the Savanna hypothesis

long term climate change

food resources limited and geographically separated, requires travel between locations and stimulates exploitation of new resources

is bipedalism a response to changing environments

1. loss of habitat forces shift to using new resources

2. loss of habitat forces travelling between resources

why bipedalism

• seeing over the grass

• carrying food/offspring

• efficient locomotion

• holding weapons and tools

• thermoregulation

was there a savanna expansion

as a result of their different photosynthetic pathways C3 and C4 plants have very distinct carbon isotope signatures

expansion of the C4 savanna ecosystem

Expansion of grasslands from 9-7 million years ago (% Poaceae). Followed by a more gradational increase in desert/semi-desert pollen from ~7 Ma.

timescale for C4 savanna global expansion

~10-2 million years ago

oldest hominin candidate

Sahelanthropus tchadensis - Northern Chad

Sahelanthropus tchadensis

discovered in 2002, lived ~7 million years ago in Northern Chad

limited fossil record

fragments on nine crania, 2 partial ulnae and a femur shaft

strong variability in the climate

sediments at fossil sites are fluvial, lacustrine and aeolian

Late Miocene climate

warmer (possibly wetter) than today

Candidate 2

Orrorin tugenensis

Orrorin tugenensis information

lived in Kenya ~6-5.8 million years ago

Orrorin tugenensis remains

teeth and lower jaw fragments, pieces of the humerus, partial femora and a phalanx

why bipedalism

size and shape of femur

evidence for bipedality - Ardipithecus ramidus

upper pelvic anatomy consistent with bipedality

lower pelvic anatomy ape like

Ardi evidence for arboreal activity

foot anatomy

evidence for a mixed lifestyle

more than 150,000 specimens of fossilised plants and animals from nearby localities - 36 Ardi individuals found so far

home for Ardi

river-margin forest in an otherwise savanna

riparian attracts

number of fauna - modern afar tribes

bipedal like behaviour

observed in chimpanzees and orangutans - hands for stabilisation/feeding