human origins and evolution
DESCRIPTION
Human origins and evolution. Gil McVean, Department of Statistics, Oxford. Questions about human origins. What defines a human? What does the fossil record tell us? What are the genetic changes that make us human? What are the genetic changes that make people(s) different?. - PowerPoint PPT PresentationTRANSCRIPT
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Human origins and evolution
Gil McVean, Department of Statistics, Oxford
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Questions about human origins
• What defines a human?
• What does the fossil record tell us?
• What are the genetic changes that make us human?
• What are the genetic changes that make people(s) different?
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What makes us human?
• Upright posture, bipedalism
• Advanced tool-making capability
• Big brain, relative to body size, and small canine teeth
• Global dispersal
• Use of fire to modify environment
• Language and ‘consciousness’ (self-awareness)
• Complex culture
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Human brain size
• Humans have an encephalisation quotient of about 6.5 – 8.0– The biggest of any mammal!
Mammals: Ebrain = 0.12 x Mbody
2/3
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What is culture?
• Language
• Beliefs
• Rituals
• Law
• Morality
• Manners
• Visual arts
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Great ape phylogeny
• Human and chimp ancestors split about 6 MYA
Hacia JG (2001)
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A recent X?
• Suggestion of a more recent divergence time for X chromosome– Patterson et al (2006)
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Some terminology
• Hominid is a term used to describe Humans and any lineages that share a common ancestor with humans more recently than the human-chimp split
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An overview of the fossils
Human – chimp splitOrigin of H. sapiens sapiens
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Australopithecus
• 3.9 – 3 MYA
• Both gracile and robust forms (latter called Paranthropus)
• Evidence for sexual dimorphism within these species
Australopithecus boisei
Australopithecus robustus
Australopithecus afarensis (Lucy)
Australopithecus africanus
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The Laetoli footprints
• First evidence of bipedalism
• 3.7 MYA
• Three sets of tracks in volcanic ash
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Lucy
• 3.2 MYA from Ethiopia
• Lucy was bipedal, an adaptation for travel across savannah woodlands and grasslands
• Big teeth, still not a big brain.
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Early Homo species
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Homo habilis
• 2.6 – 1.4 MYA
• Some, but not all, have slightly bigger brains
• Maker of tools (Oldowan tools)
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Striding out, standing tall, and at last, a big brain
• Turkana boy (1.5 million years ago): the earliest individual with estimated brain size (909cc) significantly above primate allometry curve.
• Homo ergaster
Brain size versus height
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H. ergaster lived in Eurasia at same time as in Africa
• Dmanisi fossils date to 1.7 mya
• Caucasus Mountains, Republic of Georgia (well north of the tropics)
• Associated Oldowan tools
• H. ergaster is the first species of hominin adapted for endurance running.
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Why leave Africa? The role of Pleistocene climate change
Onset of Lower Pleistocene glaciations at ~2 million years ago with formation of permanent ice sheets and sharp cooling. Africa becomes drier.
Warm
• Middle Pleistocene climate: colder and more variable; long cold glacial periods punctuated by short, warmer interglacials
• Migrations of many species between Africa and Eurasia during interglacials
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What is life in the Pleistocene like for early Homo species?
• Hunting game as well as scavenging
• Control of fire– Possibly from 2MYA
• Improving the tool kit – more elaborate
Acheulean stone tools: e.g. handaxe for butchering
• Increasingly complex social behaviour
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Homo erectus did cross sea barriers
• Archaeology on Flores, dates to 840,000 years ago
• H. florensiensis (Hobbits) on Flores date to as recently as 20,000 yrs ago
Hobbit on left compared with modern human
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Homo sapiens neanderthalensis
• From H. heidelbergensis (0.75 – 0.25 MYA)
• Only found in Europe and near East
• Diverged from AMH lineage about 0.8 MYA
• More robust than AMH, but shared many features of culture– Music – Jewellery– Complex tools– Ritual (burial of dead)– Language ability?
