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

TodayExtinction -Background extinction rates vs “big 5” mass extinctionsThe “K/T” boundary - asteroid hypothesis; dinosaur extinctions, radiation

of mammalsAre humans causing the 6th mass extinction?Species selectionTransitional forms - tetrapods, bird feathers: “exaptation”Human evolutionEvolutionary origins of Homo sapiens: fossils & molecular evidence

The big 5mass

extinctions

• Evidence from analyses of extinction (red) and blues(diversity) or families of marine invertebrates

• Permian-Triassic - 96% species extinction, 8/27 orders ofinsects; Volcanism in Siberia?

• Cretaceous-Paleogene (“K/T”), 65 Myr - demise ofdinosaurs & large terrestrial animals => mammalianradiation

The asteroid impacthypothesis - Luiz & Walter

Alvarez, UC Berkeley(see Science, 5th March, p1214)

Faunal turnover Carbon cyclechange

Iridium spike

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

Macro-evolution: Species selection

Evolution of particular trait (red)consistently associated withincreased rate of diversification (fromRabosky & McCune 2010 TREE)

BUT -diversificationrate (S-E) -higher in SI(purple) thanSC (blue)

E.g. self incompatibility (SI) in hermaphroditic plants isoften disdadvantageous within species compared toself-compatibility (SC)

E E Goldberg et al.Science 2010;330:493-495

SI

SC

Understanding the transition of tetrapodvertebrates from water to land

Fig 34.20

Tiktaalik

Acanthostega

Modification of existing structures for newpurposes: ears and feathers

Fig. 34.31. Bones of inner ear ofmodern mammals are derived fromjaw joint of ancestors (see also Fig.25.6

LateJurassicfeathereddinosaur

Feathers:for displayor warmthbeforeflight?

Recent discovery:dinosaur featherswere colored -display?

Evolution ofhominins: fossil

evidence I

See also Fig. 23.52 in Hillis et al.

• Hominins split fromcommon ancestor withchimps about 7Myr;African origins, diversityexpands 4-2Myr

• Key features: bipedalism,smaller canines (largebrain later)

• A. ramidus - neitherchimp nor human - seedisplay in VLSB

• “Australopiths” probablyparaphyletic with Homo

“Lucy”

Robustanthropoids

Evolution of hominins: fossilevidence II

• Homo - key features:increasing brain size,reduced jaw, lower sexdimorphism, moreterrestrial

• African origins; H. erectus-> europe >1.8Myr ->Indonesia (“Java man”).Extinct 200 Kya?

• H. floriensis - >1M? -12Kya. Related to H.erectus?

• Neanderthals - Europeand near east, 200-24Kya

?

Evolution ofhominins: fossil

evidence IIIH. floriensis• Possibly persistent

relative of H. erectus [ormalformed H. sapiens?]

• Exemplifies humansevolve as other species:dwarfing of largemammals on islands -eg. Stegodon “pygmyelephants & hugelizards! (Varanus)

• Putative tools >1Myr,fossils to 12Kya -overlapping H. sapiens

H. floriensis MicrocephalicH. sapiens

Migration of H. sapiens• Out of Africa - about

100Kya• Rapid spread across Sth

Asia to Australia & centralAsia

• One or 2 colonizationsacross Bering bridgeduring last ice age ->rapid spread to SthAmerica

• Polynesian migrationsacross Pacific are recent:1500 BC to 300 AD(Hawaii)

Modern humans &related species -hybridization orreplacement?

Genetic evidence largelysupports single origin & “out-of- Africa” over independentorigins from differentpopulations of H. erectus(multi-regional).

But did modern humanshybridize with, or simplyreplace neanderthals?

Paleogenomics: Neanderthal v modern humans• 60-38Kya bones of neanderthal

sequenced - compared to differenthuman populations

• 2-3% neanderthal genes ineurasian-papuan, not africans

• Several genes - eg skin &pigmentation, skeleton, metabolismunder recent selection in humans

• Refs: Green et al. 2010 Science 328:710,Gibbons 2010 Science 328:680

‘Denisovans’ - anotherrecent Homo

Neanderthals

(bottleneck?)

Modernhumans

440-270 Kya

2.5%

4-5%

Reich et al. 2010 Nature 468:1053

50-30Kyafossil