Goals: Review patterns and processes of macroevolution by applying them to humans.
Related Textbook Material: Freeman and Herron (2001) Chapter 16
Lab Manual Questions over this material are in Lab Manual Chapter XXII
The Lecture:
In this lecture we'll review some of the topics in macroevolution that we've covered during the course by seeing how they apply to humans. Much of the evolutionary history of humans, including identification of possible ancestral species, has been done by finding and studying fossils, and this continues to be an active area of research (in fact reports of a new, important human fossil came out just last week -- in April 1999.) We're not going to consider much of that research here -- our goal will be to continue looking at what we can learn about macroevolution by studying modern groups.
A first question we can ask is how humans are related to other living species phylogenetically. Based on morphological characteristics, humans clearly belong within the group that includes the great apes -- orangutans, gorillas, chimpanzees, and bonobos (a smaller species of chimpanzee). In fact we turn out to have so many morphological similarities to the great apes that it is very hard to find enough characteristics that differ to study phylogeny. Using DNA sequences, however, people have found some variation that allows phylogeny to be studied. Humans, chimpanzees, bonobos, and gorillas have about 98% of their DNA in common, though -- we are very similar, genetically, to these great apes -- indicating that we are all closely related. To study the phylogeny of humans and the apes, because of the close relatedness, people have used primarily non-coding mitochondrial DNA since it evolves rapidly and will therefore show enough variation to study phylogeny of these closely related species ( Click here if you need to review the lecture on rates of evolution of different areas of DNA to see why this is an appropriate choice )
The phylogeny that is best supported by these DNA data looks like this:
So you can see that our closest relatives appear to be the two chimpanzee species, the chimpanzee and the bonobo; the gorilla is a little more distantly related and the orangutan is still more distantly related.
Another area we can study is how human structure has evolved; we can ask whether there are examples of heterochrony that have affected human structure. A possible example is the shape of the human head. Humans and the great apes start out development with round heads, flat faces, and high foreheads; the great apes subsequently develop a projecting muzzle and low forehead so their heads no longer appear as round as ours -- their faces are elongated. So humans stop at an earlier developmental stage in head shape; the great apes progress to a later developmental stage. To determine whether this has occurred through terminal addition or paedomorphosis, note that the orangutan is the outgroup to the humans and other great apes; it, like the rest of the great apes, progresses to the elongate face stage. So humans have the derived state, and this is to stop at an earlier developmental stage. So this is an example of paedomorphosis. (Click here if you need to review the lecture on heterochrony, including terminal addition and paedomorphosis.)
Another aspect of human evolution that has been studied by looking at phylogenies is the geographic origin of humans. It is possible to study the likely history of the geographic areas used by species and populations just the way we study the evolutionary history of other traits -- by making a phylogeny and using it to determine, from the areas used by modern populations, what areas the ancestors most likely lived in. (Click here if you need to review the lecture on phylogenetic tests of adaptation, where this method was introduced.) A number of studies have been done to study the phylogeny of modern human populations in different geographic areas to try to determine how humans moved, historically. They are typically based on mitochondrial DNA both because it is rapidly evolving and because it does not recombine, so if one does a phylogeny of the mitochondrial DNA in humans each DNA haplotype (haploid genotype, the genotype of the mitochondria) reflects just one historical line -- our nuclear DNA, in contrast, gets recombined among individuals through sexual reproduction and would reflect many different lines. The results of one of the first such studies, based on non-coding mitochondrial DNA, is shown here:
This suggests that modern humans originated in Africa and later some lines moved out of Africa while others stayed there. Since the original fossil humans are also African, we expect humans to have originated in Africa. However, another thing that was done with this phylogeny was to try to determine the date at which humans left Africa. This was done using the molecular clock hypothesis ( Click here if you need to review the lecture on the molecular clock hypothesis. ) The date of speciation between humans and chimpanzees was determined from fossils; this date was used to determine the rate of evolution of the mitochondrial DNA. This rate was then used to determine the time of separation of the mitochondrial DNA lines in different human populations. Based on this, the date of movement out of Africa was determined to have occurred no more than about 200,000 years ago; in contrast, fossils suggest that the date of movement out of Africa occurred no later than about 500,000 years ago, and probably more like a million years ago.
What could cause the difference between the estimated time of movement out of Africa based on fossils and based on the phylogenetic/molecular clock analysis? Some people have suggested that it means that the people who dispersed out of Africa 200,000 years ago took over and killed off the human populations that had come out of Africa earlier. There are, however, other possible explanations. One possibility that has some support is that the phylogenetic/molecular clock results are incorrect because of the way the study was done, for some of the following reasons: