The Evolution of Homo sapiens Current Views and

The Evolution of Homo sapiens: Current Views and Provocative Hypotheses

• • • Outline I – Definitions etc II – 2 Fallacies III – “Important” genera IV – 2 Hypotheses for origin of H. sapiens

Cladistics A type of phylogenetic analysis based on two premises: • Synapomorphies define branching points. • Synapomorphies are nested. Synapomorphy (def) -- Shared, derived trait in phylogenetic “tree”

• What types of traits can be used as synapomorphies? • Morphological • Developmental • Molecular

• Reliable trees are built using multiple traits • Consensus trees.

• Alternative trees of “great apes” • What do these trees show? • What’s different about them?

• Growing tendency to use horizontal representations. • Why? • Basal = ancestral (“primitive”) • Derived = descendant

• Timeline on tree • How do we date divergences?

• Dating divergences: • Fossil record + radiometric dating • What if no fossil of common ancestor? (e. g. , first animal, first mammal, first primate) • Molecular clocks

• Some DNA/aa sequences diverge at linear rates over Molecular clocks time. • These can be used to date divergences among lineages.

Order Primates Monkeys have 1 -3 young; apes have singletons. • Adapted to arboreal existence • Dexterous hands, feet; partially or fully opposable thumbs/big toes • Frontal eyes; color vision • Short jaws • Low fecundity

Primate Classification/Terminology • Prosimian = “premonkeys” • Anthropoid = monkeys and apes (45 mya) • Hominids = great apes (25 mya) = chimps, gorillas, orangutan, human

• Hominids = great apes • Hominins = modern humans and their ancestors since the split with the lineage chimps (5. 4 – 7 mya) • Estimates of nuclear DNA similarity: 95 % • Homo = humans

II. Fallacies of human evolution

Fallacy 1: Hominins evolved early to hunt large game. Reality: Early hominins scavenged and ate plants.

• Fallacy 2: hominins an anagenetic lineage leading to humans.

• Fallacy 2: hominins an anagenetic lineage leading to humans.

• Fallacy 2: hominins an anagenetic lineage leading to humans.

• Reality: a branching tree • coexistence of species • rapid disappearance of some forms

III. “Important” Hominin Genera Australopithecus (4. 5 - 2. 2 mya) Homo (2. 3 mya – present) • All fossils > 1. 8 my from east and southern Africa Australopithecus

First innovation: Bipedalism (> 4 mya) • (Drier climate led to receding forest (? ) ) hominins moved on savannah • Possible advantages of bipedality in open terrain

Australopithecus afarensis (2. 3 mya) • “Ape brain and human body” • Upright posture; knees and hips similar to ours • Lived in small groups • Ate plants, scavenged • Over 100 fossils

A. afarensis • Arms remained long, curved fingers • Feet similar to ours

Additional australopithecine innovations • A. garhi – tool maker (for extracting bone marrow), meat eater • Best candidate for species directly ancestral to Homo

Current controversy: When did Homo evolve? H. e. Wood B, Collard M (1999) Science 284: 65 -71.

• Homo evolved about 2. 3 mya (? ) • Rapid increase in relative cranial capacity • Further jaw reduction flatter faces • H. (or A. ) habilis – “handy man”

“Homo” habilis • made and used sharp tools – Early Stone Age • (Recent ? about its status as ancestor)

Homo ergaster/ H. erectus • Evolved 1. 9 mya • Big ↑ in cranial capacity • Modern dentition • Sexual size dimorphism decreasing

• Modifications for longdistance walking/running: • Upper body – balance • Mid body – hip/gluteus • Lower body – joints/tendons/feet • Evidence of diet changes: jaw/teeth/rib cage

H. e: Out of Africa 1 • Made multi-purpose hand axes • First to use fire and expand into temperate region ( ≥ 1 mya). – Drought in Africa deforestation – Larger home range of carnivores • May have cooked meat

Homo neanderthalensis • 300 – to 30 kya • European • Brain size larger (1500 cc) than modern humans • Heavier build; cold-adapted • Historical debate of status as ancestor

IV Evolution of Homo sapiens: A contemporary controversy • Multiregional hypothesis • “Out of Africa 2” hypothesis (“African Replacement”)

Competing Hypotheses • Multiregional – H. e’s colonized widely, and adapted to local conditions, gradually evolved into H. sapiens • By this view, H. e Neanderthal Cro. Magnon; other people diverged in parallel fashion

