Why did humans become bipedal?

Question

Somewhere in evolutionary history homo started walking upright and became bipedal. You hear these hypotheses that, by walking upright, they could see better across the grassy savannas to escape predators, find food, find fellow humans etc. However, the most parsimonious way to accomplish looking farther across the grassy plains is by growing taller - much less adaptations necessary and hence more favorable from an evolutionary perspective. Other hypotheses say homo freed his hands to carry things. Quite fine, but some apes are known to carry around stuff. Moreover, walking on all fours increases running speed (you don't have to see far if you can escape faster) and apes climb trees better (what's a better place to hide from predators?). Hence my question: Why has bipedalism been favored in evolution?

Answer 1

As already pointed it out, there is no scientific consensus on the answer to this question. This is because it is hard to test proposed explanations and it's likely that several factors interacted.

All of these have been claimed to be the explanation for the evolution of bipedalism:

1. Freed hands for increased tool use. (Encyclopedia Britannica)

2. Freed hands for the use of weapons in hunting or the use of containers to transport plant foods obtained by gathering. There are three variants: the “man the hunter” version which sees male hunting as the key stimulus (Washburn 1967 in Haraway 2013); the “woman the gatherer” version which sees women needing their hands free to carry gathered vegetable foods as the stimulus; and the “man the provisioner” version, which stresses males carrying food (either meat or plants) to provision females and offspring. All of these explanations are probably flawed because they invoke present utility to explain the origins of bipedalism.

3. Bipedalism was created by female sexual selection of males with large, visible genitals. Although unlikely, this may be correct, but it is impossible to test. (Encyclopedia Britannica)

4. Bipedalism evolved to aid in avoiding predators. Higher head allows you to see longer distances over the grass. (Encyclopedia Britannica)

5. Bipedalism evolves because it allows energy efficient locomotion between forest patches in an increasingly open environment.

6. “Radiator theory”: Natural selection favored bipedalism in open environments because it decreases exposure to the sun and helps keep the brain cool. (Wheeler, discussed in Haviland 2007 and Encyclopedia Britannica) Brain is highly sensitive to heat stress. This allowed hominins to be active in mid-day, when competing predators are less active. (Essentials of Physical Anthropology, p. 165). Since the same carnivorous animals with which hominins were in competition (both would scavenge dead game) could attack hominins, hominins were probably safer from predator attacks during midday as well.

7. Terrestrial bipedalism may be a continuation of an upright body posture inherent in a tree-climbing/brachiating adaptation. Gibbons, the most arboreal of the apes and the best brachiators, walk bipedally on the rare occasions they come to the ground. (Encyclopedia Britannica)

The earliest hominin fossils appear to have been bipedal, with no clear evidence of a transition through a knuckle-walking stage, the principal mode of locomotion of modern African ape. (Kivell and Schmitt 2009, here's a secondary source, and here's a primary source that disagrees, Richmond and Strait 2000)

If this is true, then the modern African ape mode of locomotion (knuckle-walking) is a derived trait, and vertical body posture may be the ancestral trait in the African hominoid evolutionary lineages. In other words, if this is true, the ancestor of all great apes including humans was already bipedal, and modern African apes have evolved to knuckle-walk since then. Also if this is true, the previously mentioned factors would reinforce and refine bipedal locomotion in the hominin lineage.

It is likely that explanations 4, 5, 6 and 7 all interacted to select for bipedalism. They correspond best to the paleontological and ecological data. Numbers 1 and 2 are unlikely, because bipedalism appears long before evidence of tool use.

Answer 2

The thunder chimp's answer summarized the answer very well. To try to pull things together a bit, if you have seven minutes, check out this video. It shows a man chasing down a kudu. They both run for 8 hours, until the kudu literally collapses and the man calmly walks up to it and slays it. The video shows the evolutionary adaptions that the man has - he sweats all over his body, cooling himself. The kudu needs shade to cool, and sweats only a little. The man can carry water, to replenish himself. The animal cannot.

Perhaps more importantly, the man walks upright on two feet, thus shifting his center of gravity in a energetically favorable way. The animal is restricted to galloping, which is faster for a sprint but not energetically favorable for long distance running.

Here are some articles for further reading:

Evolution of Marathon Running

The Energetic Paradox of Human Running and Hominid Evolution

Endurance running and the evolution of Homo

They say similar things as the video: that we evolved for persistence hunting in an arid environment. This is an activity that the other primates don't engage in. Our ability to hunt effectively allow us to eat more meat, and thus favored efficient long distance running.

Answer 3

I doubt you are going to be satisfied with any answer since the authorities on the matter can't test the many hypothesis and don't all agree. Around the time of homo erectus (1.89 million years ago), hominids began walking upright on a more permanent basis but this wasn't the only evolutionary event. Additionally, hominids began growing taller now as well [1].

Charles Darwin hypothesized that upright walking was linked to tool use in 1871 [2]. In 2009, Lovejoy reformalized Darwin's hypothesis and came to the conclusion that upright walking comes down to food and sex [3].

To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. But earlier this year, researchers offered some support when they found that chimpanzees tend to walk bipedally when carrying rare or valuable foods.

At this point, Lovejoy suggests, a mutually beneficial arrangement evolved: Males gathered food for females and their young and in return females mate exclusively with their providers. To be successful providers, males needed their arms and hands free to carry food, and thus bipedalism evolved. This scenario, as with all bipedalism hypotheses, is really hard to test. [1]

In 1980, Peter Rodman and Henry McHenry claimed that bipedalism carries bioenergetic advantages [4]. Then in 2007, another group of researchers validated the energy advantage of walking upright [5].

Raichlen notes that if the same variation existed in our early ape-like ancestors as does in today's chimps, with some finding it easier to walk than others, this could have helped to drive adaptation. "That's what natural selection would be able to work on," he says [5].

Since there is no agreement and testing many of theses hypothesis are impossible since early hominids no longer exist, no one in their right mind can say, "Chris, this is why we evolved to walk." Even if they were correct, it would take some time to cycle through the academic community before it gained enough support to be the consensus.