Read Chapter 20

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Human evolutionary originspart 1.
Read Chapter 20 I will post 2 homeworks today
the first one will be due next M, the second one
next W.
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Mammalsdiversifyfor the first time (?)
Cambrian explosion mostrecognized animal phyla
appearfor first time in fossil record overabout
a 40 million year period.
First multicellular organisms
Diversification of unicellular life
First life?
Earth forms
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Bears, cats, canids, seals
Rhinos, horses
Whales, pigs, camels
Rabbits, hares, pikas
Monkeys, humans
Tree shrews
Elephants, sea cows
Porcupines, guinea pigs
Rats, beavers, squirrels
Armadillos, anteaters
Hedges Kumar 1999 Science 285 2031a
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Marsupials in orange
Each point representsa family of mammals
This line isthe KTboundary
Hominids (us!)
Bininda-Edmonds et al. 2007
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Primates
Loris
Lemur
Macaque
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More primatesSome closer relatives great apes
Common chimpanzee
Bonobo (pygmy chimp)
Human
Gorilla
Orangutans
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What are the evolutionary relationship among
primates?
Most scientists acceptthis phylogenetic treeas
a good representationof the evolutionary
historyof Old World monkeys,apes, and
humans.Other primates (like thelemurs and
lorises) areoutside of this grouping.
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One of the reasons is that humans and African
great apes share significant morphological
similarities, including elongated skulls,
enlarged brow ridges,shortened by stout canine
teeth, similar changes in the bones of the upper
jaw and in the wrist, enlarged ovaries and
mammary glands, and reducedhairiness.
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A phylogenetic tree based on immune reactions to
albumin (Sarich Wilson, 1967)
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Since the paper of Sarich and Wilson, there has
been considerable disagreementover the
evolutionary relationships between gorillas,
chimps, and humans. There are at least four
reasonable relationships.
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Here are three phylogenetictrees based on DNA
sequences,produced in three different
laboratoriesin the early 1990s.
Mya
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Outstanding and mysterious question what has
happened to our speciessince the divergence from
the lineage leading to chimpanzees?
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Was Lucy bipedal?
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Other evidence for bipedalism.
Chimp pelvis
Lucy pelvis
Angle that the femurmakes with kneealso is
important, and different in bipeds thanin
chimps.
This arrangement moreefficient for bipedalism.
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Is Lucy (Australopithecus afarensis) the only
fossil hominid species found?
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Another group is called the robust
australopithecines. These were short butstout
individuals, characterized by enormous teeth and
jaws, some with bony crests on the top of their
heads where jaw muscles attached.
Australopithecus robustus1 2 million years ago
Australopithecus boisei1.4 2.3 million years
ago
Australopithecus aethiopicus1.9 2.7 million
years ago
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This bony ridge is thought to have servedas an
attachment for jaw muscles.
How jaw muscles are arranged in modern humans.
Australopithecus aethiopicus
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Finally, a few members of our own genus,
Homo.These three species are more ancient
representatives of Homo.
Homo ergaster1.5 1.8 million years ago
Homo habilis1.6 1.9 million years ago
Homo rudolfensis1.8 2.4 million years ago
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More recent members of the genus Homo.
Homo sapiens 0.1 million years ago to present
Homo neanderthalensis0.03 0.3 million years ago
Homo heidelbergensis0.2 0.6 million years ago
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How are all these species related to each other?
An hypothesis.
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How to get from ergaster to sapiens? Much of the
argument centers on the geographic distributions
of different species
Its certain that the australo-pithecinesevolv
ed in Africa.
Still contentious, but most anthropologists
agreethat N. and S. America werecolonized
15,000 and 25,000years ago.
But more recently than about 2 million yearsago,
the picture becomes confusing. This is because
some species migrated out of Africaand this has
caused enormous debate about where and how
anatomically modern Homo sapiens evolved.
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The main controversy has been about the
distribution and fate of Homo ergaster.
