Why does a man need a tail: atavisms and rudiments

We are talking about atavisms and rudiments - these concepts often coexist with each other, sometimes cause confusion and have a different nature. The simplest and probably the most famous example, in which both concepts are adjacent, refers to, so to speak, the lower part of the human body. The tailbone, the end of the spine, in which several vertebrae fused, is recognized as rudimentary. This is the rudiment of the tail. Many vertebrates, as you know, have a tail, but we, Homo sapiens, seem to have no use for it. However, nature for some reason has preserved to man the remainder of this once functional organ. Babies with a real tail are extremely rare, but still born. Sometimes it’s just a ledge filled with adipose tissue, sometimes the tail contains transformed vertebrae, and its owner is even able to move his unexpected acquisition. In this case, we can talk about atavism, the manifestation in the phenotype of an organ that was in distant ancestors, but was absent in the immediate ones.

So, rudiment is the norm, atavism is deviation. Living creatures with atavistic deviations sometimes look frightening, and because of this, and also because of the rarity of the phenomenon, they cause great interest from the general public. But evolutionists are even more interested in atavisms, and precisely because these “deformities” provide interesting clues about the history of life on Earth.

The eyes of moles living underground, as well as those of Proteas - amphibians living in water in dark caves, are rudiments. There is little benefit from them, which can not be said about the wings of an ostrich. They play the role of aerodynamic rudders when running, are used for defense. Females protect wings of chicks from the scorching rays of the sun.

The secret hidden in the egg

None of the modern birds have teeth. More precisely, this way: there are birds, for example, some species of geese that have a number of small sharp outgrowths in their beaks. But, as biologists say, these “teeth” are not homologous to real teeth, but are outgrowths that help keep, for example, a slippery fish in its beak. At the same time, the ancestors of birds must have had teeth, because they are descendants of theropods, predatory dinosaurs. The remains of fossil birds, in which teeth were present, are also known. It’s not exactly clear for what reasons (perhaps due to a change in the type of food or in order to make the body easier to fly), natural selection deprived the birds of their teeth, and it could be assumed that the genome of modern feathered genes responsible for tooth formation no longer left. But that was not true. And long before humanity learned something about genes, at the beginning of the 19th century, the French zoologist Etienne Geoffroy Saint-Hilaire expressed a hunch that modern birds can grow like teeth. He observed some outgrowths on the beak of parrot embryos. This discovery aroused doubts and sense, and was eventually forgotten.

And almost ten years ago, in 2006, American biologist Matthew Harris from the University of Wisconsin noticed outgrowths resembling teeth at the end of the beak of a chicken embryo. The embryo was susceptible to the deadly genetic mutation talpid 2 and did not have a chance to survive until hatching from the egg. However, during this short life in the beak of a failed chicken, two types of tissues have developed which form the teeth. The genes of modern birds do not code building material for such tissues - this ability was lost by the ancestors of birds tens of millions of years ago. The tooth embryos in the chicken embryo did not look like blunt-ended mammalian molars - they had a pointed conical shape, just like crocodiles, which, like dinosaurs and birds, are included in the group of archosaurs. By the way, they tried to grow molars in chickens and successfully, when the genes responsible for the development of teeth in mice were introduced into the chicken genome by genetic engineering. But the teeth of the embryo that Harris was examining appeared without any outside interference. "Dental" tissue arose due to purely chicken genes. This means that these genes that did not appear in the phenotype dozed somewhere deep in the genome, and only the fatal mutation awakened them. Harris conducted an experiment with already hatched chickens to confirm his assumption. He infected them with a virus that was artificially created by genetic engineering - the virus imitated the molecular signals arising from the talpid 2 mutation. The experiment yielded results: teeth appeared on the beak of the chickens for a short time, which then disappeared without a trace into the beak tissue. Harris's work can be considered evidence of the fact that atavistic traits are a consequence of disturbances in the development of the embryo that awaken genes that have long been silenced, and most importantly, the genes of long-lost traits can continue to be in the genome almost 100 million years after evolution has destroyed these traits. Why this happens is not known exactly. According to one hypothesis, “silent” genes may not be completely silent. Genes have the property of pleiotropicity - this is the possibility of simultaneous influence not on one but on several phenotypic traits. In this case, one of the functions can be blocked by another gene, while the others remain completely “working”.

