Human Evolution

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Human Evolution
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What is a Hominid?

• 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 their status is debated

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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

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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

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Why did bipedalism become the primary adaptation
of hominids?

• 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

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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!

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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

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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

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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?

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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!
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Hominid Evolution
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Australopithecus anamensis

• 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-to-chew foods

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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)

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Australopithecus afarensis

• 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
• Small brain case averaging 430 cc
• Prognathic (jutting out) face
• U-shaped palate vs. the parabolic shape of modern
  humans
• Reduced canines

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Australopithecus afarensis
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A. afarensis Skeleton - Lucy
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Paranthropus aethiopicus

• 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

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Paranthropus aethiopicus
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Paranthropus boisei (KNM-ER 406)

• 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

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Paranthropus boisei (KNM-ER 406)
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Paranthropus boisei (KNM-ER 406)
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Paranthropus boisei (OH 5)
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Kenyanthropus platyops

• 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, human-like face appeared early
  in evolution alongside range of other facial
  forms not outcome of progressive, linear
  evolution

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Australopithecus africanus

• 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

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Australopithecus africanus
Australopithecus africanus
Ms. Ples best known A. Africanus cranium
front view
Ms. Ples best known A. Africanus cranium
lateral view
Taung Child
STS 71 Most complete A. africanus skull
Partial skeleton of A. africanus discovered in
1950s
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Paranthropus robustus

• 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

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Paranthropus robustus
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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. africanus (above),
reinforcing notion that the 2 species represent a
single ancestor-descendent lineage
Note remarkable similarity
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Hominid Evolution
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Australopithecus garhi

• 2-3 mya lack of adequate hominid fossil record
  in Eastern Africa
• 2.5 mya Recently discovered Hominid cranial
  dental remains in Ethiopia
• Recognition of new species of Australopithecus
• Descended from Australopithecus afarensis a
  candidate ancestor for early Homo

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Australopithecus garhi

• 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

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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)
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Australopithecus garhi
Not yet clear whether A. garhi falls on the
direct lineage leading to modern humans, or lies
on an extinct side-branch
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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

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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

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Homo habilis (KNM-ER 1813)

• 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

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Homo habilis (KNM-ER 1813)
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Homo rudolfensis (KNM-ER 1470)

• East Africa, 2.2-1.8 MYA
• Distinguished from other specimens of early Homo
  by
• Slight supra-orbital brow ridge across the
  forehead with no trough behind
• 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

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Homo rudolfensis (KNM-ER 1470)
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Hominid Evolution