Early Hominins
Hominins ultimately distinguished by brain size, bipedal
locomotion and toolmaking behavior
But these did not develop simultaneously: mosaic evolution
The only reliable indicator of earliest hominin status: bipedal
locomotion
Miocene forms (earliest Australopiths)
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Sahelanthropus tchadensis (7-6 mya)
• Small brain, huge supraorbital torus
• Face not prognathic and canines not large
• Foramen magnum more anteriorly positioned than in
apes
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Orrorin tugenensis (6 mya)
• Postcranial evidence suggests bipedal, but questions
remain
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Ardipithecus kadabba (5.8-5.2)
• Mix of apelike and hominin dental traits
• Postcranial remains imply bipedal, similar to later form A.
afarensis
All these forms quite primitive, as expected. But appear to have
been bipedal.
Key: changes in environment. Cooler, drier. Forests less
continuous.
Hypothesis: some late Miocene hominoids: exploited areas that
included some grasslands. These become hominins. Other
hominoids: exploited wetter, more densely forested areas:
ancestors of African apes.
Why did hominins become bipedal?
Ecological theories: patchier forests
But, ground-dwelling doesn’t require bipedalism!
Also, hominins did not suddenly become completely
terrestrial. A. kadabba associated with a wet, wooded area
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with some wooded grassland. S. tchadensis associated with
lakeside gallery forest and some grasslands.
Various
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scenarios:
Carrying (objects, infants)
Scavenging
Seed and nut gathering
Feeding from bushes
Surveillance
Long-distance walking
Male provisioning
Darwininian model has been used: natural selection for
increasingly better adaptation to a specific habitat: stresses
consistency of selection over time
Alternative View of Adaptive Change: Variability Selection
What if a population faces many remodelings of its habitat
over geologic time?
May evolve in response to wide range of adaptive conditions –
selection may be inconsistent over long time spans
Environmental records: worldwide fluctuations increasingly
extreme from Miocene to Recent: glacial-interglacial, aridmoist cycles over longer and longer intervals, up to 100,000+
years during last half of Pleistocene.
Hypothesis: significant changes during hominin evolution
may not have resulted from adaptation to any one habitat
type or general environmental trend:
First: environmental variability Æ strong disparities in
adaptive settings over time
Second: resulting inconsistencies in natural selection had
powerful influence on evolutionary change
Consider: generally two ways of responding to habitat
change:
1. Mobility or dispersal: track habitat or key resource
2. Broaden range of conditions in which can live
How can adaptive flexibility be achieved?
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Several ways: genetic polymorphism, phenotypic plasticity
and…
Third way: complex structures or behaviors designed to
respond to novel and unpredictable adaptive settings:
Variability Selection
V.S. refers to evolution of such adaptive mechanisms. V.S.
emphasizes effect of inconsistent selection over long spans to
time.
Examples:
• Locomotor system designed to allow wide repertoire
of movement
• Particular dental structure or foraging strategy that
enhances shift to newly available food types
• Large brain or specific neurological structure –
effective data processing and complex cognitive
responses
• Species-specific social behaviors
Existential poker, once again:
Remember key: maximizing homeostatic ability
In case of V.S. alleles favored by V.S. must avoid losing the game
(i.e., extinction) in each specific environmental setting they
encounter.
BUT—these alleles may not be associated with highest
reproductive success in any given setting!
What really matters: alleles permit survivorship in a given setting
AND episodic change in surroundings ultimately lower fitness of
habitat-specific alleles that once had greater advantage.
V.S. a process by which adaptive mechanisms emerge that buffer
episodic change in an environmental setting
Effects on Hominin Evolution
Early australopith body plan: terrestrial bipedalism and tree
climbing: a mosaic of human-like and ape-like characters:
Pelvic and knee modifications for two-legged striding
Powerful forelimbs and joint maneuverability
indicating arboreal activity
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Fossil evidence also suggests: foot more flexible than
ours with respect to climbing
This theme endures for 2+ million years
Gives locomotor versatility
Early australopith environments: variable: forest, savanna,
woodland
Testing hypothesis of V.S. for locomotor system: what were the
habitats like where australopithecine locomotor system first
evolved?
