Biologists and anthropologists have wrestled with this question for centuries and have postulated a diverse series of explanations for why we became bipedal. Nearly all of the speculation has fallen far short, however, of a coherent, powerful understanding, with little explanatory or evolutionary relevance, of this unique adaptation.
Most explanations target a single cause such as standing up to look over tall grass on the savannah of Africa. This is, in fact, logically one of the least rewarding of all explanations. Our earliest bipedal ancestors were small in stature. For example, Lucy was only three-and-a-half feet tall as an adult and weighed roughly 60 pounds. Imagine this puny little hominid venturing out of the forest, where she and her ancestors had lived for millions of years, onto the new, challenging, and highly dangerous world of the grasslands. A major shift in habitat is not a good time to experiment with a totally new mode of locomotion; leaving the speed and agility of quadrupedalism behind for upright walking would have been a terrible choice.
Standing up in the grasslands to get an enhanced view would have immediately announced to carnivores galore that a new entrée had appeared on the menu! Hyenas, saber-toothed cats, lions, large, wild dogs, and other fierce creatures populated the savannah. Each specialized carnivore would have been capable of devouring a tiny, bipedal hominid. Being slow, clumsy runners, the early hominids would have quickly fallen prey. Likewise, lacking the ability to fall back onto all fours and outrun the carnivores would have signaled the end of human evolution.
Standing up to reach fruit in low-hanging branches is also a lame explanation. Quadrupeds like chimpanzees can stand up on two legs and then fall to all fours to escape danger with great speed. Why become bipedal if it is possible to stand and retain the benefits of quadrupedalism?
Upright posture assisted our ancestors in keeping their bodies cool, and some have offered this as an explanation for bipedalism. Sure, it's hot under the African sun, but why not simply rest in the shade of a nice acacia tree during the hottest part of the day like baboons do, restricting foraging to the cooler times in the morning and afternoon?
Next, the aquatic "theory" is perhaps one of the all-time worst explanations. No primates collect food in the water, while most avoid being in water at all costs. If this were a good explanation, then we should have lost our limbs like the whales, which evolved from quadrupeds. Furthermore, African lakes and rivers are dangerous places occupied by hippopotami, which kill more humans each year in Africa than any other animal, as well as snakes, crocodiles, giant otters, and other dangerous creatures.
If we stood up to make tools, then why is there a disconnect between the earliest evidence for bipedalism and the first stone tools? Evidence for bipedalism extends as far back as 4.2 million years ago, perhaps even six million years ago, but stone tools do not appear in the archeological record until 2.6 million years ago—so we can rule out tool-making as an explanation.
The simple reason that these explanations fail is that most scholars have asked the wrong question: Why did our ancestors become upright? The correct question is: What were the advantages for early hominids that resulted in such a behavioral change from quadrupedalism to bipedalism?
The real advantage
Most animals expend the greatest amount of energy in reproduction, feeding, and ensuring safety. Owen Lovejoy's explanation points out that if males contributed to the survival of their mates and their offspring, then their own genes would survive into the next generation. Bipedalism would have allowed males to provide their mates and offspring with high-quality food as well as to help protect them from any looming dangers. This explanation makes biological and behavioral sense. Natural selection cannot create a behavior like bipedalism, but it can act to select the behavior once it has arisen. Surely some early hominids were better at upright walking than others, a behavior that permitted them to forage widely and bring food back to a home base.
I believe that our ancestors, pre-Lucy, became upright in the protection of the forests, a familiar environment in which they faced fewer dangers. Then, armed with a new type of locomotion, bipedalism, they were pre-adapted to move onto the savannahs and expand their territory. Once we moved into more open environments, we brought a whole package of advantages with us. Our hands were free to make and use tools, we could walk long distances to collect and carry food (and in doing so benefit from enhanced thermoregulation to prevent overheating), we could look over tall grass if we needed to, and so on.
Bipedalism was a behavioral innovation that led the way to making everything possible for our evolution, even if it is still not perfected. Humans continue to suffer from fallen arches, hernias, severe lower back pain, and other bad side effects of bipedalism. But it was still the single step in our ancient past that led to the tool-making, brain enlargement, and intelligence that have led to our preeminence on the planet today.