Bio1B Evolution 12 Last lecture: Fossil record Fossil record - significance & interpretation Extinction - Background extinction rates and the big 5 mass extinction The K/T boundary - asteroid hypothesis; dinosaur extinctions, radiation of mammals Are humans causing the 6th mass extinction? Today Macro-evolution (cont.) Species selection Transitional forms - tetrapods, birds: exaptation Evolution of developmental programs - eg. vertebrate limbs Human evolution Evolutionary origins of Homo sapiens: fossils & molecular evidence 1
Are we the cause of the 6th mass extinction? (Barnosky et al. Nature 2011) 2
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) 3 SI SC
Understanding the transition of tetrapod vertebrates from water to land Tiktaalik Fig 34.20 Acanthostega 4
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? 5
Evolution of developmental genes => phenotypic novelty Molecular homology: genes with common ancestry controlling development (top right) Changes in timing and spatial pattern of expression => change in phenotype E.g Ubx suppresses leg development in flies, but not shrimp Fig. 25.22 6
Origin of novelties: The vertebrate limb Are the fish fin and vertebrate limb homologous? Very different anatomy, yet Similar patterns of Hox gene expression Anatomic differences could be due to modification of timing/duration of expression? Shubin et al. 2009 Nature 457:818 7
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 Fig. 34.40 8
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 Fig. 34.40 9?
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 10
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) 11
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? 12
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 13
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 14