THE EVOLUTION OF HUMANS
& OTHER PRIMATES
The following outline was adapted from a presentation, with slides, at a symposium Teaching Evolution (and Confronting Creationism) in the College Classroom at the meetings of the American Association of Physical Anthropologists, Salt Lake City, Utah, April 3, 1998. It was also presented at the Convention for the National Association of Biology Teachers in Reno, NV, November 4, 1998, and in somewhat abbreviated form at the NABT Convention in Ft. Worth, TX, October 28, 1999.
The full text of his presentation
was published as "Humans As a Case Study for the
Martin Nickels, Illinois State University
SEVEN LINES OF EVIDENCE FROM BIOLOGY:
1. HIERARCHICAL (TAXONOMIC) CLASSIFICATION (Linnaeus)
2. COMPARATIVE ANATOMY
3. COMPARATIVE EMBRYOLOGY (Ernst Haeckel)
4. COMPARATIVE BIOCHEMISTRY (1950's)
5. ADAPTIVE COMPROMISES or"IMPERFECTIONS"
- [See some examples in the Blocks & Screws lesson]
6. VESTIGIAL STRUCTURES
FIVE LINES OF EVIDENCE FROM PALEONTOLOGY:
9. HOMININ FOSSIL SEQUENCE & PATTERN
- [See our Chronology Lab]
10. FOSSIL INTERMEDIATES: There are plenty of NON-"Missing
11. ECOLOGICAL COHERENCE OF FOSSIL ASSEMBLAGES
12. CHRONOLOGICAL SEQUENCE OF STONE TOOLS
by Martin Nickels, Illinois State University
NCSE Reports, vol.18, no.5, Sep/Oct, 1998, pp.24-27
As physical anthropologists we are either blessed or cursed when it comes to teaching about evolution. The reason for the dilemma is that, on the one hand, we focus on the organism that is clearly the most problematic and difficult for many people to accept as having evolved and, on the other hand, we deal with the species that is probably unparalleled in terms of the sheer amount of scientific information and evidence supporting the idea that evolution has occurred on this planet.
Personally, I regard our focus on human evolution as an unparalleled and golden opportunity for teaching about evolution and addressing important aspects of the creation - evolution controversy. There are several reasons for this.
First, we get to deal with the organism that more people and students are most inherently interested in than any other: themselves. This means that we can take advantage of this interest and use it to deal with one of the most important ideas in all of science, namely evolution.
Second, because of the amount of scientific evidence that exists for human evolution, we are in the enviable position of being able to draw upon knowledge from many areas of scientific research and build one of the strongest cases for evolution in all of biology.
Third, because of our primary focus on humans, we can underscore and reinforce the idea that humans are indeed animals (that is to say, we are a part of the natural world rather than a creature set apart from it. This idea becomes even more important, of course, when we make the case that humans are a natural product of biological processes.
Fourth, by making the case convincingly for human evolution, we pretty much assure that making the case for any other species will be that much easier. After all, having already dealt with the single most problematic species of all, there can't be too many objections to thinking that other - indeed all other - organisms have also evolved.
Fifth, we have the opportunity to illustrate several important aspects of the nature of science and scientific knowledge. These include using such criteria as independent lines of evidence, concordance or consistency of evidence and the predictive power found in the patterns inherent in nature to advance scientific understanding of the world we live in and have emerged from.
The focus of this discussion is to illustrate both the strength of the many lines of scientific evidence supporting the idea of human evolution and the importance of the concordance or agreement that exists among them. Some of the most important criteria by which the strength of any scientific theory is assessed include the number of independent lines of evidence that are concordant with one another and the ability to use knowledge of one line to predict the pattern we should find in another. Thus, using humans as a case study in evolution also allows us to illustrate some broader aspects of the nature of science and how one can judge the overall strength of any scientific theory or explanation.
