Human evolution: the mystery of testicles and the chin

The human body is a machine whose many components, from the microscopic structures in our cells to our limbs, eyes, liver and brain, were assembled in fits and starts over four billion years of history.

Scientists are still trying to understand why we evolved into this particular form. Why are humans alone in having a chin, for instance? And why is a human testicle, relative to body weight, three times the size of a gorilla’s but one-fifth the size of a chimpanzee’s?

As I explain in my new book, The Tree of Life, we are still looking for answers to many of these "why" questions. Yet we are beginning to find answers to some of them.

Tracing human evolution through the tree of life

The story of evolution shows how each species was built from simple beginnings, and when each component of a living creature was added to its blueprint.

Climbing the evolutionary tree of life takes us along a winding path through the increasingly specialised branches to which a species belongs. Humans, for instance, were animals before we became vertebrates, and mammals before we evolved into primates, and so on.

The species with which we share each branch reveal the order in which our body parts appeared. A body and a gut, inventions of the animal branch, must have preceded a backbone and limbs, which belong to the vertebrate branch. Milk and hair came with mammals, before fingernails appeared in primates.

There is a way to investigate the separate question of why each of these body parts evolved, but it works only when the feature has arisen more than once on separate branches of the tree of life.

This repeated evolution is known as convergence. It can frustrate biologists by confusing our understanding of how species are related. Swallows and swifts, for example, were once classified as sister species. DNA and comparisons of their skeletons now show that swallows are more closely related to owls than to swifts.

Testicle size matters in evolution

Convergent evolution becomes useful, however, when we treat it as a kind of natural experiment. Primate testicle size is a classic example. Adult male Abyssinian black-and-white colobus monkeys and bonnet macaques are roughly the same size.

Like chimps, humans and gorillas, though, these similarly sized monkeys have very different testicles. Colobus testicles weigh just 3 grams, whereas macaque testicles weigh a hefty 48 grams.

Several plausible explanations could account for this difference. Large testicles might be equivalent to a peacock’s tail: not useful in themselves, but appealing to females.

Perhaps the most convincing explanation, however, concerns mating. A male colobus competes fiercely for access to a harem of females that mate exclusively with him.

Macaques, by contrast, live in peaceful mixed troops of about 30 monkeys and take a different approach to love: everyone mates with everyone else. Males mate with multiple females, known as polygamy, and females with multiple males, known as polyandry.

A colobus with a harem can manage by producing the bare minimum of sperm: if a droplet is enough to produce a baby, why make more? For a male macaque, the reproductive contest takes place between his sperm and that of other males that mated before or afterwards.

A male macaque with large testicles should produce more sperm, increasing his chance of passing on his genes. This is a reasonable explanation for their different testicle sizes, but is it true? That is where convergent evolution can help.

Across the mammal branch of the tree of life, many groups have evolved testicles of widely varying sizes. In almost all these separate cases, larger testicles are consistently found in promiscuous species and smaller ones in monogamous species.

A small-testicled male silverback gorilla has sole access to a harem. Large-testicled chimps and bonobos are indeed highly promiscuous. Dolphins, meanwhile, may have the largest testicles of any mammal, accounting for as much as 4% of their body weight, equivalent to human testicles weighing roughly 3 kilograms.

Although the sex lives of wild dolphins are naturally difficult to study, spinner dolphins at least meet our expectations: they take part in mass mating events called wuzzles.

The multiple observations made possible by convergent evolution allowed us to identify this consistent correlation between testicle size and sex life across mammals. Human testicle size falls somewhere in the middle – make of that what you will!

Why did the human chin evolve?

The purpose of the human chin has provided fertile ground for scientific debate. As with testicles, there are half a dozen plausible ideas about how the human chin evolved. It may have developed to strengthen the jaw of a fighting caveman.

Perhaps it evolved to emphasise the magnificence of a manly beard. It could even be a by-product of the invention of cooking and the softer food that resulted: a functionless facial projection left behind as the jaw weakened and receded.

Intriguingly, no other mammal has a chin, not even our closest cousins, the Neanderthals.

Because the Homo sapiens chin is unique, we have a rich selection of possible explanations for its evolutionary purpose but, without convergent evolution, no sensible way to test them.

Some aspects of human nature may be destined to remain mysterious.

Max Telford, Jodrell Professor of Zoology and Comparative Anatomy, UCL

This article is republished from The Conversation under a Creative Commons licence. Read the original article.