The Evolution of Homo sapiens: Intriguing Views and Controversial Theories

1. The Evolution of Homo sapiens: Current Views and Provocative Hypotheses

2. Outline • I – Definitions etc • II – 2 Fallacies • III – “Important” genera • IV – 2 Hypotheses for origin of H. sapiens

3. Cladistics A type of phylogenetic analysis based on two premises: Synapomorphies define branching points. Synapomorphies are nested. Synapomorphy (def) -- Shared, derived trait in phylogenetic “tree”

4. What types of traits can be used as synapomorphies? • Morphological • Developmental • Molecular

5. Reliable trees are built using multiple traits  • Consensus trees.

6. Alternative trees of “great apes” • What do these trees show? • What’s different about them?

7. Growing tendency to use horizontal representations. • Why? • Basal = ancestral (“primitive”) • Derived = descendant

8. Timeline on tree • How do we date divergences?

9. Dating divergences: • Fossil record + radiometric dating • What if no fossil of common ancestor? (e.g., first animal, first mammal, first primate) • Molecular clocks

10. Molecular clocks • Some DNA/aa sequences diverge at linear rates over time. • These can be used to date divergences among lineages.

11. Order Primates • Adapted to arboreal existence • Dexterous hands, feet; partially or fully opposable thumbs/big toes • Frontal eyes; color vision • Short jaws • Low fecundity Monkeys have 1-3 young; apes have singletons.

12. Primate Classification/Terminology • Prosimian = “pre-monkeys” • Anthropoid = monkeys and apes (45 mya) • Hominids = great apes (25 mya) = chimps, gorillas, orangutan, human

13. Hominids= great apes • Hominins = modern humans and their ancestors since the split with the lineage  chimps (5.4 – 7 mya) • Estimates of nuclear DNA similarity: 95 % • Homo = humans

14. II. Fallacies of human evolution

15. Fallacy 1: Hominins evolved early to hunt large game. Reality: Early hominins scavenged and ate plants.

16. Fallacy 2: hominins an anagenetic lineage leading to humans.

17. Fallacy 2: hominins an anagenetic lineage leading to humans.

18. Fallacy 2: hominins an anagenetic lineage leading to humans.

19. Reality: a branching tree • coexistence of species • rapid disappearance of some forms

20. III. “Important” Hominin Genera Australopithecus (4.5 - 2.2 mya) Homo (2.3 mya – present) • All fossils > 1.8 my from east and southern Africa Australopithecus

21. First innovation: Bipedalism (> 4 mya) • (Drier climate led to receding forest (?)) hominins moved on savannah • Possible advantages of bipedality in open terrain

22. Australopithecus afarensis (2.3mya) • “Ape brain and human body” • Upright posture; knees and hips similar to ours • Lived in small groups • Ate plants, scavenged • Over 100 fossils

23. A. afarensis • Arms remained long, curved fingers • Feet similar to ours

24. Additional australopithecine innovations • A. garhi – tool maker (for extracting bone marrow), meat eater • Best candidate for species directly ancestral to Homo

25. Current controversy: When did Homo evolve? H.e. Wood B, Collard M (1999) Science 284: 65-71.

26. Homo evolved about 2.3 mya (?) • Rapid increase in relative cranial capacity • Further jaw reduction  flatter faces • H. (or A.) habilis – “handy man”

27. “Homo” habilis • made and used sharp tools – Early Stone Age • (Recent ? about its status as ancestor)

28. Homo ergaster/ H. erectus • Evolved 1.9 mya • Big ↑ in cranial capacity • Modern dentition • Sexual size dimorphism decreasing

29. Modifications for long-distance walking/running: • Upper body – balance • Mid body – hip/gluteus • Lower body – joints/tendons/feet • Evidence of diet changes: jaw/teeth/rib cage

30. H. e: Out of Africa 1 • Made multi-purpose hand axes • First to use fire and expand into temperate region ( ≥1 mya). • Drought in Africa  deforestation • Larger home range of carnivores • May have cooked meat

31. Homoneanderthalensis • 300 – to 30 kya • European • Brain size larger (1500 cc) than modern humans • Heavier build; cold-adapted • Historical debate of status as ancestor

32. IV Evolution of Homo sapiens sapiens:A contemporary controversy • Multiregional hypothesis • “Out of Africa 2” hypothesis (“African Replacement”)

33. Competing Hypotheses • Multiregional – H.e’s colonized widely, and adapted to local conditions, gradually evolved into H. sapiens • By this view, H.e  Neanderthal  Cro-Magnon; other people diverged in parallel fashion

34. Competing Hypotheses • Out of Africa 2– Homo sapiens evolved in Africa, migrated to Europe by 40,000 ya • Cro-Magnon resulted from a second migration into western Europe • May have caused extinction of Neanderthal

36. Fossil evidenceFavors OOA 2 • Anatomically modern human present in Africa (160 kya) well before H.e. disappeared • Homo sapiens idaltu (discovered 1997) • Thrived during inter-glacial period, 130 – 74 kya

37. Molecular evidence favors OOA @ • Haplotype – a combination of linked alleles (transmitted together during meiosis) • Haplogroup – a group of closely related haplotypes • Haplogroups can be used to track intraspecific radiations, esp. using mitochondria or Y- chrom HLA

38. z a b y x “Mitochondrial Eve” – common matrilineal ancestor lived in Africa about 200 - 150 kya. • “Y- chromosome Adam” in Africa 90 – 60 kya (?). w

39. Differences in age of “Eve” and “Adam”: variation of male vs female RS • Woman with Guinness record: • Mrs. Vasilyev (1700’s: 16 sets twins; 7 sets triplets; 4 sets quads) !! • Man with greatest number: • ??? • 16 million living men may carry Ghenghis Khan’s Y-chromosome

40. For every man with above average RS, others must have below average RS. • Human males also have higher mortality rates from infancy on.

41. Mitochondrial and Y- DNA haplogroups useful for tracing human migrations

43. Current Status of Debate • Genetic bottleneck 60,000 – 75,000 ya • Disease, volcanic winter?

44. Mt. Toba Eruption: 70 -76 kya • Candidate for bottleneck. • 6 yr volcanic winter • 2000 – 20,000 humans survived ….mostly in Africa. • Recolonization. • Effect of bottleneck: loss of genetic diversity

45. OOA2: Radiation of Homo sapiens sapiens • 60,000 ya -- Australasia • 40,000 ya – Europe • 30,000 ya – Americas

46. Neanderthals Survived the bottleneck event Located on the migration route out of Africa.  Interbreeding  H. sapiensacquired “temperate –zone adapted” genes

47. Reproductive isolation < 1 Abi-Rached, L et al. Nature on line; Yotova V et al (2011) Mol Biol Evol 28: 1957 – 1962. Unresolved: Extent to which coexisting species interbred

48. Evidence that Neanderthals could talk: FOXP2 allele Shape/position of hyoid bone Comparison with other hominids

49. How old is spoken language?

50. 40 kya several extant hominins • Denisovans – Siberia • H. neanderthalensis – NW Europe • H. floresiensis – Indonesia • H. sapiens – widespread • Interbreeding ↑ temp-zone adaptation, but may have also ↑ auto-immune disease Abi-Rached et al 2011. Science 334:89 -94.