Chapter 20. Human Evolution. Focus on material related to these chapter questions: #2 (Which came first, bipedality or large brains? Bipedality), 5 (Did humans evolve from chimpanzees? No, humans and chimps share a MRCA not shared by other extant primates), 7 (Fig.20.4: How does human genetic variation compare to close relatives? Remarkably less genetic variation in humans compared to other apes. What populations harbor the most genetic diversity, including the oldest alleles in the human gene pool? Most African populations.), 8 (Did knucklewalking evolve independently in Pan and Gorilla, or was it lost in the human lineages? It was lost in the human lineage), 12 (What features of Sahelanthropsis are suggestive of a very early hominid, near the divergence with Pan and Homo? Small brain case yet with a flat facial profile, slender teeth, and a brow ridge. The small brain case is ape-like, the others are human or chimp-human like. The 6 Ma hominid appears to have been bipedal because of such traits as a foram magnum in a forward position.), 13 (Should the outcome of the debate on the African replacement versus Multiregional evolution hypotheses affect ethical attitudes towards issues of race? No, scientific results show have no bearing on ethics.), 14 (Do genetic diversity studies suggest African races are in some way primitive? No, African populations harbor old alleles that are not found outside Africa), 15 (What are the social or ethical implications for the finding that no human race is monophyletic? None.).

1) Modern humans (Homo sapiens)are monophyletic and of recent African origin. Monophyly. Ancient DNA extracted from Neanderthals (with large brain cases, hyoid bone, etc.) form a sister clade to all samples taken from modern humans (e.g., Fig. 20.22). Recency. The MRCA of humans is so recent that Fst values among North American wolf populations are much greater than those measured between the most divergent human populations (e.g., Africans and Native Americans). Humans have a maximum mtDNA divergence of about 1% of mostly transition point mutations. This implies 0.5% divergence from the MRCA of modern humans, which is equivalent to less than 250,000 years. Pages 776-777 (Box20.2) show evidence for the relatively limited genetic variation of modern humans. Modern human fossils distinguished by a protruding chin, and uniquely associated with ornamental tools and weapons, burial sites (Neanderthals show evidence of this), artistic portrayal of the environment, and evidence of migration across expanses of ocean appear only within the last 100,000 years. African Origins. Pages783-785 (Box 20.3) show African populations harbor more genetic variation than elsewhere. Phylogenetic analysis mitochondrial, Y, and autosomal loci show the earliest branching lineages (alleles) to occur exclusively in Africa, such that Africa is optimized as the source area in all of these phylogenies. The oldest anatomically modern human fossils are found in eastern Africa, which agrees with the genetic signature of an African source area.

2) No modern human race or any other group based on phenotype, culture,or geography is monophyletic, in contrast to most species. (see mtDNA phylogeny including Biology 403 sample). In contrast to most species, especially primates, modern humans are exceptional in having a widespread distribution with no evidence of monophyletic allopatric races or populations. We achieved our worldwide distribution much more quickly than mutation and drift could have given rise to monophyletic races.

3) Modern humans are the sole survivor of a diverse phylogenetic family of bipedal cousins that co-existed for much of the last 4 Ma in Africa (figures 20.10 through 20.14, pages 766-770, and figure20.15, page 771). These included the herbivorous (robust) and omnivorous bipedal apes (Paranthropus and Australopithecus, respectively, with brain capacities less then 600cc), the first humans or Habilines (brain capacity 600-700 cc, first humans to leave Africa by 2 Ma ago), Ergasts (brain capacity 900-1100 cc, continuous prominent brow ridge, first to use campfires, widespread in Africa and Asia),and Neanderthals (stocky limbs, large nose, brain capacity 1400 cc, bones with healed wounds, European and adjacent Asia/Africa), and modern humans. Modern humans migrated out of Africa possibly as recent as 100,000 years ago, and replaced all previous forms of Homo. Genetic evidence from the Neanderthal genome project, the sister species of modern humans according to fossil evidence, suggests the gene pools of these earlier forms of humans have little legacy in the modern human gene pool (figure20.22).

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