The Story of the Human Body: Evolution, Health, and Disease

Before anything else that separates humans from other primates, there came bipedalism: walking on just two legs, with the trunk upright. That happened long before the big brain, the stone tool, or speech. The first bipedal ancestors already walked this way, while the rest of the body still looked quite ape-like.

Walking upright frees the hands, which can now carry babies, fruit, sticks. It also changes the energy bill: walking on two feet burns far fewer calories per mile than moving on all fours, which matters when food is scattered and far away.

The price came along with it: spine, knee, and hip had to reorganize to support an entire body stacked over a small base, the feet. That same design, built for walking long distances slowly, is what suffers today when daily life turns into decades of sitting in a chair and very little walking.

The australopithecines, who lived in Africa between about 4 and 2 million years ago, are a good portrait of this transition phase. They already walked upright, but still had long arms and curved hands, good for climbing trees when danger came. The brain remained about the size of a chimpanzee's.

This group's diet relied on fruit, leaves, roots, and seeds, chewed with huge teeth and jaws adapted to hard, fibrous food. With no knife and no fire, the body itself had to grind the food down before swallowing.

It's tempting to imagine the australopithecine as an unfinished human, but the book invites a different view: it was a successful species in its own way, adapted to a niche that combined ground and trees. Humanity as we know it today would only appear much later, with the genus Homo.

With Homo erectus, the body really changes: longer legs, narrower hips, a system for sweating across the whole skin that no other primate has to the same degree. This combination was tailor-made for running long distances in intense heat, when most other animals have to stop to cool off.

This is the basis of the endurance running hypothesis the author defends: instead of beating prey with short-distance speed, the human body won through accumulated fatigue, chasing the animal for hours in the midday heat until it collapsed from exhaustion, unable to cool itself. It's a hunt of endurance, not of sprinting.

Along with this came a taste for meat and animal fat, energy-dense food that fueled a body burning far more calories per day than an ape of the same size. Eating meat also opened room for another change: the gut could shrink, because digesting meat takes less work than digesting raw leaves, leaving energy for another expensive organ.