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HumanEvolution

What were ourancestors like?

Where did we evolve? Why big brains?

Relationships between populations?

Mammalian traits and implications for humansLarge brain for body size, for vertebratesPlacentation, internal gestationLactation

Primate traits and implications for humansLarge brain for body size, for mammalsLong lifespanInvasive placentationLive in social groupsExcellent vision, eyes forward (humans, sclera)First digits opposable (can grasp)Fingernails (homologous with claws)Origin and evolution of modern humans representsessential background for understanding evolutionof human genes and phenotypes related to healthand disease-> what has evolved forms substrates forwhat becomes maladapted

Who are the closest living relativesof humans? How do we know?

Bonobo Chimp Gorilla Orang

Early evidence from immunology

Mitochondrial gene phylogeny

Very short branch; for some genes we are closer to Gorilla

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Humans (we) are a species of chimpanzee

Chimpanzees Humans

~6 millionyears, based onfossils andmolecular clocks

Hominids or hominins

Comparing chimp and human traitsSpecies 1 Species 2

Hug, kiss, back-pat, hold hands Hug, kiss, back-pat, hold hands

Use tools and medicines Use tools and medicines

Use gestural, vocal communication Use gestural, vocal communication

Dominance, status among males Dominance, status among males

Male friendships and kin bonds Male friendships and kin bonds

Female & female-male friendships Female & female-male friendships

Territorial, have warfare Territorial, have warfare

Eat fruit,vegetables,meat,primates Eat fruit,vegetables,meat,primates

Large groups, fluid subgroups Large groups, fluid subgroups

Consorts and promiscuity Consorts and promiscuity

IMPLICATION? ALL OF THESE TRAITS WERE PRESENT IN COMMONANCESTOR OF CHIMPS AND HUMANS, & ALONG THE HUMAN LINEAGE

Within the human lineage:protein-coding genes

Based on 120 protein-coding genes in 1,915 populationsCavalli-Sforza & Feldman (2003) Nature Genet. 33, 266-275

Relationshipsamongcontemporaryhumans:mitochondrialDNA

Europe, Asia,Australia

Asian / Australian

African

See the outgroup?

See the bootstraps?

Mitochondrial EVE - simplified example

Among all humans, 0.4% difference in mtDNA, basallineages on tree are all African (tested as hypothesis)

Among group of humans isolated for 50K years, 0.1%different

CLOCK CALIBRATION: 2% per million years (0.10 div 0.05million)

INFERENCE: EVE lived about 200,000 years ago

WHAT THIS MEANS: Non-African populationsolder than about 200,000 years did not contributemtDNA to modern humans

WHAT THIS DOES NOT MEAN: one female alive then, or weall have same mtDNA, or Eve was anatomically orbehaviorally modern

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Humans are a recently-evolvedspecies, and human genetic diversity is

very low compared to other apes

Human genetic diversity is distributedmainly within populations

Most variationbetweenpopulations

Most variationwithinpopulations

Templeton (1999) Am. J.Anthropol. 100, 632-650

Implication: “racial’ differences in humans(in skin, hair, facialfeatures)are genetically minor(though there is much evidence for local adaptation in phenotypic traits)

Fossil data fits with DNA data

Found only in Africa Found both in Africa and outside, or only outside Africa

Recent human phylogeny (best guess)

Do we share genes with Neanderthals or Homo erectus?

mtDNA, whole genomefrom Neanderthals;

Need genome from directhuman ancestors!

Some apparent gene flow here?

Origins of modern humans

• Anatomicallymodern humansin Africa ~130KYA

• In Israel by ~90KYA

• Not enormouslysuccessful

Omo I, Ethiopia, ~130 KYA

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Origins of modern humans

• Modern humanbehaviour starts todevelop in Africa after~80 KYA

• By ~50 KYA, featuressuch as complex toolsand long-distancetrading are establishedin Africa

The first art? Inscribed ochre, South Africa, ~77 KYA

‘Multiregional’ vs ‘replacement’models for origin of modern humans

RESULTSUNCLEAR, DISPUTED Bottleneck!

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Tripled!

The Human Brain: It’s not just bigger

(1) Increased anatomical and functionallateralization (left hemisphere ‘for’language, right hemisphere ‘for’emotion, visual-spatial tasks)

(2) Increased proportion of fat (DHA, AA)

(3) Disproportionate expansion ofheteromodal association cortex (thethinking parts), cerebellum, someother areas

(4) Expansion and elaboration of the‘social brain’

Sizes of mammalian, primate, Hominoid,and human brains

Big brains and small guts

Better food; Cookingof food

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Evolution of this suite of characters human and chimp lineages

Upright before big brains; teeth smaller

The stages of human preadult development, including transition landmarks and endocrine factors mediating growth and development at different stages. IGF2 also strongly mediates prenatal growth. Adapted from Bogin (1994, 1997, 2006) and Hochberg (2010).

From Crespi 2011; Evolution of Child Health, PRSLB

Evolution of human life stages(1) Emerge as fat fetus, physicallyaltricial but neurologically advanced;‘displays’ of health, vigor? (analogous to hCG)

(2) Relatively early weaning (6 - 3 years);‘complementary foods’ early (~6 months)->effects on demograpy->mother-offspring conflicts

(3) Spend a LONG time in childhoodwith large brain and small body

(4) Undergo growth spurt in adolescence

(5) Long adult lifespan, with post-reproductiveperiod in females, substantial generation overlap in extended family networks

Evolution of neoteny (a form of heterochrony) in humans: retain juvenile form into adulthood, such that human adults are big babies w/regard to head size, shape

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RELATING HUMAN PHENOTYPIC EVOLUTION TO HUMAN DISEASE

Human-evolved adaptation Human disease/disorderwith losses of function

Large brain MicrocephalySocial brain AutismHigh intelligence Intellectual disabilityLanguage Specific Language ImpairmentExpansion of dopaminergic system Schizophrenia, Parkinson’s

Low conception rate/cycle InfertilityDeep placental invasion, Pre-eclampsiaspiral arteries modifiedFat babies Intrauterine growth restriction

Early weaning Attachment disordersLong preadult stages Early adrenarche, puberty->

negative effectsPubertal growth spurt OsteosarcomaMenopause Premature ovarian failure

RELATING HUMAN MOLECULAR EVOLUTION TO HUMAN DISEASE

Crespi 2010, Evol. Appl.

Genes subjet to recent positive selection in humans are involved in neurological diseases