I want to underscore the importance of using the term "evidence" rather than the more colloquial term "proof" in normal scientific discourse. Scientists deal with evidence, not proof, in the sense that we deal with information and data that must be made sense of or interpreted rather than being, pardon the expression, self-evident. Mathematicians and logicians may deal in undeniable proof because of the nature of the abstract ideas and concepts that they deal with, but scientists must discover the patterns inherent in the natural world and then explain them in light of our understanding of the natural processes that we must use to account for those patterns.
Scientists have, in turn, developed criteria to assess and evaluate the relative merits of alternative explanations of the evidence. These criteria include valuing concordance among independent lines of evidence and the ability to predict one line of evidence from another as ways to distinguish better explanations from worse ones.
Now, let me turn to 12 lines of Evidence for Human Evolution. I've grouped them into 7 that represent evidence from the biological present and 5 that represent evidence from the geological and biological past. I will make observations about their significance and inter relatedness rather than explain what each line means since I think they're mostly self-explanatory in that regard.
Category Number 1 (Hierarchical Taxonomic Classification) is a good example of a pattern that can, of course, be explained by special creation. Linnaeus did just that. But Darwin - a century later - explained the same set of orderly relationships between organisms as being the result of divergent evolution and shared ancestry. More important, though, is the fact that organisms created de novo need not show varying degrees of similarity to one another. Each creature could be constructed completely differently from every other creature and made from very different materials. Humans need not look like apes, but we do. We show varying degrees of similarity to them and we are made of the same stuff. We could have been created this way but we must look like this if, indeed, we have evolved and diverged from a relatively recent common ancestor.
Another important and seldom appreciated characteristic of the evolutionary explanation for the existence of organisms in naturally nested or hierarchical groupings is that it allows us to predict that organisms with certain combinations of features - such as chimpanzees with wings, flowers with bony skeletons, or humans with hooves instead of feet - are biologically impossible because of the unbridgeable gaps produced by the major divergent evolutionary events that separate chimps from birds, flowers from vertebrates, and humans from horses. An all-powerful creator, of course, could create almost any combination of such fantastic and fanciful creatures.
Number 2 (Comparative Anatomy) and Number 3 (Comparative Embryology) are similar to Number 1 in that organisms could have been deliberately formed to resemble one another but they need not have been. But if organisms share varying degrees of evolutionary kinship with one another, then such anatomical and embryological similarities are mandatory. There is probably no more powerful or striking example of such similarity than that seen among the fetuses of primates, especially the hominoids.
Category Number 4 (Comparative Biochemistry) is of special interest and importance. This is due to the fact that the agreement or concordance of the biochemical evidence with the anatomical evidence illustrates another important consideration when evaluating the strength of evolutionary theory: namely that our 20th century ability to compare the biochemical similarities among species provided a test of evolutionary theory which had been mainly based on the evidence from 19th century comparative anatomical studies, biogeography and a very limited fossil hominin record. If the same overall pattern of biochemical similarities did not agree with the pattern based on anatomical comparisons, evolutionary theory would have been in serious trouble. But the patterns do agree and evolutionary theory is all the stronger because of that.
Number 5 (Adaptive Compromises) and Number 6 (Vestigial Structures) are both very difficult to explain as being the result of deliberate design or special creation since they represent such "poor" engineering. But they are exquisite examples of the constraints inherent in biological systems evolving over time and having only existing ancestral structures available for modification in the face of new and often competing selective pressures. The human examples I've listed under Number 5 - the pelvis and the larynx - are two of the better examples of adaptive compromises between competing selective pressures that I know of.
Item Number 7 (Biogeography) refers simply to the observed fact that similar-looking species tend to be found in close proximity to one another - as illustrated by the primate examples I've listed. The special case of biogeography pertinent to human evolution, of course, is that in 1871 Darwin used the work of Huxley and others which showed that humans most resemble chimpanzees and gorillas who live only in Africa to predict where we would most likely find fossils of our own earliest ancestors - Africa.
