Biology 265 EVOLUTION

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Biology 265EVOLUTION

• Lecture 15

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Overview

• Primates
• Out of the trees
• Pelvis
• Brain
• Larynx
• Lots of speculation
• MBP million years before present

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The tree of life - does it include humans?
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Humans Evolved - Homology

• Similar embryos
• Mammary glands
• Vestigial pointed ear
• Nictitating membrane

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Arboreal Primates

• Binocular vision - 60 MBP
• Manual dexterity (opposable thumb), large
  forehead and cerebral cortex, short snout - 50
  MBP
• Color vision (cone cells in retina) evolved in
  all primates helped with depth of field perception

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Brachiating Apes

• Swinging in trees
• Straightened spine
• Legs parallel to spine
• Preadaptation to bipedalism?

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Australopithecus

• 4 - 2.5 MBP
• Brain size not much bigger than chimpanzee (450
  cm3)
• Did tool use lead to evolution of more manual
  dexterity, bipedalism, and intelligence?

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A. afarensis (Lucy)
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Hadar, Ethiopia
Lucys homeland 3-3.6 MBP
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Climate change

• About 2-3 MBP cooling temperatures reduced
  tropical woodland
• Hominids may have adapted to savannas
• Modern humans finally emerging from Africa about
  100,000 BP

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Pelvis and Bipedalism

• Many possible advantages - tool use, vision,
  locomotion
• Also costs (trade-off)
• Narrower birth canal due to strengthening pelvis
  problematic because of bigger cranium
  Obstetrical dilemma
• Infants born early in development
• Long maternal association - learning?

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Genus Homo

• 2.33 MBP
• Closely associated with crudely flaked stone
  tools
• the oldest, securely dated co-occurrence of a
  human fossil and stone artifacts in Africa.

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Early tools
2.6 MBP
1.6 MBP
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H. erectus

• This skull was found in Kenya
• Dated around 1.7 MBP
• Brain 850 cm3

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To H. sapiens

• No clear dividing line
• Archaic H. sapiens found in Africa
• 500,000 BP ?

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China - H. sapiens?

• two skulls found near the Han river in Yunxian,
  an area of central China.
• The Yunxian skulls are between 350,000 and
  500,000 BP?

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Multiregional or Out-of-Africa

• The geographic origins of Homo sapiens are
  controversial
• Multiregional dispersal of early Homo set a
  complex stage for Homo sapiens to emerge at
  connected points across much of the Old World.
• Africa both the early and later species of Homo
  had the ability to disperse across the
  continents.

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Africa

• Given that Homo sapiens fossils appear to be much
  older in Africa than on any other continent.
• Having probably emerged in Africa, Homo sapiens
  dispersed to Eurasia, replacing older populations
  of Homo.

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Lower sea levels may have helped dispersal
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Brain size

• Increased dramatically about 2.5 MYA at about
  time of first tools and appearance of Homo
• Australopithecus, 3 MYA, 450 cm3, maybe stone
  tools
• Homo habilis, 3 MYA, 650-750 cm3
• Homo erectus, 1.8 MYA, 800-900 cm3, language??

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Brain size cont.

• Homo sapiens sapiens, 100,000 BP, 1,400 cm3 (our
  brain size)
• Homo sapiens neanderthalensis, existed at same
  time as H.s.sapiens and if anything had a bigger
  brain
• Cro-Magnon man, 40,000 BP is a fully modern human
  that may have overlapped with Neanderthals.

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Encephalization Quotient

• Compares brain to body size ratio
• In humans compared to other primates, the ratio
  is 3
• But chihuahua has high encephalization quotient
  compared to Great Dane (because we bred it to
  have a small body)
• Primate brains grow at same rate as other species
  but their bodies grow slower
• Human brain continues to grow for longer

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Brains are expensive

• Brain consumes 20 of bodies energy but only 2
  of total body weight (figures based on human at
  rest).
• Brains dont rest
• Brains use roughly the energy consumed by a light
  bulb all the time (even while asleep)
• Lots of fatty myelin - expensive material (meat
  eating needed?)

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Prefrontal cortex

• Human visual cortex (at back of brain) is
  relatively small - similar to apes
• Prefrontal cortex is very large but does not
  directly take sensory information
• What does it do?
• Phineas Gage - 1848 an iron bar pierced his head.
  Survived, walked, talked, but personality
  altered.

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Why big brains?An adaptation for what?

• Not essential for social behavior or hunting
• Sexual selection? Song?
• But why pick on brain size, and why only in
  humans?

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Neoteny

• Humans resemble baby apes
• Arrested development
• Little sexual dimorphism (e.g. compare large
  canines of male gorillas)
• Humans, the naked apes