Human Evolution
Primate phylogeny
Primates branched off other mammalian lineages
~65 mya
(mya = million years ago)
Two types of monkeys within lineage
1. New World Monkeys (Americas): spider
monkey; howler
2. Old World Monkeys (Africa, Asia/Polynesia):
rhesus; grenons; Sooty mangabeys (source of
HIV-2)
OW/NW split 35-40mya (fossil record)
OW/humans and apes split (25-30mya)
Tree based on strength
of immune reactions of
human serum albumin
antibodies in other
primates. Assuming
constant rate of
evolution, and calibrated
with OW split at 30 mya.
Phylogeny of human,
chimp and gorilla was
not resolved.
Origin of humans
• Split between human and chimp lineage occurred
at least 5 mya (9-7 mya)
• Hominid = extinct and extant species more
closely related to humans than to chimps
• Examination of Hominid evolution was originally
based on fossil evidence
•BUT:
- Fossils are rare
- Fossils are incomplete
•Difficult to differentiate evolutionary differences
from within-species variation (esp. sexual
dimorphism = differences between sexes; e.g. size )
•Oldest Hominid fossil?
•cranium, jaw fragment, and tooth from Chad dated to
6-7 mya
•some researchers disagree with classification as
Hominid, say it is the skull of an adult female gorilla
•NOTE: if on Hominid lineage is inconsistent with
estimates of divergence with chimp
•Few fossils between 5-4 mya found in Ethiopia –
designated Ardipithecus
•Most fossils dated after 4.2 mya and before Homo are
called Australopithecus
•Australopithecus afarensis includes famous fossil
“Lucy” (after “Lucy in the Sky with Diamonds”)
-dated 3.2 mya
-also Laetoli footprints (3.5 mya) which
showed bipedalism (some paleontologists think that
bipedalism was the most important step in human
development; others say it was speech)
•Australopithecus was present in many areas in
Africa from 4-2.5 mya
•Homo erectus – earliest Hominid found outside
Africa. Found in Indonesia and China
•Fragmentary fossil
evidence
- reconstructed phylogeny
from 77 morphological traits
from 7 anatomical regions
Note:
1. Evolutionary tree is
branched – older fossils are
not necessarily ancestors of
anything living
2. can get some
convergent evolution
(=same change/adaptation
independently)->not
necessarily good measures
New Hominid
-Early Homo erectus – Turkana Boy (11 year old) – 1.6 mya
-Believed to be first hominid that spread out of Africa –
probably 1 mya
-By 730,000 BP (before present) H. erectus had colonized Middle
East and S. Europe
-500,000 BP – Germany, Britain, Peking
- brain size increased
Neanderthal Man
-evolved from H. erectus outside Africa
-Early European
-Appeared ~120,000 BP
-originated in Middle East
-evidence for presence 115,000-35,000 BP (some mixture with
modern humans)
Transition to Modern Humans
-some H. erectus evolved into H. sapiens
-in Africa – excellent fossil sequence ancient ->
modern
-H. sapiens probably arose 200,000-100,000 BP
-Was this transition unique to Africa?
Two main models: (plus intermediates)
1.
Multiregional
2. Out of Africa (African replacement,
monoregional)
Multiregional
Predictions:
-last common ancestor of
modern humans is H. erectus
that radiated from Africa
-Why evolve same
characteristics independently
in different places?
->Behavioural; cultural
exchange; universal selection
forces
Out of Africa
Predictions:
-last unifying event more
recent – only 100,000 BP
(ten times less)
-H. sapiens radiated from
Africa
-only arose once
-extinction of other Homo
species
Which model is supported by the
molecular evidence?
1. The relationship of modern humans to
archaic humans
-
The multiregional model predicts that modern Europeans are
descendents of archaic Europeans (Neanderthals).
DNA was extracted from the Neanderthal skeleton bones by
Svante Paabo. Analysis showed that it is completely distinct from
modern humans (not close to Europeans) and diverged very long
ago.
The timing and pattern of relationships are consistent with the Out
of Africa model.
The timing of the divergence of modern humans
•The predicted date of the last common ancestor of
modern humans is very different between the two
models.
•The multi-regional model predicts that the last
common ancestor existed prior to the migration of H.
erectus out of Africa (1.5-2 mya).
•The Out of Africa (replacement) model predicts a
much more recent common ancestor, at the time of
H. sapiens’ migration out of Africa (100,000-200,000
ya).
•Estimate this date using a molecular clock
Genetic evidence supports African
origin
-most genetic diversity is seen
within Africa – oldest group of
humans
-H. sapiens originated in Africa
-some individuals migrated – took
a subset of genetic variation with
them
-some members of lineage didn’t
migrate -> see diversity unique to
Africa
Last common ancestor of H.
sapiens is too recent to be
compatible with multiregional
hypothesis.
Molecular data indicate that H.
sapiens left Africa ~ 100,000 BP