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Bio1B Evolution 12
Last lecture: Fossil record
Fossil record - significance & interpretation
Extinction Background extinction rates and the “big 5” mass extinction
The “K/T” boundary - asteroid hypothesis; dinosaur extinctions, radiation
of mammals
Are humans causing the 6th mass extinction?
Today
Macro-evolution (cont.)
Species selection
Transitional forms - tetrapods, birds: “exaptation”
Evolution of developmental programs - eg. vertebrate limbs
Human evolution
Evolutionary origins of Homo sapiens: fossils & molecular evidence
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Are we the cause of the 6th mass
extinction? (Barnosky et al. Nature 2011)
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Macro-evolution: Species selection
E.g. self incompatibility (SI) in hermaphroditic plants is
often disdadvantageous within species compared to
self-compatibility (SC)
BUT diversification
rate (S-E) higher in SI
(purple) than
SC (blue)
E E Goldberg et al.
Science 2010;330:493-495
Evolution of particular trait (red)
consistently associated with
increased rate of diversification (from
Rabosky & McCune 2010 TREE)
SI
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SC
Understanding the transition of tetrapod
vertebrates from water to land
Tiktaalik
Fig 34.20
Acanthostega
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Modification of existing structures for new
purposes: ears and feathers
Feathers:
for display
or warmth
before
flight?
Late
Jurassic
feathered
dinosaur
Fig. 34.31. Bones of inner ear of
modern mammals are derived from
jaw joint of ancestors (see also Fig.
25.6
Recent discovery:
dinosaur feathers
were colored display?
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Evolution of
developmental genes
=> phenotypic novelty
• Molecular homology: genes
with common ancestry
controlling development (top
right)
• Changes in timing and
spatial pattern of expression
=> change in phenotype
• E.g Ubx suppresses leg
development in flies, but not
shrimp
Fig. 25.22
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Origin of novelties:
The vertebrate limb
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Are the fish “fin” and vertebrate
“limb” homologous?
Very different anatomy, yet…
Similar patterns of Hox gene
expression
Anatomic differences could be due
to modification of timing/duration of
expression?
Shubin et
al. 2009
Nature
457:818
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Evolution of
hominins: fossil
evidence I
• Hominins split from
common ancestor with
chimps about 7Myr;
African origins, diversity
expands 4-2Myr
• Key features: bipedalism,
smaller canines (large
brain later)
• A. ramidus - neither
chimp nor human - see
display in VLSB
• “Australopiths” probably
paraphyletic with Homo
Robust
anthropoids
“Lucy”
Fig. 34.40
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Evolution of hominins: fossil
evidence II
• Homo - key features:
increasing brain size,
reduced jaw, lower sex
dimorphism, more
terrestrial
• African origins; H. erectus
-> europe >1.8Myr ->
Indonesia (“Java man”).
Extinct 200 Kya?
• H. floriensis - >1M? 12Kya. Related to H.
erectus?
• Neanderthals - Europe
and near east, 200-24Kya
?
Fig. 34.40
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Evolution of
hominins: fossil
evidence III
H. floriensis
• Possibly persistent
relative of H. erectus [or
malformed H. sapiens?]
• Exemplifies humans
evolve as other species:
dwarfing of large
mammals on islands eg. Stegodon “pygmy
elephants & huge
lizards! (Varanus)
• Putative tools >1Myr,
fossils to 12Kya overlapping H. sapiens
H. floriensis
Microcephalic
H. sapiens
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Migration of H. sapiens
• Out of Africa - about
100Kya
• Rapid spread across Sth
Asia to Australia & central
Asia
• One or 2 colonizations
across Bering bridge
during last ice age ->
rapid spread to Sth
America
• Polynesian migrations
across Pacific are recent:
1500 BC to 300 AD
(Hawaii)
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Modern humans &
related species hybridization or
replacement?
Genetic evidence largely
supports single origin & “outof- Africa” over independent
origins from different
populations of H. erectus
(multi-regional).
But did modern humans
hybridize with, or simply
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replace neanderthals?
Paleogenomics: Neanderthal v modern humans
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60-38Kya bones of neanderthal
sequenced - compared to different
human populations
2-3% neanderthal genes in
eurasian-papuan, not africans
Several genes - eg skin &
pigmentation, skeleton, metabolism
under recent selection in humans
Refs: Green et al. 2010 Science 328:710,
Gibbons 2010 Science 328:680
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‘Denisovans’ - another
recent Homo
50-30Kya
fossil
Reich et al. 2010 Nature 468:1053
4-5%
2.5%
440-270 Kya
Neanderthals
(bottleneck?)
Modern
humans
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