Bipedalism and brain size are two of humanity’s most distinctive adaptations. Among close relatives such as chimpanzees and gorillas which are able to walk bipedally, we stand out as being the most efficient users of bipedal motion. Our bodies have evolved to make it not just easy but necessary for us to stand, walk and run on two legs; we are able to use our hands to manipulate tools and objects, and our arms and torsos enable us to put great strength into motions like swinging and throwing. Our brains, the pride and joy of the human species, are among the most complex of any creature on earth, and we have one of the largest brain-to-body ratios around. Our brains are so big that we have to be born before they can even fully function; many of the abilities that in other animals are instinctual and develop quickly take years for human babies to learn, let alone the computation-intensive capabilities of language, imagination and reasoning. How, when, and why did these peculiar mutations develop? The common thread between these different adaptations is the central story of the human race: the story of constantly increasing intelligence.
The earliest definitely known human ancestors are a race of ape-like bipeds called Australopithecus Afarensis, which lived 3 or 4 million years ago. They were spread over an area of Africa which included both arboreal and savanna environments. Australopithecus had a small brain compared even to today’s great ape species, but was definitely bipedal. They may have used primitive tools in the same way that chimpanzees can use rocks to crack nuts and twigs to ‘fish’ in termite mounds. About 1.5 to 3 million years ago, A. Afarensis started evolving into a number of distinct species. These included Homo Habilis, the most primitive of the Hominids, which had a much larger brain and has been found with the earliest known examples of stone tools. Early hominids also evolved a complicated hand structure with more versatile thumbs, and may have had the ability of speech. There are various theories as to why the Australopithecines evolved into the early Hominids. There would need to be some sort of selective pressure that reinforced the adaptations of more efficient bipedal motion and bigger brains for increased intelligence. These adaptations have drawbacks that still plague us today: a vertical spine is less efficient than a horizontal one and leads to problems such as spinal compression, hernias and balance problems; and a larger brain is not only more difficult to pass through the birth canal, but also requires more energy and thus an increased intake of proteins and fats.
The most popular theory about Hominid evolution is that ancient tree-dwelling primates started losing their habitat due to climate change. As the forests where they lived began to die off, there was increased competition for territory and resources. Some Australopithecenes started to venture on to the savanna, where there was less readily available food and greater danger from predators. Only the smartest and fastest could survive there; and over generations, their descendants began to stand upright and to develop large brains. Early Hominids adapted to be able to run faster, hit harder, and use tools with greater precision. They began to hunt larger prey for meat. Their brains started to develop in size because this allowed more complex tool use, more successful hunting patterns, richer communication abilities, and a more intense social life among isolated and embattled tribes. Early Hominids started to select mates based on their intelligence, leading to more intelligent generations of children. The accumulation of greater and greater intelligence led to the development of Homo Erectus, a species which created much more complex tools and was able to migrate and spread out of Africa to Europe and Asia. Homo Erectus was very human-like in size and stature; although they had a somewhat smaller brain than modern humans, they are believed to have had the ability to speak due to the size of the part of the brain known as Broca’s Area. By 300 000 to 100 000 years ago, the modern form of Homo Sapiens began to evolve. Their extremely large brains, their capacity for abstract thoughts and reasoning, highly sophisticated language and tool-using capabilities, and bodies very well conditioned to the condition of bipedal motion guaranteed that they would dominate the Hominid line and spread to all the corners of the world; and as their knowledge and culture began to evolve, they slowly developed the beginnings of the civilization we see around us today.
Sources:
The Evidence: Hominid Fossils
Britannica: Theories of Bipedalism
Human Balance, the Evolution of Bipedalism, and Disequilibrium Syndrome
The Socialization of Human Birth as Protection for Bipedalism