Competing Hypotheses • Out of Africa 2 – Homo sapiens evolved in Africa, migrated to Europe by 40, 000 ya • Cro-Magnon resulted from a second migration into western Europe • May have caused extinction of Neanderthal

Fossil evidence Favors OOA 2 • Anatomically modern human present in Africa (160 kya) well before H. e. disappeared • Homo sapiens idaltu (discovered 1997) • Thrived during interglacial period, 130 – 74 kya

Molecular evidence favors OOA @ • Haplotype – a combination of linked alleles (transmitted together during meiosis) • Haplogroup – a group of closely related haplotypes • Haplogroups can be used to track intraspecific radiations, esp. using mitochondria or Y- chrom HLA

a b z “Mitochondrial Eve” – y x w common matrilineal ancestor lived in Africa about 200 150 kya. • “Y- chromosome Adam” in Africa 90 – 60 kya (? ).

• Differences in age of “Eve” and “Adam”: variation of male vs female RS • Woman with Guinness record: • Mrs. Vasilyev (1700’s: 16 sets twins; 7 sets triplets; 4 sets quads) !! • Man with greatest number: • ? ? ? • 16 million living men may carry Ghenghis Khan’s Y-chromosome

• For every man with above average RS, others must have below average RS. • Human males also have higher mortality rates from infancy on.

• Mitochondrial and Y- DNA haplogroups useful for tracing human migrations

Current Status of Debate • Genetic bottleneck 60, 000 – 75, 000 ya • Disease, volcanic winter?

Mt. Toba Eruption: 70 -76 kya • Candidate for bottleneck. • 6 yr volcanic winter • 2000 – 20, 000 humans survived …. mostly in Africa. • Recolonization. • Effect of bottleneck: loss of genetic diversity

OOA 2: Radiation of Homo sapiens • 60, 000 ya -Australasia • 40, 000 ya – Europe • 30, 000 ya – Americas

Neanderthals • Survived the bottleneck event • Located on the migration route out of Africa. • Interbreeding • H. sapiens acquired “temperate – zone adapted” genes

Reproductive isolation < 1 • Unresolved: Extent to which coexisting species interbred Abi-Rached, L et al. Nature on line; Yotova V et al (2011) Mol Biol Evol 28: 1957 – 1962.

Evidence that Neanderthals could talk: • FOXP 2 allele • Shape/position of hyoid bone Comparison with other hominids

How old is spoken language?

Abi-Rached et al 2011. Science 334: 89 -94. • 40 kya several extant hominins • Denisovans – Siberia • H. neanderthalensis – NW Europe • H. floresiensis – Indonesia • H. sapiens – widespread • Interbreeding ↑ tempzone adaptation, but may have also ↑ auto-immune disease

Phylogeny of extant humans DNA sequence data • Debate about question: are there human races? • Blog: Why Evolution is True by Dr. Jerry Coyne (Univ. Chicago) • Answer depends on how define race • Splitters vs lumpers

Unresolved: Which species were ancestral? Problems: Splitters vs lumpers Need more fossils Need more DNA Lewin R, Foley RA (2004) Principles of Human Evolution

Additional questions • When did hominins lose fur? • Loss of most body hair about 1. 2 mya ↑ skin pigmentation (protection against UV radiation in tropics) • When did hominins begin to wear clothing? • Divergence of head and clothing lice ≈ 170 kya

When did hominins start cooking food/meat? • Raw meat is hard to process (chew & digest) • Chimps: 1 hour to chew. 5 kg • Cooked food (including meat) much easier to process/digest. Clues provided by changes in jaw/dentition, gut, and brain.

• Evidence of diet changes: jaw/teeth/rib cage • Brain size changes Function of endurance running?

Diet and Relative Brain Size • H. sapiens brains (2. 5% body mass) use about 20% BMR • Other mammals 10 13% • Did reliance on cooked food allow for brain expansion? • Cooked food may be as old as OOA 1. • Suite of human traits linked: • Endurance running and furlessness • Fire and cooked food (including meat) • • Brain expansion

Summary • A: Upright posture evolved first. • Freed hands for gesturing (“sign language”).

• H: Relative cranial capacity increased rapidly. • Importance of neocortex for abstract/symbolic thought • Inventiveness/ forethought • Complex language