Bipedalism
Humans are unique from other animals in many ways: our big brains, our language abilities, our cultural adaptations, our social structures, etc. All of these unique traits evolved in the last 7 million years, but the very first of these traits to evolve, the trait that we use to distinguish members of the human lineage (hominins) from other apes, is bipedalism. Bipedalism means walking on two feet. (In Latin, bi=two and ped=foot, which is why bicycles have two wheels, and we put our feet on the pedals). Humans are bipeds, animals whose normal standing position involves only two feet.
Most mammals are quadrupeds (four-footed), but humans are not the only bipedal mammals. Kangaroos are bipeds, as are a number of hopping rodents, like jerboas. Lots of mammals will occasionally stand or walk on their back legs. For example, meerkats and prairie dogs will stand to get a better view, and bears can stand on their back legs while using their front paws to catch fish. In general, monkeys and apes have a more upright posture than other mammals, frequently sitting with their hands free and off the ground. Many can stand briefly in this upright posture -- in order to reach food, for example -- or even walk short distances bipedally in order to carry food from one location to another.
Unlike all other primates, humans are obligate bipeds. We are unable to effectively and efficiently walk or run on all four of our limbs. As children, we can crawl, but, for most of us, as we get older that method of locomotion becomes harder and harder. Our earliest ancestors were facultative bipeds, meaning they were able to walk bipedally and probably did so most of the time, but were also able to move quadrupedally when that was a better choice. It appears that our last common ancestor (LSA) with chimpanzees was a facultative biped. From that last common ancestor, chimpanzees evolved to become quadrupedal knuckle-walkers, while hominins evolved to become obligate bipeds.
We know a lot about locomotion in our ancestors because the human skeleton changed to allow bipedalism. We can look at when those changes appear in the fossil records.
Skeletal Changes with Bipedalism
The human skeleton is different from other apes in a number of ways that make us better bipeds. Here is a list of the main changes, from top to bottom:
our foramen magnum is underneath the skull, allowing the spinal cord (which enters the skull here) to be at a nearly right-angle to the skull. In other apes, the foramen magnum is near the back of the skull and the spinal cord angles backward.
our spine is an S-curve, rather than a C-curve like an ape's. This helps balance our weight over our feet.
our lumbar spine is flexible and curved, not fused and straight. This frees up our lower back, allowing us to rotate and sway our hips
in general, our pelvis in narrower than that of a chimpanzee, to make our walk more efficient
our ilium is short and flared, not long and thin, which moves our muscle attachments forward so our muscles are better positioned to pull our leg forward
our ischium is shorter than an ape's. The ape's long ischium gives them climbing leverage, as if they are permanently crouching, and gives them a larger attachment area for the hamstring muscle. Human hamstring muscles are much smaller than in apes.
we have huge butt muscles (gluteus maximus), which are very small in other primates. This allows us to extend our legs fully.
compared to apes, our legs are much longer relative to our body, giving us a longer stride
our knees are angled inward relative to the position of our hips, unlike an ape's which are a straight line from hip to ankle. This allows us to center our weight over our foot for better balance while walking.
we have a strong, robust astragalus in our ankle, to take the full weight of our body
we have a strong, non-opposable big toe, unlike a chimp's big toe which looks a lot like a thumb. We don't use our feet for grasping/climbing but rather to take our weight as we walk.
our feet have arches, rather than hand-like palms as in an ape. This helps us to distribute the weight as we walk, making our movement more efficient.
In addition to the changes above, which help us to walk more efficiently, our skeletons also changed when we stopped climbing trees. These changes didn't occur as early in our evolution. In other words, first we developed the ability (and eventually the necessity) to be bipeds, and then we lost the ability to climb trees as well as our ape cousins. Some of the changes to our skeleton that marked the end of our climbing ability include:
compared to apes, our arms are much shorter relative to our body-size. Chimpanzees have longer arms than legs. Our legs are longer than our arms.
our fingers are shorter than in most apes. Longer fingers give apes more power for climbing, but our short fingers give us more control over our fine finger movements.
our fingertips are wider than those of an ape. This, too, gives us more fine-motor control as well as more sensitivity to sensations in our fingertips.
our finger bones are straight rather than curved. An ape's curved fingers give them more power for climbing.
Why Bipedalism?
Bipedalism first evolved around 7 million years ago, which we know from the skeletons of the earliest hominins (see the reading on Basal Hominins). It has stuck around since. Obviously, bipedalism must have increased the reproductive fitness of those early hominins or the trait would not have survived. And yet, bipedalism has a number of down sides. Because evolution works with the material it has (in this case, starting with a quadrupedal ape), we don't have a perfectly designed skeleton or body for bipedalism. Instead, we have a quadruped's skeleton that has been jury-rigged to fit our bipedal adaptation. This has led to a number of problems, including:
We are less stable than quadrupeds, more prone to slip, trip, or be pushed down
We can't run as fast as chimpanzees or other quadrupedal apes
The changes to our skeleton make us prone to back pain, such as slipped discs and lumbar problems. These don't occur in apes because their spines are curved differently, and they don't put as much weight on their spine.
The changes to our skeleton make us prone to foot pain, such as fallen arches, bunions, and plantar fascitis. These don't occur in apes because they don't put all of their weight on their back feet all the time.
Our skeletons are not well-structured for pregnancy. Chimpanzee babies hang toward the ground below the mother's spine, and her whole skeleton supports that weight. Human babies sit in the bowl of their mothers' pelvis. This means the weight of the baby puts pressure on a mother's back and legs. The baby's weight can cut off blood circulation to the legs in certain positions and can press against the sciatic nerves, causing pain and difficulty walking. The pelvis begins to "unknit" late in pregnancy, allowing more flexibility for the baby to push through the birth canal, but the separation between the halves of the pelvis makes it harder for pregnant women to walk.
The combination of our big brains and our narrow pelvis make it harder for infants to fit through the birth canal. Human mothers have higher death rates than other animals because of this problem. It also takes longer for women to recover from childbirth.
Normally, if a trait negatively affects pregnancy and childbirth, we would expect it to be weeded out by natural selection. After all, anything that causes reproduction to be more difficult should lower reproductive fitness by definition! But, despite the problems with pregnancy and childbirth, bipedalism remained. It must have given our ancestors some very important advantages to overcome the disadvantages.
The most likely advantage that bipedalism gave us was efficiency. We don't climb as well as a chimpanzee, and we can't run as fast as a chimpanzee, but we can walk and run for far, far longer because our stride is much more efficient. Humans can walk, or even run, for hours without stopping. We have the ability to cover far longer distances than any of our ape cousins. This was the new adaptation our ancestors developed with two-footed walking: the ability to hit the open road.