- Количество слайдов: 40
Human origins and evolution Gil Mc. Vean, 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?
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
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
What is culture? • Language • Beliefs • Rituals • Law • Morality • Manners • Visual arts
Great ape phylogeny • Human and chimp ancestors split about 6 MYA Hacia JG (2001)
A recent X? • Suggestion of a more recent divergence time for X chromosome – Patterson et al (2006)
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
An overview of the fossils Origin of H. sapiens Human – chimp split
Australopithecus • 3. 9 – 3 MYA • Both gracile and robust forms (latter called Paranthropus) • Evidence for sexual dimorphism within these species Australopithecus robustus Australopithecus africanus Australopithecus afarensis (Lucy) Australopithecus boisei
The Laetoli footprints • First evidence of bipedalism • 3. 7 MYA • Three sets of tracks in volcanic ash
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.
Early Homo species
Homo habilis • 2. 6 – 1. 4 MYA • Some, but not all, have slightly bigger brains • Maker of tools (Oldowan tools)
Striding out, standing tall, and at last, a big brain Brain size versus height • Turkana boy (1. 5 million years ago): the earliest individual with estimated brain size (909 cc) significantly above primate allometry curve. • Homo ergaster
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.
Why leave Africa? The role of Pleistocene climate change • 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 Warm Onset of Lower Pleistocene glaciations at ~2 million years ago with formation of permanent ice sheets and sharp cooling. Africa becomes drier.
What is life in the Pleistocene like for early Homo species? • Hunting game as well as scavenging • Control of fire – Possibly from 2 MYA • Improving the tool kit – more elaborate Acheulean stone tools: e. g. handaxe for butchering • Increasingly complex social behaviour
Homo erectus did cross sea barriers • Archaeology on Flores, dates to 840, 000 years ago • H. florensis (Hobbits) on Flores date to as recently as 20, 000 yrs ago Hobbit on left compared with modern human
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?
Homo 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
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
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.
What does genetic variation tell us about human evolution? • Modern humans appear in the fossil record about 200 K years ago • The mitochondrial Eve dates back to about 150 K years ago • The Y-chromosome Adam dates back to about 70 K years ago • AMHs left Africa about 70 KYA • For most of our genome, however, the common ancestor is about 500 K – 1 M years ago – This predates the origin of Homo sapiens considerably
Did early humans interbreed with Neanderthals? Neanderthals mt. DNA sequences say no… Ovchinnikov et al (2000)
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)
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 >160 KYA • There is an inversion found primarily in Europeans that is roughly 3 MY old Haplotype 1 Haplotype 2 Stefansson et al (2005)
What are the genetic differences that make us human?
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)
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)
Gene evolution • FOXP 2 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)
What are the genetic differences that make people and peoples different?
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 TGCATTGCGTAGGC TGCATTCCGTAGGC TGCATT---TAGGC TGCATTCCGTAGGC TGCTCATCAGC TGCTCATCA------GC • Minisatellites – 1 every few kb ≤ 100 bp • Repeated genes – r. RNA, histones 1 -5 kb • Large inversions, deletions – Y chromosome, Copy Number Variants (CNVs)
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
Diversity is not evenly distributed across genes II • Adaptive evolution ‘wipes out’ diversity nearby due to the hitchhiking effects of a selective sweep – e. g. Duffy-null locus in sub-Saharn africa, protects against P. vivax FY*O mutation African Pop 1 Pop 2 European Hamblin and Di Rienzo (2000) Ancestral allele Derived allele Missing data
Long haplotypes • A selective sweep at the Lactase gene in Europeans
Strong population differentiation • SLC 24 A 5 Lamason et al (Science 2005)
Classes of selected genes Voight et al. (2005)