The One Human Body Part That Science Still Can’t Explain

The human body is eerily similar to a complex machine, with countless delicate parts. These parts range from the tiniest of cells to our limbs, eyes, kidneys, and brains. These parts have slowly assembled themselves over approximately four billion years of evolution.

However, scientists are still trying to understand why humans evolved into their specific form. Why, for example, do we uniquely have a chin? And why are human testicles, relative to body weight, three times the size of a gorilla’s but only a fifth the size of a chimpanzee’s? As shown in the new book, The Tree of Life, many of these “why” questions remain unanswered. However, we are starting to uncover potential and credible explanations for some of these questions.

Evolution shows us how, beginning from simple origins, each species gradually took shape as new components were added to its blueprint. By tracing the tree of life, we can follow the winding path through increasingly specialised branches that each species occupies. Humans, for example, were once simple animals before becoming vertebrates, then mammals, and eventually primates.

The other species that occupy each branch show us the sequence in which our body parts evolved.

A body and a gut, innovations of the animal branch, appeared prior to the backbone and limbs of vertebrates, just as milk and hair in mammals came before the development of fingernails in primates.

We can study why certain body parts have evolved, but this only works when a feature has appeared independently in multiple branches of the tree of life—a phenomenon known as convergence. Convergence can frustrate biologists because it obscures true evolutionary relationships. For example, swallows and swifts were once thought to be closely related, but after examining DNA structures and making skeletal comparisons we now know that swallows are actually closer to owls than to swifts.

When it comes to evolution, size seems to matter.

Convergent evolution becomes particularly useful when it’s treated as a natural experiment. A classic example is the size of primate testicles. Adult males of the Abyssinian black-and-white colobus monkey and the bonnet macaque are roughly the same body size, yet their testicles differ dramatically. Colobus males have testicles weighing just 3 grams, while macaque testicles are an astounding 48 grams, similar to the variation seen among chimps, humans, and gorillas.

Several reasonable explanations could account for the difference in testicle size. Large testicles might function like a peacock’s tail, not directly useful but attractive to females. Yet the most convincing explanation still likely relates to mating behaviour. Male colobus monkeys fiercely compete for control of a harem, where females mate exclusively with him. Differently, macaques live in peaceful mixed troops of around 30 monkeys, where mating is much more promiscuous: males mate with multiple females (polygyny) and females with multiple males (polyandry).

A colobus male with his harem can get by producing only a minimal amount of sperm — if a single droplet is enough to fertilise an egg, what is the reason to produce more? For a male macaque, however, reproduction is a contest between his sperm and that of other males who have mated with the same females. Larger testicles allow him to produce more sperm, increasing his chances of passing on his genes. This explanation offers a reason for their differing testicle sizes, but how exactly can we be sure? This is where convergent evolution provides valuable insight.

Numerous examples of testicles evolving to different sizes can be seen across the mammal branch of the tree of life. Almost without exception, species with promiscuous mating systems have larger testicles, while monogamous species have smaller ones.

Male silverback gorillas, with their small testicles, maintain exclusive access to a harem, while chimpanzees and bonobos, with much larger testicles, are highly promiscuous. Dolphins take this to the extreme, with testicles that can account for up to 4 percent of their body weight — equivalent to roughly three kilos in humans. Though observing wild dolphin mating is difficult, spinner dolphins seem to match expectations, engaging in large group mating events called wuzzles.

Thanks to the repeated examples offered by convergent evolution, scientists uncovered a consistent link between testicle size and mating behaviour across mammals. As for humans, our testicle size is average, and what you make of that is up to you!

But then there’s the human chin…

The human chin has long ignited debate and conversation among scientists trying to pin down what exactly its purpose is. Like testicles, there are several plausible explanations for its evolution. Perhaps it developed to strengthen the jaw of a fighting caveman or to enhance the appearance of a manly beard. It might even be a by-product of cooking and the softer foods that came with it, a functionless facial protrusion left behind as our jaws gradually weakened.

Intriguingly, the chin is found in no other mammal, not even our closest relatives, the Neanderthals. Because the human chin is so unique, we have a range of possible explanations for its evolution, but without examples of convergent evolution, there’s no practical way to test which, if any, are correct.

This serves as a reminder that there are still parts of human nature that could be forever beyond our grasp.