Miocene: 22.5-5 million years ago:

• Forces of continental drift have two major effects on African geography
  • highland uplift and development of Rift Valley in east Africa
  • water gap between Africa and Eurasia closed by 17my
• Geographical changes have impact on climatic patterns (global climate getting cooler and more seasonal)
• Changing geography and climate impacts both vegetation and animal population distributions

Early Miocene (22.5-17mya)

• Apes have evolved from early Anthropoids, and found only in Africa, they are very diverse (fill many niches) and much more abundant than today
• Proconsul africanus is an example of an early Miocene ape with a generalized skeleton, and a mosaic of both primitive traits and more evolved or "derived" traits, a likely common ancestor to all living apes

Middle Miocene (17-10 mya) and Late Miocene (10-5 mya)

• Apes first colonize Eurasia after link-up of continents
• Apes diversify to fill many new ecological niches that emerge as cooling climates cause forests to breakup and grasslands to spread
• Examples of Middle and Late Miocene apes:
  • Ankarapithecus (Turkey) not discussed in class
  • Gigantopithecus (China) not discussed in class
  • Otavipithecus (Namibia) not discussed in class
  • Sivapithecus indicus (Pakistan) = an organutan-like ape 10-8 my old from Siwaliks in Pakistan

Search for First Hominid: limited fossil evidence can be interpreted in different ways

Ramapithecus / Sivapithecus controversy:

Jaws of a small ape with hominid-like dentition and a small canine tooth socket initially named "Ramapithecus" and thought to be a human ancestor, until they were matched with larger jaws (and face, and postcranial skeletal elements) of large, sexually dimorphic ape Sivapithecus

Molecular clock:

DNA evidence for branching pattern of hominoids, which assumes a constant rate of mutation through time, can be callibrated in time with reference to known fossils. Dates for fossil Sivapithecus (early on orangutan lineage) and Proconsul (ancestral to all living hominoids) imply a date for the separation of hominids from the ancestral apes (and common ancestor with chimpanzees) between 5 and 7 million years ago. There is a scramble to look for fossils from this time period.

Rift Valley Fossil Hunt:

• sedimentary basins to bury fossils
• volcanic sediments are dateable with K/Ar
• sediments exposed from Late Miocene and Pliocene (5-1.8 mya)

The earliest hominid fossils currently known:

• Australopithecus afarensis = best-known early hominid
  • from sites in Hadar, Afar triangle of Rift in Ethiopia and Laetoli Tanzania found in 1970's and 1980's by teams led by D. Johansen (Hadar) and Mary Leakey (Laetoli)
  • 4-3 million years old
  • small brained, sexually dimorphic biped (e.g. "Lucy")
  • fossilized bipedal footprints at Laetoli
  • mosaic of primitive and derived features on skull and locomotor skeleton
• Australopithecus bahrelghazalia
  • recently discovered, from Chad
  • 3.5 mya
  • very similar to A. afarensis
• Kenyanthropus platyops
  • from sites in northern Kenya near Lake Turkana, recently found by team led by Meave Leakey
  • 3.5-3.2 million years old
  • suggested ancestor for early Homo
• Australopithecus anamensis
  • from sites in northern Kenya near Lake Turkana, recently found by team led by Meave Leakey and Alan Walker
  • 3.8-4.2 million years old
  • small brained biped (dentition more primitive than A. afarensis)
• Ardipithecus ramidus = earliest hominid currently known
  • from Aramis site in Afar region of northern Ethiopia found by team led by Tim White
  • most recent discoveries (2001) = earliest specimens = 5.8 million years old (PDF file)
  • largest sample of ramidus = 4.4 million years old
  • small brained creature with teeth/jaws intermediate between chimp and later hominids (e.g. A. afarensis) (e.g. larger canines, smaller molars, thin enamel)
  • many postcranial remains have been found, but not yet described in print, so we do not yet know how it walked... stay tuned! ... but it was an upright creature, judging by the orientation of the foramen magnum in base of skull
  • probably very close to what a "missing link" between hominids and chimps might have looked like

WWW links to descriptions and images of early Hominid Fossils

• Fossil Hominids at the Talk_Origins archive