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Chapter 19

Chapter 19. Human Evolution. On the Origin of The Species. Sold out in 1 day, had only a small section on the origin of man. 12 years later The descent of Man, and Selection in Relation to Sex. The last chapter concerns the origin and evolution of our species.

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Chapter 19

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  1. Chapter 19 Human Evolution

  2. On the Origin of The Species • Sold out in 1 day, had only a small section on the origin of man. • 12 years later The descent of Man, and Selection in Relation to Sex. • The last chapter concerns the origin and evolution of our species

  3. On the Origin of The Species • It is a much debated topic with different camps of researchers drawing different conclusions • Scopes Monkey Trial 1925 in TN • Prohibited teaching evolution • 1995 AL state board of education ruled all textbooks must state Evolution is a theory not fact.

  4. On the Origin of The Species • I have taught you that evolution is a process, and this process is occurring around us. • Utilizing evolutionary theory we will examine physical, morphological, molecular, and behavioral evidence to test hypotheses about human origins

  5. Relationships of Humans and Extant Apes • Order Primates • Suborder Catarrhini = Old World monkeys and apes • Superfamily Hominoidea = great and lesser apes • Family Hominidae = great apes and humans • Homo sapiens • Our closest relatives are Pongo pygmaeus, Gorilla gorilla, Pan troglodytes, and Pan paniscus • Orangutan, gorilla, standard chimpanzee, bonobo

  6. Relationships of Humans and Extant Apes • These taxonomic classifications are not universally accepted • It is accepted that humans belong to the same clade as African great apes • Synapomorphies • Relatively large brain, elongated skulls, enlarged brow ridges, shortened canines, wrist bone fusion, reduced hair, absence of a tail, great flexibility of hips and ankles • Which is our closest relative? • Chimpanzees or gorilla?

  7. Relationships of Humans and Extant Apes • Molecular phylogeny of Sarich and Wilson had polytomy of chimpanzees, gorilla, and man • Timeline showed that split between apes and Old World monkeys was 30 Mya • Split between African great apes and humans 5 Mya

  8. Relationships of Humans and African Apes • Gorillas and chimpanzees share knucklewalking which humans lack • Humans and two chimpanzees share features of skull, teeth, limbs, and delayed sexual maturity • Some molecular analyses place humans and gorillas as sister taxa • Conflicting evidence makes interpretation of correct branching order difficult

  9. Relationships of Humans and African Apes • Difficult to identify which characters are ancestral or derived • Dryopithecus is an extinct European ape 10 My old • Shares many traits with gorillas that humans and chimps lack • If these characters are ancestral and humans and chimps don’t have them, it is most parsimonious for humans and chimps to be sisters

  10. Relationships of Humans and African Apes • Using DNA sequences, researchers attempt to answer human/chimp/ gorilla question • Three different genes gave different tree topologies • mtDNA, Y chromosome, nuclear gene • All trees support humans as sister to chimpanzees

  11. Relationships of Humans and African Apes • Criticism of these reconstructions because they are gene trees and not species trees • If ancestor was genetically variable for a particular locus, the gene tree may not accurately represent the species tree • Ancestor may pass on a subset of its alleles for a gene to each descendent species • That gene will not give an accurate estimate of phylogeny

  12. Gene Tree vs Species Tree

  13. Relationships of Humans and African Apes • Ruvolo wanted to address this issue directly • She exhaustively sampled human and ape individuals • If the gene tree and species tree is incongruent, individuals of a species would not group together • They always did group together in her study so it was not a problem for that gene

  14. Relationships of Humans and African Apes • Another way to test this issue is use as many genes as possible and see which ones are congruent • In 14 independent genes • 11 showed humans and chimps together • 2 showed gorillas and chimps • 1 showed humans and gorillas • Human/chimp relationship most likely

  15. Genome Projects • Humans 23 prs of chromosomes while chimps and gorillas have 24

  16. Timing the Split (by an undergrad)

  17. Recent Ancestry of Humans • Humans and chimps last shared a common ancestor 5.4 Mya • Most recent ancestor was a knucklewalker, ate mostly fruit, lived in many habitats, had complex social groups, and made tools • What intermediate ancestors lived between chimp/human ancestor and modern Homo and Pan?

  18. Recent Ancestry of Humans • Fossils provide some answers • Fossil record is incomplete • Scientists disagree on placement of fossils on hominid tree • Sahelanthropus tchadensis

  19. Recent Ancestry of Humans • Sahelanthropus older end of window for split. Looks like chip due to brain case but from front looks like Australopithecine. • Australopithecines are one of the earliest humans • Two main body forms • Robust and gracile • Bipedal like modern humans • Known from limb structure and footprints

  20. Recent Ancestry of Humans • Australopithecines were short with small brain cases • Modern human structure more similar to gracile australopithecines • Robust australopithecines had enormous teeth and very strong jaw muscles for biting tree branches (like gorillas do today)

  21. Gracile australopithecines

  22. Fossil Timeline

  23. Recent Ancestry of Humans • First Homo was H. habilis • Homo had larger brain cases and smaller teeth than Australopithecus • Homo was also taller with less sexual dimorphism in size • Homo rudolfensis may be the same species as H. habilis but have some skeletal differences • H. ergaster is more recent ancestor from Africa

  24. Recent Ancestry of Humans • H. erectus was first human to leave Africa • H. heidelbergenesis is its descendent • H. neanderthalensis may be our ancestor or may have been a sister species that died out • Modern Homo sapiens appeared approximately 100,000 ya

  25. Recent Ancestry of Humans • Strait reconstructed a cladogram based on fossils • Such a cladogram is difficult to read because ancestor-descendent relationships are not adequately depicted • All species appear as terminal taxa

  26. Recent Ancestry of Humans • Strait also constructed hypothesis of descendent relationships • Time scale allows testing by fossil record • New fossils should be found from ages depicted in correct hypothesis

  27. Recent Ancestry of Humans • Determining trait polarities is difficult so that deciding on which hypothesis is more likely is difficult • In the past as many as five human species coexisted • We are the sole surviving species of a much larger human radiation

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