain why Darwin did not write more about human evolution. Tattersall notes that some fossils shedding light on human evolution had been discovered at the time of Darwin’s writing but that the area was tainted by fraud and controversy. Darwin may have viewed our own fossil history as just another gap in the fossil record—one which might never be filled by hard evidence. Though he clearly accepted the idea that humans evolved from ape ancestors, Darwin may have curtailed his discussion of this transition partly because of the evidence he wanted, but did not have: fossils of so-called missing links. Here, we will dig into the concept of a missing link to see whether this is the problem for evolutionary theory that Darwin imagined and to find out what we have learned about this sort of evidence since Darwin published his ideas in 1859.
Are “Missing” Links Really Missing?
Darwin (1859) described the lack of missing links in the fossil record as “the gravest objection which can be urged against my theory.” If this was the most serious objection raised against Darwin’s ideas at the time, the counterarguments must have been weak. After all, everything we know about geological processes and biology indicates that we should not expect the fossil record to provide a complete catalog of the history of life on Earth. The fact that the fossil record is, indeed, incomplete is entirely consistent with evolutionary theory. Sure, more transitional fossils would shed additional light on the evolutionary history of many lineages, but a lack of transitional fossils in no way implies that those lineages did not evolve. As we will see, the fossil record may even be more complete than Darwin imagined.
Missing Links vs. Transitional Features
Paleontologists interested in major transitions in the history of life generally prefer the term transitional feature to missing link for several reasons. First, as described above, missing links are not really missing. Second, different features of a modern organism may have evolved at different times in that lineage’s history. The tetrapod transition from water to land, for instance, involved the evolution of many features—adaptations for moving, sensing, breathing, and bearing young in this new, dry environment. However, these changes did not all occur at once. For example, evidence suggests that the earliest tetrapods evolved four limbs long before they evolved an ear with an eardrum adapted for sensing vibrations through the air (Clack 2002). This means that the fossil organisms that represent the transition from fin to leg may not be the same organisms that most clarify the evolution of modern tetrapod ears. Hence, in many cases, it is more accurate to focus attention on a specific transitional feature than on an organism as a whole.
Show Me the Fossils
Our improved understanding of geology, fossilization, and the history of life have provided critical information about where to look for fossils with transitional features. In fact, Tiktaalik was not discovered by chance but because scientists went looking for it. Previous research had suggested that vertebrates made the transition to land-living in river ecosystems about 375 million years ago—so paleontologists began their search in 375 million-year-old rocks that had preserved a river delta. When the scientists did discover Tiktaalik, its form was not much of a surprise: Tiktaalik had the set of characteristics that they had expected to find in such an organism, based on other transitional fossils.
Links in the Chain?
In his article, Tattersall also explains that the whole idea of links (missing or not) is rooted in previous concepts of the Great Chain of Being—the idea that organisms can be arranged in a sequence from lowest to highest, with humans looking down upon other species. As Tattersall notes, Darwin was not an advocate of this idea—and yet the Great Chain of Being has sometimes been twisted into an evolutionary version of the same notion. According to this view, humans are more evolutionarily advanced than other species. It is understandable why this view is attractive. Humans view the tree of life from the vantage point of our own tiny twig. We trace the hominid branch back in time—passing long-lost relatives along the way (our Neanderthal cousins, Great Aunt Lucy…)—until we reach the ancestor linking us with other primates and marvel at how far we have come.
The notion of the Great Chain of Being is invalid, but not just because modern organisms lack any sort of ancestor–descendent relationship to one another. The whole idea of ranking organisms on a grade, from primitive to advanced, implies that there is an objective way to measure advancement. But this unbiased measure simply does not exist. We humans are familiar with our own species and tend to see our own adaptations and abilities (big brain, complex culture, unusual ability to manipulate our surroundings) as unparalleled innovations. But that value judgment is unjustified. Another organism might see it differently. Imagine, for a moment, evolutionary history from the point of view of a spider. A spider might not notice any huge difference between humans and chimps. We are just a couple of hairy mammals with big heads—and, after all, what is a little difference in brain size, compared to, say, a spider’s intricate mating rituals, impressive silk-secreting apparatus, and complex web design? That spider might be just as impressed with its own evolutionary innovations as we are with our own adaptations. The same perspective-shifting exercise could be applied to any organism on the tree of life, including chimpanzees. In fact, some recent research suggests that, in some ways (namely, the adaptive evolution of proteins), chimpanzees may have evolved more than humans have since our two lineages split (Bakewell et al 2007). In short, as players ourselves, humans are in no position to make an objective assessment of who “wins” the evolutionary game. We are not outside or above the tree of life—but a part and product of it.
