Throughout the entire history of humans, the acquisition of bipedalism was vital to the success of early humans. Australopithecus afarensis, one of human’s earliest ancestors, was first discovered in various parts of Africa, specifically Ethiopia. When they were discovered, the fossils of A. afarensis, which date back 3.2 million years ago, were the oldest, and only fossils to demonstrate that early human ancestors walked on two legs (Gibbons, 2009). In recent years, other fossils have surfaced of an older ancestor, Ardipithecus ramidus, that are 4.4 million years old (Gibbons, 2009). While a picture is forming of when human ancestors began to walk on two legs, the questions of how and why this transition occurred still remain. Many factors could have played a role in this evolutionary feat. In many examples of drastic evolutionary change, the environment often plays an important role. In regards to human ancestors, how much, and in what ways, did the environment have an influence on the evolution of bipedalism in humans? In 1974, a team of experts were conducting a dig to survey a particularly fossil rich area in Ethiopia, when they came upon different bone fragments that were later classified as A. afarensis. Before this discovering, it was unclear as to how humans began to walk upright, let alone why. This discovery by paleoanthropologist Donald Johanson and his team led scientists to questions the very origins of humans (Kimble & Delezene, 2009). This discovery revealed to scientists a distinct shift between humans quadrupedal ancestors and their new bipedal forms. Australopithecus afarensis was the key to show that humans had a differentiated themselves from their quadrupedal ancestors. This new bipedal form was more efficient and distinctly different from previous human ancestors. While fossils can show a great deal about the early ancestors of humans, they cannot tell the whole story. The remains of A. afarensis can only show so much about this important transformation. One major influence on the bipedalism of early humans was the topography that surrounded them. Before the late-middle of the Miocene in East Africa, habitats were more closed off, and densely crowded with trees. In a crowded environment like this, it is easy to imagine how there was little need for long distance traveling. The trees that dominated this environment made climbing an important tool for any organism and wouldn’t necessarily promote bipedalism. Around this time, a shift towards open environments began (Jablonski & Chaplin, 1992). Due to this shift in the environment around them, early humans were faced with new ways to gather food, avoid predators, and interact amongst themselves. The loss of trees wasn’t the only change in the topography; the earth itself got rockier, and required human ancestors to climb more than before. The rocky structures that now dominated the land made for perfect stabilization structures; like a toddler learning to walk, the rocks allowed for them to not only improve their posture, this type of topography also offered an access to food resources and a way to hide from predators (Winder, et al. 2013). This change in the environment and topography allowed for effective, long term, habitual walking (Winder, et al. 2013). These and other environmental changes in Africa required the evolution of bipedalism (Jablonski & Chaplin, 1992). While the shift to a complex topography is one current hypothesis that explains why humans evolved the adaptation of bipedalism, another claims the change occurred to better navigate flexible branches often found in the shrinking forests which were common during the time period of this evolutionary change (Thorpe et al. 2007). Before we were land dwellers, our ancestors frequented the trees. The habitat of our early hominid ancestors was dominated, for a long time, by trees and a lack of open terrain (Thorpe et al. 2007). This hypothesis says that due to the lingering presence of trees, early hominids would evolve the trait of bipedalism from traversing tree branches on two legs in search of food, while at the same time using their free hands to steady themselves and reach for different objects around them (Thorpe et al. 2007). With this hypothesis, it is easier to understand how certain traits in early hominids were still present. For example, the forelimbs of our ancestors remained useful for long and grasping movements; this skill would be most useful in an environment characterized by trees (Thorpe et al. 2007). The search for food, while it may seem an obvious factor that drove the evolution of bipedalism, the way in which our early ancestors looked for food, and the different environments that surrounded them had an even more profound effect on their evolution. Recent studies have shown that hominids who lived close to habitats dominated by water may have been the first to evolve bipedalism (Kuliukas, 2002). The drive to find food, and the subsequent traversing of these wet environments, allowed for a for a type of movement called wading (Kuliukas, 2002). While apes don’t normally like to travel by water, they will venture into it when food is scarce (Kuliukas, 2002). This type of movement is well suited for creating the posture necessary to walk on two legs (Kuliukas, 2002). What is important about this hypothesis is not the drive for food itself, but how the environment around our early ancestors affected their evolution. By continuously walking through water, it is hypothesised that early hominids evolved the ability to stand upright with ease (Kuliukas, 2002). Like other hypothesis, the environment plays a large role in shaping evolution and ultimately the outcome of our very own species. All three of these hypothesis show how deeply the environment can affect evolution, however, there is still the question as to which hypothesis can best explain why early hominids began to walk upright. With each hypothesis, there is a particular element of the environment that is said to have influenced the evolution of bipedalism; these elements differ depending on the interpretations of the state of the environment at the time of the evolution. Many scientist have agreed that trees, while thinning during the time of this evolution, were still a prominent factor in the environment (Elton, 2008). With this in mind, the hypothesis involving the use of tree branches to begin walking on two legs would be a perfect explanation as to why humans walk upright. Using branches as stepping stools, the hands of early hominids were free to grasp for food and other important items (Thorpe et al. 2007). However, if there was a thinning of forests during this time, it is also plausible to see how an open environment would foster the ability to walk upright (Winder, et al. 2013). If hominids had to traverse long ways to search for food, or even through water as another hypothesis states, it would also provide an explanation as to why bipedalism evolved. Unfortunately, without proper fossil evidence and without knowing for certain the state of the environment during this time, it is difficult to truly explain why humans began walking upright. Ultimately, the one common denominator of this evolutionary change is the environment. Without a shifting environment, our ancestors may have never began to walk upright. In order to survive, early hominids did their best to evolve to the environment around them. In the future, scientists will be tasked with finding further evidence that would prove the type of environment that early hominids evolved in. In order to properly pinpoint when and why hominids began to walk upright, more fossils will need to be discovered. With this information, the mystery of one of human’s greatest acquisitions, bipedalism, could finally be solved.