Human Evolution What were our ancestors like? Where did we evolve? Why big brains? Relationships between populations?
Mammalian traits and implications for humans Large brain for body size, for vertebrates Placentation, internal gestation Lactation Primate traits and implications for humans Large brain for body size, for mammals Long lifespan Invasive placentation Live in social groups Excellent vision, eyes forward (humans, sclera) First digits opposable (can grasp) Fingernails (homologous with claws) Origin and evolution of modern humans represents essential background for understanding evolution of human genes and phenotypes related to health and disease-> what has evolved forms substrates for what becomes maladapted
Who are the closest living relatives of humans? How do we know? Chimp Gorilla Orang Bonobo
Mitochondrial gene phylogeny Very short branch; for some genes we are closer to Gorilla
Humans (we) are a species of chimpanzee ~6 million years, based on fossils and molecular clocks Hominids or hominins Humans Chimpanzees
Comparing chimp and human traits Species 1Species 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 COMMON ANCESTOR OF CHIMPS AND HUMANS, & ALONG THE HUMAN LINEAGE
Within the human lineage: protein-coding genes Based on 120 protein-coding genes in 1,915 populations Cavalli-Sforza & Feldman (2003) Nature Genet.33, 266-275
Relationships among contemporary humans: mitochondrial DNA Europe, Asia, Australia See the bootstraps? Asian / Australian African See the outgroup?
Mitochondrial EVE - simplified example Among all humans, 0.4% difference in mtDNA, basal lineages 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 0.05 million) INFERENCE: EVE lived about 200,000 years ago WHAT THIS MEANS: Non-African populations older than about 200,000 years did not contribute mtDNA to modern humans WHAT THIS DOES NOT MEAN: one female alive then, or we all have same mtDNA, or Eve was anatomically or behaviorally modern
Humans are a recently-evolved species, and human genetic diversity is very low compared to other apes
Human genetic diversity is distributed mainly within populations Most variation between populations Implication: “racial’ differences in humans (in skin, hair, facial features) are genetically minor (though there is much evidence for local adaptation in phenotypic traits) Most variation within populations Templeton (1999) Am. J. Anthropol.100, 632-650
Fossil data fits with DNA data Found only in Africa Found both in Africa and outside, or only outside Africa
Do we share genes with Neanderthals or Homo erectus? Some apparent gene flow here? mtDNA, whole genome from Neanderthals; Need genome from direct human ancestors!
Anatomically modern humans in Africa ~130 KYA In Israel by ~90 KYA Not enormously successful Origins of modern humans Omo I, Ethiopia, ~130 KYA
Modern human behaviourstarts to develop in Africa after ~80 KYA By ~50 KYA, features such as complex tools and long-distance trading are established in Africa Origins of modern humans The first art? Inscribed ochre, South Africa, ~77 KYA
‘Multiregional’ vs ‘replacement’models for origin of modern humans
The Human Brain: It’s not just bigger Increased anatomical and functional lateralization (left hemisphere ‘for’ language, right hemisphere ‘for’ emotion, visual-spatial tasks) Increased proportion of fat (DHA, AA) Disproportionate expansion of heteromodal association cortex (the thinking parts), cerebellum, some other areas Expansion and elaboration of the ‘social brain’
Big brains and small guts Better food; Cooking of food
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 Emerge as fat fetus, physically altricial 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 childhood with large brain and small body (4) Undergo growth spurt in adolescence (5) Long adult lifespan, with post-reproductive period 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
RELATING HUMAN PHENOTYPIC EVOLUTION TO HUMAN DISEASE Human-evolved adaptation Human disease/disorder with losses of function Large brain Microcephaly Social brain Autism High intelligence Intellectual disability Language Specific Language Impairment Expansion of dopaminergic system Schizophrenia, Parkinson’s Low conception rate/cycle Infertility Deep placental invasion, Pre-eclampsia spiral arteries modified Fat babies Intrauterine growth restriction Early weaning Attachment disorders Long preadult stages Early adrenarche, puberty-> negative effects Pubertal growth spurt Osteosarcoma Menopause Premature ovarian failure
RELATING HUMAN MOLECULAR EVOLUTION TO HUMAN DISEASE Genes subjet to recent positive selection in humans are involved in neurological diseases Crespi 2010, Evol. Appl.