Evolutionary Anatomy – Walking Upright and Childbirth

A message from UCLA professor, Dr. Tony Friscia.
A message from UCLA professor, Dr. Tony Friscia.

In the last installment of our evolutionary anatomy series, we talked about vestigial structures, those hold-overs from our evolutionary ancestry. This time we’re going to talk about some specific adaptations of the human body, and how often seemingly unrelated aspects of our biology are linked.

The example I will use are those features of our skeleton that are adaptations to walking upright. Our ape-like ancestors mainly used all four limbs to support their body weight while walking, probably similar to the way modern chimpanzees move. The transition to upright walking in the hominid lineage was accompanied by many anatomical changes that made our unique form of locomotion more efficient:

A figure showing the curvatures of the vertebral column (from eOrthopod).
A figure showing the curvatures of the vertebral column (from eOrthopod).

The curvatures in our lower backs and necks keep the majority of our weight over our hips and legs. Gorilla and chimps have a spinal curvature that is concave forward along the entire length – like a c-shape (also called a kyphosis). This is fine for them, because all the limbs are supporting their weight. We have the same shape to our vertebral column when we are born, but as we learn to walk, we develop the reverse curvatures (lordoses) in the lumbar (lower back) and cervical (neck) regions. So, although we say we walk with a straight back, it actually has 4 curvatures in it (2 in each direction) which give us the appearance of a straight back.

A figure showing the shape of the pelvis in a chimpanzee (left) and a human (right). In the center is the pelvis of one of our earliest upright-walking ancestors (from TalkOrigins).
A figure showing the shape of the pelvis in a chimpanzee (left) and a human (right). In the center is the pelvis of one of our earliest upright-walking ancestors (from TalkOrigins).

Our pelves are short, along with the shortened lumbar region of our vertebral column (with it’s lordosis). This is another feature that enhances stability. In apes, the pelvis and lumbar regions are very long, and the ilium (the ‘hip bone’ you put your hands on when they are on your hips) extends far up the vertebral column, almost to the rib cage. The shortened pelvis of human acts like a bowl for the abdominal organs, and the ilia are flared laterally, giving better mechanical advantage to the muscles that stop us from falling over when we walk (the main one being the gluteus medius).

A figure showing the Q-angle (from Physiopedia).
A figure showing the Q-angle (from Physiopedia).

Our femurs slant inward from the hips to the knees making all humans a little knock-kneed (the angle they form at the knee is called the Q-angle). This keeps the knees under the weight of the torso, which prevents the body from swaying side-to-side while walking. This slant is so distinctive that some hominid fossils preserve only the distal end of the femur and from this we can tell that they walked upright.

There are a number of other adaptations to walking upright – the orientation of the foramen magnum (down, not back), the loss of the opposable big toe, the lack of curvature in the fingers and toes, etc. But there was a cost to these adaptations to walking upright – many of these features often had direct implications for childbirth.

You don’t need to tell any mother (and most fathers) than human childbirth is painful. For most other mammals this is not the case. Think of a mother gazelle on the plains of Africa – it stands for the whole birthing process, and barely seems to notice that it dropped a newborn. The reason for this difference is that rearrangment of the pelvis. The pelvic outlet (the bony ring of the pelvis that limits the size of the birth canal) has been narrowed in humans. It is now just about as wide as a newborn’s head. This makes for a painful childbirth process. In other mammals, the pelvic outlet is much larger relative to the newborn, making their childbirth far less traumatic.

A figure showing the relative size of the pelvic outlet and a newborn’s head, chimpanzee on the left, human on right, and early upright hominid in the center (from Evolution and the Prehistory of Man)

This has important implications for child rearing as well. Humans are born altricial – poorly developed. A newborn human is relatively helpless. It can’t move well on its own, can’t obtain food, and can’t communicate well. (Some might argue that this helplessness continues well into their teenage years…) A big reason for this is that humans can’t be born too developed, especially with a much larger brain, because of that limitation of the birth canal size. In contrast, think back to that baby gazelle. Soon after it’s born it can get around on it’s own, and even find it’s own food (although mother’s milk provides the main part of nutrition for a while). This is called precocial development.

A question you should ask yourself after hearing about these trade-offs is why our hominid ancestors took to walking upright in the first place. There are actually a number of theories about this, ranging from freeing the hands to carry objects from place to place, to being able to see over talk savanna grass. The reality is that there were probably numerous reasons why this transition happened, so no one theory can offer an explanation.

Next time we discuss one of the most bizarre quirks of human anatomy that can only be explained through evolution.

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