Human Evolution

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Human Evolution
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Human Classification

• Domain Eukarya
• Kingdom Animalia
• Phylum Chordata
• Class Mammalia
• Order Primate
• Family Hominidae
• Genus Homo
• Species sapiens

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• Animals
• multicellular eukaryotic organisms that ingest
  other organisms
• Chordates
• Animals that at some point during their
  development have
• a notochord (long flexible rod that runs along
  the dorsal axis of the body in the future
  position of the vertebral column)
• a dorsal, hollow nerve cord
• pharyngeal slits or clefts
• and a muscular, post-anal tail.

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• Mammals
• amniotes with mammary glands that produce milk
• Primates
• Include lemurs, monkeys, chimpanzees, gorillas
  and humans
• Main characteristics
• Opposable thumbs
• Forward facing eyes
• Well-developed cerebral cortex
• Omnivorous

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• Hominidae
• The great apes
• Includes orangutans, gorillas, chimpanzees and
  humans
• Larger than monkeys
• Relatively long arms, short legs, no tail
• Larger brain in proportion to body size
• More flexible behavior than other primates

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• Homo
• Genus that includes extinct species such as
  Homo habilis, Homo erectus, and
  Homo neanderthalensis as well as Homo sapiens,
• Had larger brains than predecessors, such as the
  Australopithicus
• Sapiens
• Modern humans
• Homo sapiens idaltu extinct lived 160,000
  years ago in Pleistocene Africa slight skull
  differences from
• Homo sapiens sapiens only extant subspecies of
  Homo sapiens

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• Australopithicus
• 1974 Lucy
• discovered

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Homo Characterized by Larger Brain Size
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Homo sapiens sapiens
Homo sapiens idaltu
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Major Physical Features Defining Humans as
Primates

• Grasping limbs, with long fingers and a separated
  opposable thumb
• Mobile arms, with shoulder joints allowing
  movement in 3 planes, and the bones of the
  shoulder girdle allowing weight to be transferred
  via the arms

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Major Physical Features Defining Humans as
Primates, contd.

1. Stereoscopic vision, with forward facing eyes on
   a flattened face, giving overlapping fields of
   view. Each eye gets a slightly different view of
   the world. The brain fuses the two views to get
   depth information.

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Major Physical Features Defining Humans as
Primates

• Skull modified for upright posture

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What is the Evidence that Humans are a Species of
Ape that Spread to Colonize New Areas?

• Closest existing relatives of humans are
  orangutans from Southeast Asia and gorillas and
  chimpanzees of Africa.

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Biochemical Evidence that Humans are a Species of
Ape that Spread to Colonize New Areas

• Study of amino acid differences for proteins and
  mitochondrial DNA show humans are more closely
  related to chimps and gorillas than to
  orangutans.

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Three phylogenetic trees were reconstructed based
on the DNA sequences of

1. 4700 bp of mitochondrial DNA
2. the testis specific protein on the Y chromosome
3. noncoding regions of the ß-globin gene

All three trees show that human and chimp DNA
sequences are more similar on average than are
human and gibbon or human and orangutan
sequences.
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Biochemical Evidence that Humans are a Species of
Ape that Spread to Colonize New Areas

• These biochemical data support the theory that
  human ancestors split from ancestors of the
  chimps and gorillas in Africa.
• The molecular data also allow approximate dating
  of the splits of various groups of humans
• Between Africans and other humans 140,000 y.a.
• Between Europeans and Japanese 70,000 y.a.

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Ancestors of modern humans migrated out of Africa
less than 500,000 y.a.

• Older fossil hominids found out of Africa must
  have been the result of earlier migrations, and
  modern humans are not descended from these
  hominids.

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Anatomical Evidence that Humans are a Species of
Ape that Spread to Colonize New Areas

• Neoteny when infant characteristics persist in
  the adult
• Adult humans show similarities in appearance to
  baby apes, including flat faces, large brain to
  body size ratio, upright heads, and little body
  hair.
• This suggests that human evolution may have
  involved a slowing down of development with long
  childhood, delayed puberty, and retention of
  juvenile characteristics into adulthood.

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Neoteny
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Neoteny and Brain Size

• The slowing down of development in the genus Homo
  included continued brain growth after birth, as
  opposed to cessation of growth after birth.
• Consequences of increased brain size include
• increased capacities for learning, complex
  thought, and memory
• possibility of language and complex tool
  manufacture
• increased amount of energy required to use brain

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Anatomical Evidence that Humans are a Species of
Ape that Spread to Colonize New Areas Bipedalism

• Bipedalism is walking on two legs
• All apes are capable of bipedalism, but are not
  as nearly as adept as humans are

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Bipedalism Adaptations Involved

• Foramen magnum, hole in skull through which brain
  and spinal cord connect, moved forward, allowing
  the head to balance on the backbone
• Arms became shorter and less powerful
• Legs became longer and stronger
• Knee changed to allow the leg to straighten fully
• Foot became more rigid, with longer heel, shorter
  toes and a non-opposable big toe

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Bipedalism Some Consequences

• Collecting food from bushes may be easier
• Walking long distances while carrying food,
  water, infants, tools, or weapons is possible
• Tree climbing is more difficult
• Go to Evolution Library The Transforming
  Leap, from Four Legs to Two
• Read the article, "The Transforming Leap, from
  Four Legs to Two."
• Summarize the different hypotheses offered to
  explain the development of bipedalism and the
  evidence that supports or refutes each.