Bipedalism --> Theories

Endurance running Hypothesis —> Behavourial Theory

Bramble & Lieberman (2004) :

• Homo evolved to travel long distances both walking and running

• Carrier argued that endurance running was for predatory pursuit

• Endurance running may have increased scavenging/hunting capacity

Threat model —> Behavioural theory

Carrier (2011)

• Many quadrupedal species stand bipedally on their hind-limbs to fight to gain a 40-50% higher strike force

Jabolonski & Chaplin (1993)

• Bipedalism could enforce social dominance, creating relationships critical to social order

• This would provide a selective advantage by relieving morbidity and mortality due to intra and inter group aggression

• In mountain Gorillas bipedal displays are central to overcoming challenge - 80% of male-male encounters involve bipedal threat

Freeing of the hands - Behavioural theory

Kingdom (2003)

• Bipedalism is a long drawn out sequence, that may have varied among different regional groups and there are many explanations

• Freeing of the hands for terrestrial defence as posited by Darwin 1871 or from the need to carry babies, food and other objects back to base

Lovejoy (1988)

• Bipedalism played an important role in the creation of the nuclear family

• Bipedal locomotion freed the hands making it possible to carry/gather more food

• Males who could walk bipedally, able to carry more than their quadrupedal counterparts, were more appealing to females

• A selective advantage was created for bipedal behaviour

Radiator theory —> Ecological theory

Falk (1990)

• The brain is a heat sensitive organ and emissary veins cool the brain under hyperthermic conditions - the network of veins act as a radiator releasing thermal constraint

• Brain size increases with increasing amount of mastoid and parietal emissary veins in Homo lineages, but Australopithecus lacks this network

• Humans in a vertical condition drain blood to their vertebral plexus of veins so selection for bipedal locomotion aids this

• Radiator theory is in keeping with the idea that gracile Australopithecines occupied a Savannah niche that had high temperatures / solar radiation

• Savannah hypothesis works in tandem with radiator theory

Savannah hypothesis —> Ecological theory

Falk (1990)

• Savanna hypothesis = early hominins abandoned forest/woodlands for the Savanna

• In the Savannah environment persistence hunting would have necessitated the development of Thermoregulatory mechanisms for dissipating heat (such as hairlessness and sweat production)

• Wheeler’s physiological hypothesis = Hominins scavenged for food in the day and risked overheating , bipedal locomotion reduced the area of the body the sun affects (40% less exposure) which would increase scavenging time

Kingdom (2003)

• Provides a criticism of radiator theory because:

  1. Human ancestors did not always live in forest environments

  2. These ancestors could not have moved out into open environments unless they were already bipedal

Seed eating hypothesis —> Ecological theory

Hunt (1994)

• Seed eating hypothesis was formulated by Jolly (1970)

• Focused on the demands of collecting small resources distributed close to ground level - bipedal locomotion saves energy as upper body does not have to be raised

• Bipedalism evolved to reduce energy costs when reaching into trees for food acquisition

• In Tanzanian Chimpanzees 80% of bipedal behaviour was during feeding

The role of ecological/ environmental inputs

Baker (1977)

• An adult male chimpanzee was observed to take up bipedal behaviour after a total paralysis of the forearm (fabian)

• This is not normal behaviour - bipedal behaviour occurs less than 1% of the time in natural habitats and Fabian was observed to have some traits others did not that would aid his bipedal locomotion such as downward tilt of his pelvis

• Fabian’s observes locomotion is the response to an environmental input (disease) and may be an example of the pliancy in Miocene and Pliocene populations to an array of ecological factors