Human Evolution
What is a Hominid?
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Modern humans & our direct
and indirect ancestors after
our lineage split from the
chimpanzee
•
Until recently, earliest
hominids were dated
between 3.5 and 2.4 mya &
placed in the genus
Australopithecus
•
In last few years, time range
of Australopithecus pushed
back to 4.2 mya, distribution
expanded to include regions
outside E. and S. Africa
•
New finds from 4.5-7 mya
are thought to be hominids
that predate
Australopithecines, although
Hominid Sites
• Earliest fossil hominid sites are in
Africa
• They now span the latest Miocene
to the early Pleistocene from about
6-7 mya to about 1.6 mya
•The major groups of sites are:
1. Ethiopia = Middle Awash valley &
Hadar (Australopithecus
afarensis)
2. Kenya = Lake Turkana
3. Tanzania = Olduvai Gorge
4. South Africa = various sites in
limestone caverns centered
around Sterkfontein
What Makes A Hominid? - Bipedalism
• Primary feature
distinguishing hominids
from other hominoids is
walking erect on two legs –
erect bipedalism
• Adaptations for bipedalism
in the the partial skeleton of
“Lucy,” an australopithecine
( 3.2 mya) clearly seen in the
hip, spine and leg bones
Why did bipedalism become the
primary adaptation of hominids?
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Carrying behavior
Reduction of overall heat stress - facilitates heat
loss through convection by exposing body to air
currents, only humans have sweat glands that
produce moisture to cool body
Most energy efficient way to travel long distances
Allows for better vision in open environments &
defensive action against predators by freeing
hands to throw objects
Evidence for Early Bipedalism
• The record of bipedalism is
most graphically preserved in the
fossilized footprints at Laetoli,
Tanzania!
• 3.6 million year old tracks left
by 2 individuals were uncovered
in volcanic ash by Mary Leakey
(1978-79)
•
Footprints were left by 2
australopithecines in damp
volcanic ash of Laetoli
•
In the Laetoli trail, prints of
the 2 individuals can be seen
walking away from us
•
Notice how close the tracks
are!
Laetoli Footprints
•
Laetoli footprints clearly
show that the creatures
who made them were fully
bipedal
•
Big toe hardly diverges
from the rest of the foot,
unlike in apes
•
Gait = “heel-strike”
followed by “toe-off” – the
way modern humans walk
Laetoli Reconstruction
•
In this reconstruction, the 2
early hominids, identified as
Australopithecus afarensis,
walk bipedally across an open
ash field produced by an
erupting volcano
•
Region is wooded, but here
trees are absent and the
volcanic ash, wetted by a
light shower of rain formed a
flat shallow layer in which
footprints are deeply
implanted
•
Footprints filled up with yet
more ash, and were thus
preserved
•
Footprints reveal that, even
at this early stage of human
evolution, our ancestors
walked upright with striding
gait very similar to our own
Orrorin: Earliest Evidence for
Walking on Two Legs?
How far back in time does the record of bipedalism extend?
•
Fossils from Turgen Hills in
Kenya have been dated to about
6 mya
•
Include: upper portion of a
femur, lower portion of the
humerus, some lower jaw
fragments, & teeth
•
Arm bone = virtually identical to
that of a chimpanzee
•
Femur = more human-like,most
important for showing
adaptations for walking on 2
legs
•
Was Orrorin a direct human
ancestor, or close to the
common ancestor of chimps and
humans?
Ardipithecus: Earliest True Hominid?
•
Between 4.5 and 5.5
mya from the Middle
Awash valley site
•
Remains very
fragmentary: limb
bones, toe bones,
jaws & teeth
•
Straight toe bones
suggest it may have
been bipedal
•
Ardipithecus &
Orrorin are
candidates for last
common ancestor of
chimps & humans
•
Each have mosaic of
features seen in later
hominids & modern
chimpanzee
IT MAY WELL BE THAT THE LAST
COMMON ANCESTOR OF CHIMPS AND
PEOPLE HAD A MIX OF FEATURES:
SOME RETAINED IN HUMANS,
OTHERS RETAINED IN CHIMPS!
Hominid Evolution
Australopithecus anamensis
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Turkana region of
Kenya
Dated to 4.2-3.9 MYA
Probably walked
upright
Teeth covered with
enamel much thicker
than that of
Ardipithecus, so diet
may have had hard-tochew foods
Australopithecus anamensis
•Lived roughly 4 mya
• Only a few
anamensis fossils
have been found
• Those shown here
include: jawbone &
part of the front of
the face, parts of an
arm bone (radius),
fragments of a lower
leg bone (tibia)
Australopithecus afarensis
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Eastern Africa 3.9 – 3.0 MYA
Pelvis and leg bones resemble modern
humans in some ways
Sexually dimorphic in body size
May have been adept at tree climbing,
based on curvature of finger and toe
bones
Ape-like features:
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Small brain case – averaging 430 cc
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Prognathic (jutting out) face
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U-shaped palate vs. the parabolic
shape of modern humans
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Reduced canines
Australopithecus afarensis
A. afarensis Skeleton - Lucy
Paranthropus aethiopicus
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Eastern Africa, dated 2.7-2.3 MYA
Note widely flaring zygomatic arches (bones arching around side of skull to
join below eyes, forming cheeks). Prominent sagittal crest – largest ever
discovered in human lineage – and cheek teeth are correspondingly large.
