The human body is not really made for swimming. It is made for walking and running. Our upright gait on two legs sets us apart from other mammals—the ones our ancestors were chasing on foot. Our unusual anatomy actually makes swimming harder for us than it is for animals that go about on four legs.

The reason humans don’t instinctually know how to swim is because of the way their body is designed. Because we stand erect, it’s harder to float naturally on top of the water. Standing upright creates drag instead of buoyancy, so some type of motion is needed to keep your head above water.

Accordingly, a number of features of modern human physiology have been proposed as means by which humans, but not our closest relatives, are able to swim. These include: an infant swimming reflex, a diving reflex, voluntary breath-holding, and buoyancy provided by increased adipose tissue.

The tree-dwelling ancestors of apes had less opportunity to move on the ground. They thus developed alternative strategies to cross small rivers, wading in an upright position or using natural bridges. They lost the instinct to swim. Humans, who are closely related to the apes, also do not swim instinctively.