Around 7 or 8 million years ago, the earth's climate underwent a dramatic cooling event which lowered land and ocean temperatures. Growth in the Antarctic ice cap during this time resulted in a dramatic drop in sea levels, including the Mediterranean Sea2. As a result of these sea level changes in the Mediterranean, water sources availability within nearby continents like Africa were severely limited. Thus, the extensive, moisture-dependent forests of these continents were reduced as their water sources dried up. This shift toward less dense forests and the subsequent growth in wodland envirionments may have been a driving force for bipedal evolution in hominins4-9.
Recent studies have determined that the early ancestors of humans probably lived in some sort of wooded habitat, perhaps a woodland savanna4-9. Climbing trees in search of food or to escape predators would have been a common behavior for organisms living a wooded or forest environment, and it is possible that early bipedal ancestors retained features (i.e., long arms, and curved fingers and toes) that were adapted to arboreal locomotion. In fact, some of the early hominin fossils do exhibit morphological adaptations conducive to tree climbing8-12.
If bipedalism is one of the defining characteristics for hominins, then bipedal characeristics may be used to pinpoint the first appearance of hominins. To put it another way, although the DNA evidence suggest that apes and human shared a common ancestor sometime between 7 and 8 Ma, characteristics of this shared ancestor remain somewhat debated. The identification of early bipedal adaptations within the fossil record may help to identify this shared ancestor, or perhaps help to determine what characters would be expected in this ancestor. Therefore, understanding the evolution of bipedalism remains an important study in the story of human origins.