Adaptations for heavy chewing
Small cranial capacity (410 cc) and prognathic face
Black skull exhibits features more like A. afarensis
Paranthropus aethiopicus
Paranthropus boisei (KNM-ER 406)
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Ethiopia, Kenya and Tanzania; dated 2.2 – 1.2 MYA
Absolutely largest teeth found in any hominid species; referred to as
hyper-robust due to massive molar and premolar teeth
Skull has extremely broad, short face with flaring zygomatic arches
(cheek bones), relatively small brain, pronounced sagittal crest in males
Skull and dental features = adaptations for heavy chewing
Paranthropus boisei (KNM-ER 406)
Paranthropus boisei (KNM-ER 406)
Paranthropus boisei (OH 5)
Kenyanthropus platyops
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Kenya, 3.5 to 3.2 MYA
Ancestral features:
 small ear canal more like
that of chimps, A.
anamensis &
Ardipithecus
shape & small size of
braincase
Derived features: relatively
flat face and small molars
Importance: flat, humanlike face appeared early in
evolution alongside range
of other facial forms; not
outcome of progressive,
linear evolution
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Australopithecus africanus
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3.3 – 2.5 MYA in Transvaal
region of South Africa
First australopithecine to be
described (1924)
More globular cranium &
slightly higher ratio of brain to
body size
Teeth & face appear less
ancestral (reduced in size
relative to earlier forms, face
less prognathic)
May represent 2 species or
one very sexually dimorphic
species
Proportions of arm to leg
lengths may be more ape-like
than in A. afarensis
Australopithecusafricanus
africanus
Australopithecus
Ms. Ples – best known A. Africanus
cranium – front view
Taung Child
Ms. Ples – best known A. Africanus
cranium – lateral view
Partial skeleton of A. africanus discovered in 1950s
STS 71 – Most complete A.
africanus skull
Paranthropus robustus
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South Africa, 2.0-1.0 MYA
Short, broad face with deep
zygomatic arches & large
temporal fossa. Larger
individuals (males?) have
sagittal crests
Very large cheek teeth
covered with thick enamel
Wear patterns on teeth
indicate herbivorous diet of
harder, more resistant, &
perhaps smaller food items
than A. africanus
Lived in secondary
grasslands near rivers and
wetlands
Paranthropus robustus
Paranthropus robustus
This recently discovered
(1999) skull of Paranthropus
robustus from Drimolen,
South Africa is the most
complete found so far
(reversed image)image)
Note the remarkable
similarity to the newly
found but older skull of A.
Note
remarkable
africanus
(above),
similarity
reinforcing notion that the 2
species represent a single
ancestor-descendent
lineage
Hominid Evolution
Australopithecus garhi
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2-3 mya = lack of adequate
hominid fossil record in
Eastern Africa
2.5 mya = Recently
discovered Hominid cranial
& dental remains in Ethiopia
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Recognition of new species
of Australopithecus
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Descended from
Australopithecus afarensis =
a candidate ancestor for
early Homo
Australopithecus garhi
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Discovered at Bouri,
Ethiopia in 1999
Found with butchered
bones – maybe oldest
toolmaker
Oldest stone tools are also
dated to about 2.5 mya
Ancestral to Homo? Right
place at the right time…
Australopithecus garhi
One surprise in the A. garhi skull
was enormous back teeth, instead
of smaller ones seen in later Homo
species (Video Image/UC
Berkeley)
Australopithecus garhi
Not yet clear whether A. garhi falls on the direct
lineage leading to modern humans, or lies on an
extinct side-branch
Australopithecus garhi
• EARLIEST BUTCHERS: Signs
that hominids scraped and
smashed animal bones, like
this tibia, 2.5 mya
• Antelope tibia shaft has
been shattered, smashed,
and cut by stone tools
• The earliest documented
percussion marks made by
hominids, presumably
extracting fatty marrow from
these bones
Rise of the genus Homo
•
Earliest come from the same African sites as Australopithecus
•
Most date between 2.4 and 1.8 mya
•
Homo habilis means “handy man”
•
Growing consensus that there may be 2 or more species of
Homo by 2 mya
“Homo habilis” (KNM-ER 1813)
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East Africa, 2.4-1.6 MYA
3 traits define H. habilis as
transitional species:
 Expanded cranial
capacity (590-710 cc)
 Reduced post-canine
tooth size
 A precision grip,
provides anatomical
basis for tool-making
One of many problems =
obtaining accurate brain
volume estimates from
crushed and/or distorted
skulls
May represent 2 or more
species
“Homo habilis” (KNM-ER 1813)
“Homo rudolfensis” (KNM-ER 1470)
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East Africa, 2.2-1.8 MYA
Distinguished from other
specimens of early Homo by:
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Slight supra-orbital brow
ridge across the forehead
with no trough behind
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Less prognathic & longer
face, squared off maxilla
Discovery of Kenyanthropus,
similar facial features, have
led some to see link between
“platyops” and “rudolfensis,”
representing a distinct
hominid lineage – placing
“1470” in the new genus as
“Kenyanthropus rudolfensis”
“Homo rudolfensis” (KNM-ER 1470)
Hominid Evolution