Bipedalism Evolutionary Theory

Aridity Hypothesis (savannah hypothesis)

• 8-6 mya forests began to shrink and were replaced by grasslands

• bipedalism offered an advantage to exploit grassland niches (sympatric speciation - new species evolves from existing species and both continue to live in same geographical area)

Forest Hypothesis

• bipedalism arose not on land, but in the trees (arboreal bipedalism)

• standing on two legs in trees allows for better surveillance

• stems from upright posture (orthograde)

• already bipedal before grasslands arrived

Turnover Pulse Hypothesis

• rapidly changing climates (dry and wet) encouraged a selection for a generalized body plan (stabilizing selection - natural selection favouring individuals with average or moderate phenotyps)

Variability Selection Hypothesis

• generalized body plans (anatomic mosaic) and adaptive behaviours allowed individuals to survive better over longer periods of time

• considers multiple events of genetic drift, stabilizing selection, and reproductive isolation