Give Me an Example of That
Whale evolution. Here, we focused mainly on fossil organisms that illustrate the evolutionary steps linking fish to modern tetrapods. This video clip and short article from WGBH takes this transition full circle, exploring some of the fossil evidence that helps us understand how some land-dwelling tetrapods made their way back into the water and became whales (http://www.pbs.org/wgbh/evolution/library/03/4/l_034_05.html)
Find out how people viewed fossils in the earliest days of paleontology in this short article from the Understanding Evolution website: http://evolution.berkeley.edu/evolibrary/article/history_04
Find out more about science’s earliest views of human evolution in this short article from the Understanding Evolution website: http://evolution.berkeley.edu/evolibrary/article/history_17
Evolutionary biologist Leslea Hlusko’s research takes her from the deserts of Ethiopia, where she hunts for hominid and primate fossils, to a baboon colony in San Antonio where she takes thousands of measurements of the primates’ imposing canines. Find out how the two projects are linked, in this research profile from the Understanding Evolution website: http://evolution.berkeley.edu/evolibrary/article/hlusko_01
A news item addressing the discovery of Tiktaalik: http://evolution.berkeley.edu/evolibrary/news/060501_tiktaalik
A news item describing a recent fossil discovery in the area of human evolution: http://evolution.berkeley.edu/evolibrary/news/070901_headlines
A news item explaining research that suggests that, in some ways, chimpanzees may have evolved more than humans have: http://evolution.berkeley.edu/evolibrary/news/070501_chimps
In the Classroom
Adventures at Dry Creek is an interactive web-based module from the University of California Museum of Paleontology, for grades 6–8. Students conduct a simulated field study at a fossil dig in Montana. This activity comes complete with an explanation of the standards addressed, lesson plans, discussion questions, and assessments (http://www.ucmp.berkeley.edu/education/explorations/reslab/newdc/index.html)
Stories From the Fossil Record is a web-based module from the University of California Museum of Paleontology, for grades 5–9. This lesson provides students with a basic understanding of how fossils can be used to interpret the past. It comes complete with an explanation of the standards addressed, lesson plans, and assessments (http://www.ucmp.berkeley.edu/education/explorations/tours/stories/index.html)
Getting Into the Fossil Record is an online, interactive module from the University of California Museum of Paleontology. It comes in two versions appropriate for grades 5–8 or 9–12. In it, students are introduced to fossils and the fossilization process by examining how fossils are formed and the factors that promote or prevent fossilization. It comes complete with an explanation of the standards addressed, lesson plans, and assessments (http://www.ucmp.berkeley.edu/education/explorations/tours/fossil/index.html)
The Evolution of Flight in Birds is an online, interactive module from the University of California Museum of Paleontology, for grades 9–12. Students examine evidence from the fossil record, behavior, biomechanics, and cladistic analysis to interpret the sequence of events that led to flight in the dinosaur lineage. This module comes complete with an explanation of the standards addressed, lesson plans, and assessments (http://www.ucmp.berkeley.edu/education/explorations/reslab/flight/main.htm)
Hominid Cranium Comparison is a lesson from the Evolution and the Nature of Science Institute, for grades 9–12. Students describe, measure, and compare cranial casts from contemporary apes, modern humans, and fossil hominids to discover some of the similarities and differences between these forms and to see the pattern leading to modern humans (http://www.indiana.edu/~ensiweb/lessons/hom.cran.html)
Investigating Common Descent: Formulating Explanations and Models is a lesson from the National Academy of Sciences, for grades 9–12. Students formulate explanations and models that simulate structural and biochemical data as they investigate the misconception that humans evolved from apes (http://www.nap.edu/openbook.php?record_id=5787&page=81)
The author wishes to thank Judy Scotchmoor and Brian Swartz for helpful comments on earlier drafts, as well as David Smith, Josh Frankel, and Kevin Padian for help obtaining images.