Human Evolution

 Lineage through the Cenozoic

o Earliest fossil primates appear in Eocene around 55 mya
 Small, arboreal omnivores
 Likely originated in North American and spread to Europe
 Molecular evidence suggests older origin
 Molecular clock suggests most recent common ancestor of primates existed 66-
69 mya
 Dates back to around Pre-K-Pg (around time of mass extinction of dinosaurs)
o Existed before dinosaurs became extinct
 Closest living creature to mammals called Dermopeterea
 Last common ancestor of all extant primates; humans split from prosimians ~66mya
 Tarsiers split off from monkeys and apes ~55mya
 Monkeys migrate from western Africa to South America ~40mya
 Big differences between new-world monkeys and old-world monkeys
o Different nose structure, prehensile tails, dentition, claws
 Animals classified with extant old-world monkeys show up around ~30mya and by
25mya apes diverge from monkeys
 We belong to the group Catarhinnni that includes old world monkeys and apes
 Great apes split from gibbons ~17mya
 Gibbons underwent rapid radiation in SE Asia
 Orangutans (Asian apes) split ~15mya
 Ancestral forms of great apes seen in Europe and then spread towards East Asia
 Strong evidence that humans are closer to chimpanzees and bonobos than gorillas
 Split from gorillas 7-8 mya
 Split from chimps 6-5 mya
 Molecular sequence data can lead to erroneous results when there is
o Too much evolution where there are homoplasies resulting from accumulation of
independently derived shared mutations
o Not enough evolution where there are conflicting data due to incomplete lineage sorting
 Incomplete lineage sorting is when variation present within an ancestral population
segregates differently among ancestors and different loci
 Produce discrepancies in the phylogenetic tree; alleles are separated into different
lineages during speciation
 A lot of the hypotheses that were evaluated were based solely on the fossils available at the time
 Members of the genus Homo – Neanderthals and Homo erectus were discovered relatively early in far
flung regions
 Early man showing up in very different places with poor dating methods
 There were no fossils filling the gap between fossil apes and early humans
 One of the first hypotheses was that these early humans evolved completely independently from local
primate populations
 Earliest version of the “multiregional hypothesis” (although this one implies a much, much deeper
divergence than any modern form of the hypothesis)
 The idealogical push for this event led to cases of scientific fraud – trying to show that intermediate
ape/human forms were regionally specific
 Discoveries in the 20th century refute the old explicitly racist version of the multiregional hypothesis
 Filling in the gaps between fossil apes and early humans, all of the action is in East and South Africa
 “Taung Child” skull shows a mix or human and ape features (2.5 mya) and is an example of
Australopithecus africanus
 Once people really started looking and figure out how to look, discoveries began to happen rapidly and
continue to this day to build a much more complete picture o the diversity and evolution of early
Hominins
 One of the most famous examples is “Lucy” which is a female Australopithecus afarensis skeleton
o One major question at the time was which human-like attributes arose first
o Lucy showed that bipedalism outdated brain size by a wide margin
 In the genus Australopithecus, we see clear evidence of transition from ape like to human like features
o This dates from 4 mya to 1.9 mya
 More recent finds push back closer to common ancestor with chimps
o Species in the genus are fully upright bipedal hominins with relatively smalls brains
 Persisted for 2 million years
o Seem to have done well throughout Africa (some recent evidence points to some even leaving
Africa)
o A diversity if Australopithecus forms arises during this time with direct relationships poorly
understood
o Australopithecus gave rise to another lineage around 2.7 mya, the robust Paranthropus
 Example of ecological divergence
 Paranthropus filled new niche feeding on heavy, harder to digest plant material
 Evolved alongside our own lineage until around 1 mya
 Potential precursors to Australopithecus are Sahelanthropus (6 to 7 mya)
o Single skull
o Suggestive of shift toward bipedalism very early on
 Oldest well studied Hominin precursor is Aridpithecus (around 4.4 mya)
o Upright posture
o Still partially adapted to climbing
o Fits almost perfectly with expectations of transitional fossil
 Around 2 mya – East Africa contained 3 genera of hominins (Australopithecus, Paranthropus, and
Homo)
o Organisms that we can start referring to as humans appear 2.1 mya (Homo habilis)
o Appearance corresponds to proliferation of a new kind of trace fossil: tools
 Some recent evidence suggests hominins were manipulating stones as far back as 3
million years
 Oldowan technology – coarse but consistent chopping implements common from 2.6-
1.7 mya
 Likely bridges from late Austalopithecines to early Homo
o As soon as the genus Homo appears, it both diversifies and migrates rapidly
 Homo habilis was contemporary with Homo rudolphensis
 These two seem to show a smaller version of the “gracile vs robust” ecological
divergence in Paranthropus
 Both are confined to Africa
 The next steps sees the first definitive movement of our genus out of Africa
o Homo ergaster/erectus appears 1.9 mya
 Immediately shows up around the globe
o This ancient migration sets up the possibility of a new softer “multiregional hypothesis”
 If Homo erectur occurred around the world so early, did modern humans evolve
gradually from these populations in each different region?
 Or did modern humans evolve in one location and subsequently displace H. erectus?
o Two hypotheses make testable predictions by molecular and fossil data
 African replacement: H. ergaster/erectus diversifies into a number of different lineages
in different locations, but H. sapiens originates in Africa and then displaces all the rest
 Multiregional evolution: H. ergaster/erectus spreads across the globe and H. sapiens
appear gradually in different regions at once
 Advent of DNA sequencing data shows strong support for a single, African origin of
Homo sapiens
o The common ancestors of all modern humans likely lived in East Africa ~150,00 to 200,000
years ago
o Earliest fossils of anatomically modern humans appear 160,000 in Ethiopia
o Humans leave Africa ~80,000 to 60,000 years ago and eventually replace all the other members
of the genus that existed at the time: Neanderthals, Denisovans, Hobbits
 Triggered by evolutionary radiation