That Darwin was correct is borne out by Category Number 8 (Paleobiogeography) as, indeed, all of the earliest-known hominins are from Africa and nowhere else. But the fact that Darwin could use evidence from biogeography to predict what the pattern should look like in a completely separate body of evidence - the fossil record - is a wonderful example of how concordance among separate lines of evidence is both a testable prediction of a scientific theory and further support for a theory - in this case, evolution - when the prediction is borne out.
Number 9 (the Fossil Sequence) for hominins is just a single case study of the general pattern present in the overall fossil record. That pattern is that modern species are not found throughout the fossil record from top to bottom - which they should be if all species were formed at one time at the very beginning of life on this planet. Instead, what we discover is less and less evidence of modern species as we go deeper and deeper into the fossil and geological record - a pattern both predicted by evolutionary theory and completely consistent with evolutionary theory. Indeed, this is the only pattern consistent with evolutionary theory. And there is no more impressive fossil series one can use to illustrate this pattern than the overall hominin fossil sequence. There is also no more pedagogically powerful example for students than that of our own lineage.
Number 10 (Fossil Intermediates) refers to the fact that, regardless of the mode or rate of evolutionary change, there should be evidence of morphological continuity over time in the fossil record if species are evolutionarily linked and related to one another. Is there a better classroom example one can use to illustrate this point than a fossil like Lucy with her mixture of ape-like and human-like features? I sometimes think that as physical anthropologists we are especially blessed to have such a wonderful example to use in our teaching.
Number 11 (the Ecological Coherence of Fossil Assemblages) is an especially powerful point to use when countering the associated claims of Flood Geology that many creationists make. The idea that the fossil and geological pattern seen on this planet is really a record of a single, recent, global catastrophe in the form of the Great Deluge and Flood postulates that no real chronological order of any consequence exists in the earth's geological or fossil record. But the fact that successive fossil assemblages actually contain ecologically-coherent groups of species common to specific environments counters this creationist claim by illustrating that environments come and go and come again many times over time but the species within them change.
The fossil record, then, is not merely a jumbled collection of drowned flood victims but a record consisting of ecological snapshots of the natural history of life on this planet. The number of ecologically-coherent paleoanthropological and archeological sites from Laetoli, Lake Turkana, Olduvai Gorge on up to the present is stunning, and all provide excellent examples for us to use in our teaching.
Finally, Number 12 (the Archeological Record) of stone tools and other artifacts is a uniquely human line of evidence available to us because we teach about human natural history. No other organism has left such a record of its behavioral evolution. More importantly, the pattern of change in the lithic prehistory of humans parallels that of the fossil record in its change from more primitive to more modern over time. The archeological record uniquely enriches our study of human evolution.
Individually, perhaps, one can claim that any given line of evidence looks the way it does because that is the way it was intended to look by a/the Creator. But such a creationist claim actually involves mixing elements of different creationist models such as the "young earth - quick creation with a flood" model and the "old earth - progressive creation without a flood" model in ways that are fundamentally incompatible and inconsistent with one another. Only an evolutionary explanation can rationally account for these lines of evidence both individually and collectively. Indeed, it is their combined strength that supports evolution so extraordinarily well.
In conclusion, the fact that there are so many lines of evidence in support of the idea of human evolution simply means that we, as physical anthropologists, have an unrivaled opportunity to teach about evolution and effectively confront creationism in our classrooms. We have the best case study for evolution in all of biology. Let us rejoice in that and use it in our teaching. The opportunity is yours, and I hope you all take advantage of it.
Acknowledgment: Many thanks to Craig Nelson of Indiana University for helping me develop and enrich my thinking about the strength of the case for evolution in general. He encouraged me to apply several of these lines of evidence to humans as a case study.
AUTHOR S ADDRESS:
Martin Nickels is Professor of Physical Anthropology at Illinois State University in Normal. His research interests include the history of human evolutionary studies, hominin paleontology and prehistory, primate behavior, and the biological bases of human behavior: He twice was selected as the Outstanding University Teacher at Illinois State University and was a Sigma Xi National Lecturer in 1995 and 1996. He co-authored The Study of Physical Anthropology and Archaeology, as well as articles in many professional journals.