Hominds part 2 Out of Africa: DNA clocks Multiregionalism: the Neanderthal within us all Bipedalism

DNA clocks DNA can vary because of: –Mutation (random copying errors which accrue with time) –Recombination (1/2 of each parent’s DNA mixed to make child)

DNA clocks: mitochondrial DNA A portion of our DNA comes only from our mothers, and experiences ONLY mutation

mT DNA does not undergo recombination; only mutation mT DNA is thought to mutate at a constant rate Small mutations in DNA through time; 2-4% per million years Calibrated against dateable fossils –Difference between 2 organisms Last common ancestor between humans and chimps: 350,000 generations ago, or about 5-7 million years DNA clocks

Out of Africa mT DNA of thousands of humans all point to a last common maternal ancestor 200, ,000 years ago -- very recent! Furthermore: –Highest gene diversity in Africa –Asian, European genes present in African pools but not others –+ statistical modeling --> Homo sapiens did not mix genetically with homo erectus.

Out of Africa Neanderthal mT DNA: –Very different from modern humans –Hard to reconcile difference with possible presence of some Neanderthal ancestry in modern homo sapiens

Neanderthals+ multiregionalism Major skull differences: Neanderthals have: –Longer, lower occipital bun –Big retromolar gap –Almost no chin –Strong browridge –Sloping face

Early modern homo sapiens in Europe after Neanderthals were gone Hybrid child Adult skeletons with a range of brow ridges, retromolar gaps, occipital bun sizes Simple explanation: some Neanderthal- homo sapiens interbreeding Geneticist response: convergent evolution (requires puropose for retromolar gap, chin, and browridge)

Other possibilities: Hybrids didn’t survive for some reason Hybrids DID survive, but we just not looking at the right part of the genome yet. Y-chromosome data: recent African origin How to use rest of genome subjected to recombination?

Why bipedalism? Less exposure to sun Easier to carry children, food Easier to reach food in trees Less energetically expensive –Quadupedal knuckle-walking: ~35% more energy than walking on 2 legs Dense forests: < 1 mile/day to gather food Savannahs: 6-8 miles/day to gather food

Bipedal movement freed up energy for brain development

Brain development supported hunting Higher calorie diet from meat allowed more brain development –C and N isotopes in bone give information about proportion of meat in diet –Neanderthal diet: almost all protein from meat