by The human body is a machine, the many parts of which – from the microscopic details of our cells to limbs, eyes, liver and brain – were assembled in the four billion years of our history.
But scientists still puzzle why we have developed into this special form. For example, why do people have a chin? And why is a human testicle in relation to body weight the size of a gorilla, but a fifth of the chimpanzees? As I show in my new book The Tree of Life, we are still looking for the answers to many of these “why” questions. But we start finding answers to some of them.
The history of evolution tells us how every kind of simple beginnings was built when each of the components that make a living being added to their blueprint. When we climb the evolutionary tree of life, we can follow a rotation path that visits the increasingly specialized branches, which includes one species. For example, we humans were animals before we became vertebrates; Mammals before they develop into primates and so on.
The groups of species that we share each of these branches reveal the order that appeared our parts of the body.
A body and an intestine (inventions of the animal branch) must have come before the backbone and limbs (vertebrate branch); Milk and hair (mammals) came in front of fingernails (primates).
There is a way to examine the separate problem why we have developed each of these parts of the body, but it only works if the characteristic in question has developed more than once on separate branches of life. This repeated development is called convergence. It can be a source of frustration for biologists because it confuses us how species are related. For example, Swallows and Swifts were classified as sister species. We now know from DNA and from comparisons of her skeletons that swallows are really closer relatives of owls than Swifts.
Size is important when it comes to evolution
However, convergent evolution becomes something useful when we consider it a kind of natural experiment. The size of the primate testicles gives us a classic example. Abyssinian black and white colobus and motor hood macaque adults are approximately the same size. But like chimpanzees, people and gorillas, these similar monkeys have very different testicles. Colobus testicles weigh only 3 grams. The testicles of the macaques, on the other hand, are a whopping 48 grams.
You could find several credible explanations for your different testicular sizes. Large testicles may be the equivalent to the cock of the peacock and not useful per se, but attractive for women. But perhaps the most plausible explanation refers in the way they mate. A male colobus monkey competes wildly to access a harem of women who only mate with him. Makaks, on the other hand, live in peaceful mixed troops of around 30 monkeys and pursue a different approach to love in which everyone combines with everyone else: men with several women (polygamy) and women with several men (polyandry).
The colobus with its harem can get away from the production of a minimal sperm – if a droplet is sufficient to produce a baby, why do more? For male macaques, the competition for reproduction is in a struggle between his sperm and the sperm of other men who have it before or after the couple. A male macaque with large testicles should make more sperm, which gives him a higher chance of passing on his genes. It is a reasonable explanation for your different testicular sizes, but is it true? The convergent evolution helps here.
When we look over the entire seam of the tree of life, we find that there are many groups of mammals that have developed testicles of all sizes. In almost all of these separate cases, larger testicles are consistently found in promiscuitive species and smaller.
A male gorilla with small test stages has sole access to a harem. Chimpanzees and Bonobos are indeed very promiscuitated. Dolphins may have the largest mammals of all of them that make up to 4 percent of their body weight (corresponds to human testicles with a weight of approximately 3 kilos). Although Wildes Delphin sex life is difficult to study, Spinner Delfines correspond to at least our expectations and named Walles events in mass pairings.
We were able to discover this consistent correlation between the testicle size and gender life directly over the mammals of the several observations provided by convergent development. And as far as people are concerned, we have the testicle size somewhere in the middle, you can do what you want!
But what about the human chin?
The human chin was fruitful for arguments between scientists. As with testicles, there are half a dozen plausible ideas to explain the development of the human chin. It could have developed to strengthen the jaw of a fighting cave man. Perhaps the chin has developed to overdo the splendor of a male beard. It could even be a by-product of the invention of the cooking and the softer food-a blends of face, which is left behind by the back flood of a weakening jaw.
Interestingly, however, a chin cannot be found in any other mammal, not even our closest cousins, the Neanderthals. Thanks to the uniqueness of the Homo Sapiens kinn, we have a number of possible explanations for its evolutionary purpose, if there is no convergent evolution, no reasonable way to test it.
Some parts of human nature can be intended to remain a mystery.
Max Telford is Jodrell professor of zoology and comparative anatomy at Ucl.
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