Human testicles are much smaller, proportionally speaking, than our primate relatives. Evolution can explain why.
But the size of other body parts remains a bit of a mystery.
The human body is a machine whose many parts – from the microscopic details of cells to limbs, eyes, liver and brain – have been assembled in spurts and pauses over the four billion years of our history.
But scientists still don't know why we evolved in this particular form.
Why are humans the only ones with protruding chins, for example?
And why, relative to body weight, is a human testicle three times larger than a gorilla's, but only a fifth the size of a chimpanzee's?
As I state in my new book, Tree of Life, we are still looking for answers to many of these 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 built – when each of the components that make up a living being was added to its blueprint.
If we climb up the evolutionary tree of life, we can follow a winding path that visits increasingly specialized branches to which species belong.
We humans, for example, we were animals before we became vertebrates, mammals before we evolved into primates, and so on.
The groups of species with which we share each of these branches reveal the order in which our body parts appeared.
The body and intestines (discoveries of the animal branch) had to come before the spine and limbs (vertebrate branch), milk and fur (mammals) came before nails (primates).
There is a way we can study the separate question of why we evolved each of these body parts, but that only works if the function in question evolved more than once on separate branches of the tree of life.
This repeated evolution it's called convergence.
It can be a source of frustration for biologists because it confuses us in what ways species are related.
Swallows and terns, for example, were once classified as sister species.
We now know, thanks to DNA analysis and comparisons of their skeletons, that swallows are actually closer relatives. Sova but a hoe.
Size matters when it comes to evolution
But convergent evolution becomes useful when we experience it as a kind of natural experiment.
The size of the testicles in primates provides a classic example.
Male Abyssinian black and white colobus monkeys and Indian macaques are about the same size.
But like chimpanzees, humans, and gorillas, these very similar apes have extremely different testicles.
The testicles of a colobus monkey weigh only three grams, while the testicles of a macaque monkey weigh an incredible 48 grams.
Several plausible explanations can be devised for the different testicular sizes.
Large testicles can be like a peacock's tail, they are not useful in themselves, but they are attractive to females.
But maybe the most plausible explanation It has to do with the way they mate.
Male colobus competes fiercely for access to a harem of females who will mate exclusively with him.
Macaque monkeys, however, live in peaceful mixed groups of about 30 individuals and have different approach to love, where everyone mates with everyone else: males with multiple females (polygamy) and females with multiple males (polyandry).
A colobus with his harem can get away with producing only the bare minimum of sperm – if a drop is enough to produce a baby, then why make more?
For many male macaques, reproductive competition occurs during a battle between his sperm and the sperm of other males who have mated before or after him.
A male macaque with large testicles should produce more sperm, which gives him a better chance of passing on his own genes.
That's a reasonable explanation for their different testicle sizes, but is it true?
This is where convergent evolution helps.
If we look at the entire mammalian branch of the tree of life, we will see that there are many groups of mammals that have developed testicles of all different sizes.
In almost all of these isolated cases, the larger testicles consistently find in promiscuous species, and smaller in monogamous ones.
Male silverback gorilla The one with small testicles is the only one who has access to the harem.
Chimpanzees with large testicles and bonobos are really extremely promiscuous.
Dolphins have maybe the largest testicles among mammals, they make up as much as four percent of their body weight (equivalent to human testicles weighing approximately three kilograms).
Although it is difficult to study the sex lives of dolphins in the wild, at least spinner dolphins fit our expectations, engaging in mass mating rituals called "vuzle".
Thanks to numerous observations provided by convergent evolution, we were able to discover this consistent association between testicular size and sexual life in all mammals.
And as for humans, our testicle size is somewhere in the middle – and you can draw your own conclusions from that.
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But what about the human beard?
The human beard was fertile soil for discussions between scientists about its purpose.
As with testicles, there are five or six plausible ideas that explain the evolution of the human beard.
It could have evolved to strengthen the jaws of warring cavemen.
Perhaps the beard evolved to exaggerate the magnificence of a man's beard as a fashion accessory.
It could even be by-product the discovery of cooking and the softer foods it produced – a functionless facial feature left behind stagnant tides jaw weakening.
To make things even more intriguing, however, the beard cannot be found. not in any other mammal, not even in our closest relatives the Neanderthals.
Thanks to the uniqueness of the beard homo sapiens, although we have a rich array of possible explanations for its evolutionary purpose, in the absence of convergent evolution, we have no convincing way to test them.
Perhaps some parts of human nature are destined to remain a mystery.
*Max Telford is Professor of Zoology and Comparative Anatomy at University College London.
Watch the video: How Darwin came up with the theory of evolution over 160 years ago
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- Charles Darwin's questions about the "heinous mystery" that tormented him for the rest of his life
- Chimpanzees and humans - we're really alike
- Modern man and Neanderthal lived together longer than we think
Bonus video:
