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
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
The asteroid impact hypothesis - Luiz & Walter Alvarez, UC Berkeley (see Science, 5th March, p1214) Faunal turnover Carbon cycle change Iridium spike
Are we the cause of the 6th mass extinction? (Barnosky et al. Nature 2011)
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;330:493-495 Evolution of particular trait (red) consistently associated with increased rate of diversification (from Rabosky & McCune 2010 TREE) SI SC
Understanding the transition of tetrapod vertebrates from water to land Tiktaalik Fig 34.20 Acanthostega
Modification of existing structures for new purposes: ears and feathers Feathers: for display or warmth before flight? Late Jurassic feathered dinosaur Fig. 34.31. Bones of inner ear of modern mammals are derived from jaw joint of ancestors (see also Fig. 25.6 Recent discovery: dinosaur feathers were colored - display?
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. 23.52 in Hillis et al.
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, 200-24Kya?
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
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)
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?
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. 2010 Science 328:710, Gibbons 2010 Science 328:680
Denisovans - another recent Homo Reich et al. 2010 Nature 468:1053 50-30Kya fossil 4-5% 2.5% 440-270 Kya Neanderthals (bottleneck?) Modern humans