Moreover, the oldest known fossils of ergaster
appear roughly simultaneouslyin several distant
parts of the world, about 1.8 1.9 million years
ago.
This map shows the locationsof fossil hominids
found inAfrica, including Homo
sapiens. Locations where H. ergasterhave been
found are in blue
Lots ofH. ergasterin here.
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Fossils of H. ergaster that are about the same
age (1.8 1.9 ma) have also been found inthe
Caucasus region of Eastern Europe and possibly on
the island of Java (Indonesia).
H. ergaster
H. sapiensor H. neander.
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Anatomically modern H. sapiens appear first in
the fossil record about 100,000years ago in
Africa and Israel, and somewhat later throughout
Europe and Asia. How did these modern H. sapiens
arise?
The widespread early distribution of H.
erectus/ergaster has led to debate. Consider
four possibilities
Africanreplacement
Hybridization andassimilation
Multiregionalevolution
Candelabra
Blue arrows indicate gene flow (interbreeding).
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Molecular evidence Neandertal DNA.
Neandertals (Homo neanderthalensis) were common
in Europe up until about30,000 years ago. They
may have disappeared either because they were
drivenextinct by newly arrived H. sapiens from
Africa (African replacement) or because
theywere assimilated into H. sapiens by
interbreeding (hybridization and assimilation).
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These two alternatives lead to different
predictions about the alleles present inhuman
and Neandertal DNA 1. If African replacement
is correct, there should be few, if any, traces
of Neandertal alleles in present-day human
populations. The Neandertal alleles
should simply have died out with the
Neandertals, and the sequence of our
genetic material should be substantially
different. 2. If hybridization and
assimilation is correct, traces of Neandertal
genes should be present in present-day human
populations. Our genes should not be that
different.
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Amazingly, it is possible to recover DNA from
some Neandertal bones. This picture is from the
paper by Krings et al. 1997 (done in Svante
Pääbos lab) showing how they sampled bonefrom a
Neandertal humerus (upper arm bone). The bone
was found near Düsseldorf, Germany,and is
estimated to be about 30,000 years old.
They recovered and sequenced several short pieces
of DNA (about 360 base pairs altogether). They
then compared this Neandertal sequence with
sequences from about2000 humans and 60
chimpanzees.
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They compared each sequence with every other
sequence and counted the number of
differencesbetween them. The histogram below
(from Krings paper) shows the result.
Humans and chimps differ onaverage at about55
bases.
Any two humanshave on average8 differences in
this region of DNA.
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Other researchers have tried to distinguish the
African replacement model from themultiregional
model using only DNA sequences from present-day
humans. This hasproved difficult because the
models are identical in many respects they both
predictthat early humans migrated to different
geographic areas and then differentiated. They
differ in when they predict this happened.
African replacement predicts this happened
recently (within the last 200,000 years) and
multiregional evolution predicts that it began
about 1.8 million years ago. So the trick is to
estimate when lineages leading to different
modernhuman groups began to diverge.
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Two other observations 1. The deepest split in
the tree occurs within African lineages. 2.
There is more total genetic variation among
African than non-African lineages. This is
consistent with early H. sapiens migrating out of
Africa and carrying away only a subset of the
genetic variation present on the African
continent.
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Other evidence forolder African
populations Higher allelic diversityin African
populations compared to rest ofworld.
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The timing of the split (170,000 years ago) has
been the most controversialaspect of this study.
Is it supported by other studies?
Estimate of how many yearsago the split happened
Study
Ruvolo et al. 1993 129,000 536,000 Horai et
al. 1995 125,000 161,000 Cavalli-Sforza
1997 about 146,000 Goldstein et al.
1995 75,000 287,000 Tishkoff et al.
1996 102,000 450,000
Conclusion although none of the tests is
definitive on its own, the balance of evidenceis
tipping in favor of the African replacement
model. That is, modern H. sapiens evolved
firstin Africa sometime in the last few hundred
thousand years, then migrated out of Africa and
displaced all other populations of Homo in the
rest of the world. An implication is that
current racial differences are of quite recent
origin.