Elixir of immortality found in a two-thousand-year-old grave

Boas and pythons have the so-called anal spurs - single claws, which are the rudiment of the hind legs. There are known cases of the appearance of atavistic limbs in snakes.

Strange survivability

It was possible to learn about toothy chickens and make a discovery almost by accident - all because, as already mentioned, the mutation killed the embryo even before it was born. But it’s obvious that mutations or other changes that cause ancient genes to come to life may not be as fatal. Otherwise, how to explain the much more well-known cases of atavisms found in completely viable creatures? Such atavisms as observed in humans, such as multi-fingering (polydactyly) on the arms and legs, poly-moscovite, which also occurs in higher primates, are quite compatible with life. Polydactyly is characteristic of horses that, with normal development, walk on one finger, whose nail has turned into a hoof. But for the ancient ancestors of the horse, multi-fingering was the norm.

There are some cases when atavism has led to a serious evolutionary turn in the life of organisms. Mites of the Crotonidae family atavistically returned to sexual reproduction, while their ancestors propagated by parthenogenesis. Something similar happened in the hairy hawk (Hieracium pilosella), an herbaceous plant in the aster family. Far from all, whom in zoology are called tetrapoda, tetrapods are actually. For example, snakes and cetaceans come from land ancestors and are also included in the superclass tetrapoda. Snakes lost their limbs completely, in cetaceans, the forelimbs became fins, and the hind limbs almost disappeared. But the appearance of atavistic limbs is noted in both snakes and cetaceans. There are cases when a pair of hind fins was found in dolphins, and the four-legged seemed to be restored.

The rudimentary pelvic bones of some cetaceans have long lost their original function, but their uselessness is called into question. This rudiment not only recalls that whales were descended from tetrapods, but also plays an important role in the breeding process.

More bone - more offspring

However, something else reminds us of the four-legged whales, and here we turn to the area of ​​rudiments. The fact is that in some species of cetaceans, the rudiments of the pelvic bones are preserved. These bones are no longer associated with the spine, and therefore with the skeleton as a whole. But what made nature preserve information about them in the gene code and transmit it by inheritance? This is the main mystery of the whole phenomenon called rudimentation. According to modern scientific concepts, rudiments can not always be spoken of as superfluous or useless organs and structures. Most likely, one of the reasons for their preservation is precisely in the fact that evolution has found rudiments for a new, previously uncharacteristic application. In 2014, American researchers from the University of South Carolina published an interesting work in Evolution magazine. Scientists examined the size of the pelvic bones of whales and came to the conclusion that these sizes are correlated with the size of the penises, and the muscles of the penis are attached just to the rudimentary pelvic bones. Thus, the size of the whale penis depended on the size of the bone, and a large penis determined success in reproduction.

The same with the human tailbone, which was mentioned at the beginning of the article. Despite its rudimentary origin, this part of the spine has many functions. In particular, the muscles involved in controlling the genitourinary system, as well as part of the bundles of the gluteus maximus muscle, are attached to it.

The appendix, the vermiform appendix of the cecum, sometimes causes a lot of troubles to a person, becoming inflamed and necessitating surgical intervention. In herbivores, it has a significant size and was "designed" in order to serve as a kind of bioreactor for the fermentation of cellulose, which is a constructive material of plant cells, but is poorly digested. The appendix has no such function in the human body, but there is another. The intestinal process is a kind of nursery for Escherichia coli, where the original flora of the cecum is preserved intact and multiplies. Removal of the appendix entails a deterioration in the state of microflora, for the restoration of which it is necessary to use drugs. Also, this organ plays a role in the body's immune system.

It is much harder to see the benefits of rudiments such as ear muscles or wisdom teeth. Or the eyes of moles - these organs of vision are rudimentary and do not see anything, but can become the "gateway" of infection. Nevertheless, it is clearly not worth rushing to declare something in nature redundant.

The article “Remember the Ancestors!” Was published in the journal Popular Mechanics (No. 10, October 2015). Do you like the article?

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