154 Bipedalism and Nakedness

Key Idea: Bipedalism provided advantages such as better provisioning, greater safety, and greater efficiencies in locomotion and thermoregulation. The first major step in the evolution of humans as a distinct group from apes was their ability to adopt the habitually upright stance we call bipedalism. Closely linked to this shift was the reduction in body hair. A number of selection pressures for hair reduction are described below (left). Early studies suggested that bipedalism and hair reduction were both evolutionary responses to the changing climate of East Africa about 7-3 mya. However, a 2009 analysis of the 4.4 mya Ardipithecus fossils indicates that these very early hominins were still primarily forest dwellers, so any current hypotheses must account for the emergence of bipedalism in a forested environment. The Ardipithecus finds indicate that bipedalism was strongly associated with provisioning, and was later reinforced by a move into less forested habitats as savannah became established throughout Africa in the later Miocene.

Hair reduction

3D

Bipedalism

Retention of head hair

Hair on the head and shoulders has been retained to reflect and radiate heat before it reaches exposed skin.

Parasite control

A reduction in body hair would have made it easier to control external parasites such as fleas and lice. This would have been increasingly important when early hominins began to use a 'home base'. Many external parasites need to complete their life cycle at a single location so that hatching eggs can reinfect their host.

Thermoregulation

About 3 mya, the vegetation patterns in East Africa began to favour open grasslands, with fewer forested areas. This environment would have provided fewer opportunities for shelter from the sun, creating a selection pressure for the refinement of several thermoregulatory mechanisms.

Shorter, finer hairs (not hair loss) in early hominins would have allowed greater heat loss via radiation from the skin surface. Well developed sweat glands in humans enable heat loss at 700 watts m-2 of skin (greater than any other mammal).

Felix Hicks

Seeing over the grass

An upright posture may have helped early hominins to see predators or locate carcasses at a distance.

Carrying offspring

Walking upright enabled early hominins to carry their offspring, so the family group could move together.

Provisioning as a selection pressure

The ability to carry food while walking seems to have been important in the initial development of bipedalism. Females would have favoured males able to provide energy-rich foods, which would improve offspring survival and increase reproductive rate. The ability to carry food from its source to a place of safety would have had a great survival advantage.

Efficient locomotion

Once bipedalism was established, changing habitats would have provided selection pressure for greater efficiency. Being able to move across the growing savannah without expending large amounts of energy would have offered a great survival advantage.

Holding tools and weapons

Tool use was probably a consequence of bipedalism, rather than a cause. Upright walking appears to have been established well before the development of hunting in early hominids.

Thermoregulation

Upright walking exposes 60% less surface area to the sun at midday and there is greater air flow across the body when it is lifted higher off the ground.

1. What advantages might an early human ancestor have gained by adopting a bipedal stance?

2. (a) What selection pressures are likely to have been important in the evolution of bipedalism initially?

(b) What environmental changes could have reinforced the advantages of bipedalism to human ancestors?