Human testicles are much smaller, in proportion, to some of our primate cousins. Evolution can tell us why. But the size of other body parts is a little bit more of a mystery.

The human body is a machine whose many parts – from the microscopic details of our cells to our limbs, eyes, liver and brain – have been assembled in fits and starts over the four billion years of our history.

But scientists are still puzzling over why we evolved into this particular form. Why do humans uniquely have a chin, for example? And why, relative to body weight, is a human testicle triple the size of a gorilla's but a fifth that of a chimpanzee? As I show in my new book, The Tree of Life, we are still searching for the answers to many of these "why" questions. But we are starting to find answers to some of them.

The story of evolution tells us how, starting from simple beginnings, each species was built – when each of the components that make a living creature was added to its blueprint. If we climb the evolutionary tree of life, we can follow a twisting path that visits the increasingly specialised branches that a species belongs to.

We humans, for example, were animals before we became vertebrates; mammals before evolving into primates and so on.

The groups of species we share each of these branches with reveal the order our body parts appeared in. A body and a gut (inventions of the animal branch) must have come before backbone and limbs (vertebrate branch); milk and hair (mammals) came before fingernails (primates).

There is a way we can study the separate problem of just why we evolved each of these body parts, but it only works if the feature in question has evolved more than once on separate branches of the tree of life. This repeated evolution is called convergence. It can be a source of frustration for biologists because it confuses us as to how species are related.

Swallows and swifts, for example, were once classified as sister species. We now know from both DNA and comparisons of their skeletons that swallows are really closer relatives of owls than swifts.

But convergent evolution becomes something useful when we think of it as a kind of natural experiment. The size of primate testicles gives us a classic example. Abyssinian black and white colobus monkey and bonnet macaque adult males are roughly the same size. But, like chimps, humans and gorillas, these similar monkeys have

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