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)
- a bit of ape from Italy
- Asian apes, including
- Gigantopithecus & Lufengpithecus (China)
- 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 (e.g. thick enamel, low-cusps) 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. Important lessons from this? (1) base phylogenetic grouping on as many specimens as possible; (2) tooth morphology may represent convergent diet (e.g. shared trait may just indicate that two animals were adapted to similar diets)
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 4 and 6 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 best-known early hominid fossil species:
- 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 (see image of knee) (see image of hand) (see image of pelvis)
- Take the Lucy Test ! (your instructor is not responsible for the "Lucy Test" site, but thinks it is a worthwhile exercise)
- READ about this species in your textbook!!!!
- read about discovery of Australopithecus anamensis in Kenya!
