Bio1B Evolution 12 Last lecture: Speciation: outcomes of secondary contact Fossil record - significance & interpretation (Ch 18)

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1 Bio1B Evolution 12 Last lecture: Speciation: outcomes of secondary contact Fossil record - significance & interpretation (Ch 18) Today Extinction - Background extinction rates vs big 5 mass extinctions The K/T boundary - asteroid hypothesis; dinosaur extinctions, radiation of mammals Are humans causing the 6th mass extinction? Species selection Transitional forms - tetrapods, bird feathers: exaptation Human evolution Evolutionary origins of Homo sapiens: fossils & molecular evidence

2 The big 5 mass extinctions Evidence from analyses of extinction (red) and blues (diversity) or families of marine invertebrates Permian-Triassic - 96% species extinction, 8/27 orders of insects; Volcanism in Siberia? Cretaceous-Paleogene ( K/T ), 65 Myr - demise of dinosaurs & large terrestrial animals => mammalian radiation

3 The asteroid impact hypothesis - Luiz & Walter Alvarez, UC Berkeley (see Science, 5th March, p1214) Faunal turnover Carbon cycle change Iridium spike

4 Are we the cause of the 6th mass extinction? (Barnosky et al. Nature 2011)

5 Macro-evolution: Species selection E.g. self incompatibility (SI) in hermaphroditic plants is often disdadvantageous within species compared to self-compatibility (SC) BUT - diversification rate (S-E) - higher in SI (purple) than SC (blue) E E Goldberg et al. Science 2010;330: Evolution of particular trait (red) consistently associated with increased rate of diversification (from Rabosky & McCune 2010 TREE) SI SC

6 Understanding the transition of tetrapod vertebrates from water to land Tiktaalik Fig Acanthostega

7 Modification of existing structures for new purposes: ears and feathers Feathers: for display or warmth before flight? Late Jurassic feathered dinosaur Fig Bones of inner ear of modern mammals are derived from jaw joint of ancestors (see also Fig Recent discovery: dinosaur feathers were colored - display?

8 Evolution of hominins: fossil evidence I Hominins split from common ancestor with chimps about 7Myr; African origins, diversity expands 4-2Myr Key features: bipedalism, smaller canines (large brain later) A. ramidus - neither chimp nor human - see display in VLSB Australopiths probably paraphyletic with Homo Robust anthropoids Lucy See also Fig in Hillis et al.

9 Evolution of hominins: fossil evidence II Homo - key features: increasing brain size, reduced jaw, lower sex dimorphism, more terrestrial African origins; H. erectus -> europe >1.8Myr -> Indonesia ( Java man ). Extinct 200 Kya? H. floriensis - >1M? - 12Kya. Related to H. erectus? Neanderthals - Europe and near east, Kya?

10 Evolution of hominins: fossil evidence III H. floriensis Possibly persistent relative of H. erectus [or malformed H. sapiens?] Exemplifies humans evolve as other species: dwarfing of large mammals on islands - eg. Stegodon pygmy elephants & huge lizards! (Varanus) Putative tools >1Myr, fossils to 12Kya - overlapping H. sapiens H. floriensis Microcephalic H. sapiens

11 Migration of H. sapiens Out of Africa - about 100Kya Rapid spread across Sth Asia to Australia & central Asia One or 2 colonizations across Bering bridge during last ice age -> rapid spread to Sth America Polynesian migrations across Pacific are recent: 1500 BC to 300 AD (Hawaii)

12 Modern humans & related species - hybridization or replacement? Genetic evidence largely supports single origin & outof- Africa over independent origins from different populations of H. erectus (multi-regional). But did modern humans hybridize with, or simply replace neanderthals?

13 Paleogenomics: Neanderthal v modern humans 60-38Kya bones of neanderthal sequenced - compared to different human populations 2-3% neanderthal genes in eurasian-papuan, not africans Several genes - eg skin & pigmentation, skeleton, metabolism under recent selection in humans Refs: Green et al Science 328:710, Gibbons 2010 Science 328:680

14 Denisovans - another recent Homo Reich et al Nature 468: Kya fossil 4-5% 2.5% Kya Neanderthals (bottleneck?) Modern humans