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Homo sapiens sapiens
• Anatomically modern humans (AMH)
• Modern anatomy at 200,000 years ago (Ethiopia – Omo I and II)
• Out-of-Africa event 70,000 years ago
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Out of Africa: diversity among early modern Homo sapiens
(a) Skhul 5, Israel, 90,000 Yrs
(b) Cro-Magnon 1, France, 23-27 KYrs
(c) Kow Swamp, robust Aboriginal Australian, 9-13 KYrs
Shared features: Cranial vault height high and domed, brow ridges lighter or absent, chin present
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Dispersal of AMHs out of Africa
• Into Middle East by 90,000 years ago, and then retreat. (Neanderthal distribution expands)
• Reach Australia by 60,000 years ago, apparently via south Asian coastal route.
• 40,000 years ago: substantial presence of moderns in Europe and Asia (little evidence in archaeological record at earlier dates)
• Last Neanderthals about 25,000 years ago
• Bottleneck in dispersal out of Africa - implicated by genetic data– Note that this bottleneck is not associated with speciation,
only with modest structure between sub-Saharan and other human populations.
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What does genetic variation tell us about human evolution?
• Modern humans appear in the fossil record about 200K years ago
• The mitochondrial Eve dates back to about 150K years ago
• The Y-chromosome Adam dates back to about 70K years ago
• AMHs left Africa about 70KYA
• For most of our genome, however, the common ancestor is about 500K – 1M years ago– This predates the origin of Homo sapiens considerably
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Did early humans interbreed with Neanderthals?
Ovchinnikov et al (2000)
Neanderthals
mtDNA sequences say no…
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But…
• There is some evidence for this in the presence of unusual haplotypes found in Europe composed of SNPs not found in non-European populations
Plagnol and Wall (2006)
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Deeper trees in the human genome
• There is growing evidence that some regions of our genome have truly ancient common ancestors
• Dystrophin has an ancient haplotype found primarily outside Africa suggesting a colonisation of >160KYA
• There is an inversion found primarily in Europeans that is roughly 3MY old
Stefansson et al (2005)
Haplotype 1
Haplotype 2
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What are the genetic differences that make us human?
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Chromosomal changes
• Human chromosome 2 is a fusion of two chromosomes in great apes
• There are several inversion differences between the chromosomes
Feuk et al (2005)
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Gene loss
• Loss of enzymes that make sialic acid – Sugar on cell
surface that mediates a variety of recognition events involving pathogenic microbes and toxins
• Myosin heavy chain– Associated with
gracilization
Wang et al (2006)
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Gene evolution
• FOXP2 is a highly conserved gene (across the mammalia), expressed in the brain. Mutations in the gene in humans are associated with specific language impairment
• Across the entire mammalian phylogeny, there have only been a very few amino acid changing substitutions
• However, two amino acid changes have become fixed in the lineage leading to modern humans since the split with the chimpanzee lineage
Enard et al. (2002)
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What are the genetic differences that make people and peoples
different?
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How do we differ? – Let me count the ways
• Single nucleotide polymorphisms– 1 every few hundred bp
• Short indels (=insertion/deletion)– 1 every few kb
• Microsatellite (STR) repeat number– 1 every few kb
• Minisatellites– 1 every few kb
• Repeated genes– rRNA, histones
• Large inversions, deletions– Y chromosome, Copy Number Variants (CNVs)
TGCATTGCGTAGGCTGCATTCCGTAGGC
TGCATT---TAGGCTGCATTCCGTAGGC
TGCTCATCATCATCAGCTGCTCATCA------GC
≤100bp
1-5kb
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Detecting recent adaptive evolution
• Let’s look closely at the dynamics of the fixation process for adaptive mutations
• The fixation of a beneficial mutation is associated with a change in the patterns of linked neutral genetic variation
• This is known as the hitch-hiking effect (Maynard Smith and Haigh 1974)
• Looking for the signature of hitch-hiking can be a good way of detecting very recent fixation events
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Diversity is not evenly distributed across genes II
• Adaptive evolution ‘wipes out’ diversity nearby due to the hitch-hiking effects of a selective sweep– e.g. Duffy-null locus in sub-Saharn africa, protects against P. vivax
Pop1
Pop2
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European
African
FY*O mutation
Ancestral alleleDerived alleleMissing dataHamblin and Di Rienzo (2000)
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Long haplotypes
• A selective sweep at the Lactase gene in Europeans
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Strong population differentiation
Lamason et al (Science 2005)
• SLC24A5
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Classes of selected genes
Voight et al. (2005)