The Bipedal Adaptation
Striding gait: weight placed alternately on single, fully extended
hind limb. (Both feet on ground only 25% of time, decreases with
increasing speed)
Drastic structural/anatomical alterations in basic primate
quadrupedal pattern:
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Hominin pelvis comparatively much shorter and broader
and extends to side: to stabilize line of weight
transmission from lower back to hip joint
Broadening two sides an extend around to produce basinshaped structure to support abdominal organs
• These also reposition attachments of key muscles
that act on hip and leg: especially gluteus maximus
Gluteus maximus in quadruped: pulls thigh to side;
in biped it acts, along with hamstring, to extend
thigh, pulling to rear when walking and running
Foramen magnum repositioned farther under skull. Skull
balanced on spine
Spine has two distinct curves – thoracic and lumbar.
Keep trunk (and weight) centered above pelvis
Lower limbs elongated: in humans 20% of body height,
only 11% in gorillas)
Femur angles inward; modified knee permits full
extension of this joint
Big toe enlarged and brought in line with others
(adducted); distinctive longitudinal arch: absorbs shock
and adds propulsive spring
Hominin bipedalism becomes habitual and obligate.
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All major structural changes required for bipedalism seen in early
hominins from East and South Africa. In particular, pelvis dramatically
remodeled.
Pliocene forms
Set I: Early, primitive Australopiths
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Ardipithecus ramidus
• Dated to 4.4 mya
• In partial cranium, foramen magnum positioned further
forward than in quadrupeds
• Humerus differs from quadrupeds: didn’t function to
support weight in locomotion
• Above evidence suggests obligate bipedalism
• But, clearly primitive: flattening of cranial base and
relatively thin enamel caps on molars; canines large
relative to postcanines
• Estimated weight: 93 lbs.
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Australopithecus anamensis
• Dated to 4.2 – 3.9 mya
• Thick tooth enamel like all later Australopithecines
• Canines slightly larger than in A. afarensis but with
humanlike vertical roots
• Postcanine tooth rows parallel
• Very small acoustic meatus as in Ar. ramidus
• Postcranial skeleton similar to A. afarensis
• Probably had relatively long forearms compared to
modern humans
• Possibly descended from Ar. ramidus and ancestral to A.
afarensis
• Teeth and supporting bones suggest change in diet: from
one like chimp in Ar. ramidus to harder foods.
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Australopithecus afarensis
• Dated 4.2 – 2.5 mya
• Heavily enameled back teeth
• Postcanine tooth rows range from nearly parallel to
slightly divergent
• Foramen magnum under skull
• Footprints from Laetoli (3.7 mya) believed from this
species – bipedal, perhaps “strolling” with short stride
• Canines large, lower first premolar semisectorial
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Cranium: hint of sagittal crest; 380-500 ml cranial
capacity, with average 420 ml (chimp avg. 395, gorilla
506)
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Kenyanthropus platyops
• Dated 3.5 – 3.2 mya
• Site environment comparable to Laetoli and Hadar (mixed
grassland/wooded)
• Skull: mosaic of features – thick enamel on molars (like A.
afarensis) as well as derived characteristics such as
flatter mid-facial region and tall, vertically oriented cheek
region
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Australopithecus bahrelghazalia
• Dated 3.5 – 3.0 mya
• Found in Chad, in north-central Africa
• Extends known range of hominins of period 1500 miles
west
Set II: Later, more derived Australopiths
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Australopithecus africanus
• Dated 3.5 – 1.7 mya
• Presumably descended from A. afarensis
• Retains long arms
• Large, thickly enameled back teeth
• Retains flexibility of foot
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Australopithecus garhi
• Dated 2.5 mya
• Femur longer than other australopiths – step towards
long legs of modern humans
• Possible use of tools to process animals for food
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Paranthropus
• Dated 2.4 – 1.3 mya
• P. aethiopicus ancestor to later P. robustus and P. boisei
• Exceptionally large back teeth with thick enamel
• Specialized diet of hard foods: seeds, nuts but also meat
• Extremely powerful chewing apparatus: massive jaws,
broad faces, sagittal crest
• Use of bone digging tools, possible stone tools
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Set III: Early Homo (first hominans)
• Homo habilis
• Dated 2.3 – 1.6 mya
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Homo rudolfensis
Dated 2.4 – 1.6 mya
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These forms the first to show encephalization
Altered cranial shape – higher vault, more rounded
Smaller teeth (especially molars); narrower premolars
Diet more omnivorous
Toolmaker: Oldowan industry