BIPEDALISM

=Bipedalism= Walking upright distinguishes humans from other primates, and this distinction is expressed anatomically in many of the unique skeletal and other features of the human form. Bipedalism formed the backdrop for our divergence form the rest of the apes. [image:http://i.imgur.com/Ab7S8QKl.png?11] Changes in the locomotion (movement) of primate species had already led to a more upright posture (illustrated above). Primates also developed the tendency to sit upright. While only humans are habitually bipedal, apes and monkeys will even stand on two legs under certain circumstances. For instance, some monkeys will stand to look over tall grass in order to spot potential food sources, predators and other monkeys. Many primates stand when fighting or displaying dominance, they do this because standing makes them appear larger. Some monkeys and apes will even stand up for short periods of time in order to carry things of throw something. This tendency toward a more upright posture was the foundation that allowed for the fully upright, bipedal ape to emerge. However, it was an environmental change that finally triggered the divergence of the Hominin lineage (our direct ‘human’ ancestors) The relatively rapid divergence of new species such as the Hominin branch is known as an '''adaptive radiation'''. This often occurs when there is a significant environmental change and new species rapidly evolve to take advantage of an unoccupied niche. ==Selection for Bipedalism== [image:http://i.imgur.com/4xONZyM.png?1] It's believed the earliest Hominins emerged in Eastern Africa. Most of the earliest fossil finds come form the rift valley, highlighted green in the map (left). This story begins approximately 5-6 million years ago when this region of Africa experienced considerable environmental changes. Africa became much dryer and the forest, which was home to the apes, became a wooded savannah (grassland). This massive shift forced some of the apes / early Hominins out of the trees in search of alternate food sources. It also meant that movement throughout a continuous tree canopy was no longer possible. [image:http://i.imgur.com/O9LWzpT.jpg?1] As climate and habitats changed, bipedalism had considerable advantages. First and foremost bipedalism was more energy efficient. Even a small reduction in the energy used for movement would be a huge selective advantage. This energy could be invested into rearing young / increases the chances of survival. Bipedalism also made it easier to regulate body temperature (thermoregulation). Being able to see over tall grass or simply see further over the horizon may have helped early Hominins to locate food or avoid predation. Early Hominins would have been scavengers, being able to collect food and carry it to a home base is a selective advantage especially since it reduces the threat from competing scavengers. Freeing up of the hands allowed the further development of tools and weapons. Whilst tools are a definite selective advantage, they are probably a consequence of bipedalism rather than a cause. Specialization of tasks done with the hands would have contributed to the social interaction and cultural evolution of early Hominins. [image:http://i.imgur.com/t9q6PvZ.png?2] '''Advantages of Bipedalism:''' *'''Seeing over the grass''' may have helped to spot predators or locate carcasses at a distance. *'''Holding tools and weapons''' (probably a consequence of bipedalism, rather than a cause). *'''Carrying food''' to a ‘home-base’ / position of safety. *'''Thermoregulation''': Smaller surface area presented to the sun at midday (60% less)& greater air flow across the body when lifted higher off the ground. *'''Efficient Locomotion''': Energy efficient method that favors low speed, long distance movement – walking. ===Why did Bipedalism Evolve so Rapidly=== In a sense bipedalism is an extension of a tendency shown by most primates towards a more upright posture. Monkeys sit semi-upright, apes brachiate with the body suspended vertically, and nearly all primates suckle their young sitting upright. However humans are the only primates that habitually walk on two legs. In evolutionary terms bipedalism actually developed very rapidly (over approx 2.2 million years). We have already looked at some of the advantages of a bipedal lifestyle, however, these may not fully explain the speed at which bipedalism developed. [image:http://i.imgur.com/X1btJkk.png?1] As Africa became warmer and drier, walking up-right also meant that less of the body was exposed to direct sun-light from above, aiding thermoregulation. Standing upright also increased air-flow across the body, making it easier to cool down (thermoregulation). This also explains why selection favoured the reduction in body hair and an increase in the number of sweat glands. However, infant chimps and gorillas hang onto their mothers’ long hair using not only their hands, but prehensile (grasping) feet. Human babies do not have prehensile feet, and moreover their hands have no hairy maternal body to cling to. Thus they have to be carried by their mother. This may have resulted in self-accelerating push towards bipedalism. Partially bipedal hominins, with a big toe tending to face slightly forward, would have less prehensile feet. As infants they would therefore be less able to grasp the mother, who would have to use her arms to carry them. Since her arms would be less available for walking, she would depend more on her legs, thus increasing the advantage of having a forward facing big toe. This may have resulting in a ‘positive feedback’ cycle. A greater dependency on the mother to carry the infant may also have influenced the social behaviour of our early ancestors. Most primates do not share food (except with relatives). However, if females were forced to carry their young their hands may not have been free to forage for food. Thus males may have played a greater role in gathering food and carrying it back to a home base to be shared. Later the freeing up of the hands would also allow for the development of tools, which would in turn result in a better diet that would in turn allow for the development of a larger brain. This would in turn result in better tools, a better diet… and so forth. Another positive feedback cycle that led to the rapid expansion of the brain.