The Many Theories Behind Human Bipedalism
What is bipedalism?
Bipedalism, as defined by the Oxford dictionary of archaeology, is “the habit of walking on two feet”, and was one of the first features to evolve that define humans as a species, and a member of the genus Homo. Humans, and their ancestors, are the only mammals to have evolved to be fully bipedal, and it is this feature which distinguishes us from our immediate fossil ancestors, like the chimp and the gorilla. This evolution could be due to a variety of reasons. Hominin bipedalism evolved during a long period of ecological change: global temperatures dropped and savannahs began to replace the forests. This meant that there was a new environment that our ancestors had to adapt to as best as was possible in order to survive, and various theories have been put forward on how and why our ancestors managed this.
Theories on the origin of bipedalism
Many of the most basic theories on the origins of bipedalism are based on feeding requirements. The first example of this is the postural feeding hypothesis: this theory suggests that bipedalism developed as a feeding technique first, only later evolving into a form of locomotion. Like chimpanzees, it suggests that human ancestors started to stand on their hind legs in order to reach food that was higher up in trees and bushes. Finding that this was an effective stance, early hominins may have then adopted this for further activities, later including locomotion.
Other theories based on feeding technique include man the hunter-scavenger, woman the gatherer and man the provisioner. Man the hunter-scavenger is based on the idea that at a slower pace, bipedalism allowed for greater stamina. This would have allowed early hominins to track and kill prey effectively as their bodies would have been greater adapted to endurance chases, meaning that they could follow migrating herds in order to obtain a greater amount of prey (they could pick off the weaker animals at the back of the herd). The only problem that we encounter with this theory, however, is that at the time when bipedal features started to appear in our history, around 4.2 – 3.9 million years ago, there is no evidence of stone tools to butcher the carcasses that they would have obtained having hunted in this way, and there is no widespread evidence of meat eating amongst the specimens found to date.
Darwin’s ideas support the theory of woman the gatherer/man the provisioner: he said that bipedalism allowed for our ancestors to both carry food and to use tools such as sticks to forage with. It is assumed that this was usually the women that took part in foraging, as they could do this while looking after any offspring – this is called social foraging, and bipedalism would have aided them as it allowed them to multitask. In addition to this, the same theory meant that males could gather food regularly, as the provisioners for a species, and take it back to share with the females: which would have promoted pair bonding and sexual fidelity with a partner, faster reproduction and decreased male competition. It also meant that males could both gather food and be alert with weapons so that they could defend their tribes if necessary. However, as with most proposed theories, this one has a problem – there is no great sexual dimorphism among humans as there is among other apes, and if chimps have developed the ability to carry things and walk on three legs (which is a lot easier skill to develop in evolutionary terms), then why did humans take the more energy consuming route towards bipedalism?
The last of the main bipedal theories based on feeding requirements is the theory put forward by Rodman and McHenry (1980). This is the environmental food distribution theory, which states that food became more dispersed due to the changing environment around the world – mainly the receding forests, the main early habitat for humans – meaning that it was necessary to travel further to find adequate food sources. The periods of heating and cooling led to the drying out and thinning of the forests, producing patches of grassland among patches of forest. This changed the distribution of food for many species at the time. For humans, of all forms of travel, bipedalism was the easiest to develop, as it allowed for quicker transport between patches of forest as well as the ability to carry food home. But, we have to wonder if this theory is correct as ‘cheaper’ evolutionary changes would have been to alter group size, so the males would be providing for a smaller community, or to alter the main component of the hominin diet to something that was more readily available in the new, changing, environment.
There are also many other suggested reasons for the evolution of bipedalism. One such reason is the basis of the thermoregulation theory. This theory, put forward by Wheeler in 1984, suggested that bipedal posture developed as hominids had to forage in the newly created savannahs all day, when the sun was hot and overhead. Being bipedal would have meant that they wouldn’t have had to stop their foraging to look for shade as often as a quadruped would have had to, as bipeds show 60% less body surface area to the sun than quadrupeds do. Therefore, bipedalism would have meant that early humans had to deal with less thermal stress and the possibility of overheating was greatly reduced (as heat dissipation would have been promoted due to the reduced amount of skin exposed to the sun). Another advantage would be that there would have been less need for water than those species that were quadrupedal. However, this theory only works when the sun is directly overhead. Also, bipedalism is proved to have evolved predominantly in the woodlands, not the savannah, and there is no evidence in the Australopithecine skeletons found that would prove this: all the skeletons found from the appropriate period still have some arboreal adaptations, indicating that they remained in the forests.
The aquatic ape theory is yet another that explains the origins of bipedalism to some extent. Around 5 – 1.5 million years ago, there was prolonged flooding across the world due to climate change that drove the weaker human ancestors that were not equipped to deal with the new environment out of the forests and into the sea, the only place where accessible food would have remained. Bipedalism would have allowed these hominins to wade through the water, and efficiently swim and dive for food.
Although this theory was generally rejected by the scientific community when first published, it has recently been reconsidered by many scientists, and there is some proof for this theory. Firstly, humans, unlike all other apes and mammals, are relatively hairless – we do not have fur as other animals do, but just a fine covering of hair for warmth. This, and the shape of our bodies when bipedal, means that we are quite streamlined, allowing for competent swimming and diving, and therefore the efficient collecting of food if this theory is correct. However, with this theory probably more than any, there are clear problems that mean this theory may not be entirely correct: there is no supporting evidence from any flora or fauna found with bipedal fossils of the correct ages, as well as the fact that the body proportions most ideal to swimming did not appear till the Homo genus evolved.
Finally, the last of the most debated theories is the bipedal threat display theory. Here, being bipedal allowed these hominins to hold a weapon as well as appear threatening to any danger, whether it be competition for a food source or a predator. This is a behaviour that could have prompted some groups of apes (the ancestors to the current human) to adopt the bipedal posture more often as it allowed them to look prominent and threatening to predators and opposing species as well as, for males, proving their dominancy in the community. It allowed greater vigilance against predators too due to the increased height and increased visual range that bipedalism would have given them.
So which theory is correct?
It is hard to know, when initially presented with such a variety of theories on the origins of bipedalism, which one is correct. Although many of them seem very feasible and often also have supporting evidence, there is not one theory yet put forward that does not have a flaw. However, we cannot know whether this is just due to a lack of evidence as there are new archaeological finds being uncovered every day. While there is still much debate on which theory is the most popular among scientists, it seems that the current most widely accepted theories on the origins of bipedalism are those which state that bipedalism adapted as either 1. a way to cope with the environmental changes on the African continent that all humans ancestors came from (involving theories to do with being able to hold tools or weapons while in motion as well as those about being more able to travel to find food) or 2. as an adaptation for locomotion on more flimsy branches for arboreal ancestors.