Introduction to Bipedalism: Bipedalism Geological Age & Climate

Around 7 or 8 million years ago, the earth's climate underwent a dramatic cooling event which lowered land and ocean temperatures. As a result, changes occurring in the growth of the Antarctic ice cap created successive drops in the sea level of the Mediterranean Sea². These sea level drops acted to limit the availability of water sources within nearby continents like Africa, thus producing a situation wherein the extensive, moisture-dependent forests of these continents were reduced as their water sources dried up. This shift toward less dense forests and more woodland may have been a driving force for the evolution of bipedalism in hominins^4-9 .

Recent studies have determined that the early ancestors of humans probably lived in some sort of wooded habitat, perhaps a woodland savanna^4-9. 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 climbing^8-12.

It may prove to be the case that the origin of bipedalism is the first major morphological change that signals the appearance of hominins. Early researchers originally hypothesized that brain enlargement was the first change that occurred in the hominin lineage, but this hypothesis has been falsified following the discovery that early hominins retained ape-sized brains, but had bipedally-adapted morphologies.