Mysteries of Evolution: Ancient Animals That Didn't Die Out

2024 Author: Seth Attwood | [email protected]. Last modified: 2023-12-16 15:55

The evolution of life on Earth contains many mysteries. One of them is evolutionary leaps, during which, in a short time by paleontological standards, new groups of living beings or new signs appeared that radically change the "structure" of an organism. An example is the origin of birds from dinosaurs.

But there are examples of the opposite property: evolution seemed to stop for hundreds of millions of years.

The phenomenon of "living fossils" remains one of the most controversial in modern biological science, and a huge amount of topics and material for discussion has accumulated. We know one of the textbook stories from school: until the end of the 30s of the 20th century, the superorder of cross-finned fish was considered extinct in the Cretaceous period.

However, in 1938, an amazing creature was pulled out of the Indian Ocean, from a depth of 70 m, later called coelacanth. It turned out that fish, in whose fins there were muscular lobes, survived to the modern era. Particularly keen interest in the find was caused by the fact that science considered cross-finned fish a transitional form from fish to amphibians, and "muscular" fins were perceived as a step to the paws, with which you can move on land.

Also, the cross-finned, as it turned out, had a close common ancestor with the fish of the superorder lungs - that is, they can breathe both oxygen dissolved in water and atmospheric air. This branch left descendants in the modern fauna in the form of horn-toothed fishes - and they can also be considered a kind of living fossils, because the other numerous representatives of the superorder exist only in the geological chronicle.

Thus, living creatures are usually referred to as living fossils, which either morphologically almost do not differ from known ancient animals (plants, bacteria), or have inherited some archaic features from distant ancestors.

What happened to the clock?

The existence of such "twin pairs", uniting the inhabitants of ancient Earth and our contemporaries, has become one of the difficult questions of evolutionary theory. After all, evolution, according to modern concepts, is based on a kind of biological clock. Over long time scales, genomes should accumulate a comparable number of mutations. And if some creatures have remained practically unchanged for hundreds of millions of years, then their "clock" has stopped.

The phenomenon of "living fossils" was seized upon by creationists who deny the evolutionary mechanisms identified by science. Let for hundreds of millions of years genetic mutations and natural selection have turned some branch of dinosaurs into eagles and tits, but why did these objective laws of nature leave the cross-finned, albeit relative, but unchanged?

As if in response to this kind of reasoning, many biologists today are inclined to generally consider the term "living fossils" (going back, by the way, to Darwin himself) incorrect. And because he does not have a clear definition, and because he inaccurately denotes the essence of the phenomenon. After all, there is no question of stopping evolution. More recently, a study was published, prepared by scientists at the University of Michigan, on sturgeons that live in the American Great Lakes.

This fish, which has a rather archaic appearance, was considered one of the candidates for living fossils - sturgeons have existed on our planet for about 100 million years. However, as we managed to find out, the inhabitants of the Great Lakes throughout history demonstrated colossal rates of evolutionary changes - while retaining the main morphological features, they constantly changed in size. The Great Lakes were home to both dwarf and giant fish, as well as sturgeon of many intermediate sizes.

The submarine ship Nautilus - an inhabitant of the depths of the Pacific and Indian oceans - is one of the most spectacular representatives of "living fossils". It belongs to the Nautiloidea - a superorder of cephalopods, whose fossils have been known since the Cambrian (500 million years ago). Unlike other cephalopods such as octopuses or squids, the nautilus have preserved their shells of amazing beauty for half a billion years. Of the whole variety of nautiloids, only a few species remain.

The same conclusions were made by modern science for the classic examples of "living fossils" - the same coelacanth. Patrick Laurenti, an evolutionary biologist at the French National Science Foundation CNRS, was one of those who established that there are noticeable anatomical differences in size, in the structure of the skull, spine and other morphological elements between the coelacanths - representatives of the Cretaceous fishes - and modern coelacanths. And most importantly, the rate of change in the genome is quite comparable to the changes in the DNA of creatures that have undergone radical metamorphoses in the course of evolution.

Shields - small freshwater crustaceans of the suborder Notostraca - first appeared on Earth about 265 million years ago and have since retained their appearance unchanged. However, the assumption of stopped evolution did not work here either. Researchers from the University of the British city of Hull have sequenced several genes from the DNA of about 270 individuals of the living shields.

As a result of this work, it turned out that today shields form not 11, as was previously thought, but 38 separate species, and these species belong to two different branches, which were divided in the Jurassic period - about 184 million years ago. At the same time, active speciation and the corresponding changes in the genome occurred regularly, without affecting the basic morphology.

The green continent has become the place on Earth where the most unusual groups of mammals have evolved in isolation for a long time.

Quiet place and fine tuning

But if evolution regularly introduces, albeit not immediately noticeable, but constant constructive changes, why does the phenomenon of "living fossils" arise? To illustrate this mechanism, let us turn to human history. Large migrations like the Great Migration of Nations, the formation of states and empires, the spread of world religions - all this led to the mixing of ethnic groups and a constant change in the way of life of people from generation to generation.

But there are cases when, as a result of macro-processes, any separate tribe ended up on a remote island, or in the depths of the jungle, or in other conditions that led to an isolated existence, but did not greatly contribute to the development of civilization. And while railroads were being laid somewhere, modern cities were being built, planes were flying into the sky, the isolated tribe continued to live as its ancestors lived, perhaps thousands of years ago.

Roughly the same thing, only on different time scales, happened in the history of wildlife. The ancestors of most "living fossils" belonged in the distant past to much more extensive related groups of creatures. This numerous relatives in the past, having fallen under the ax of natural selection, either adapted to the changed conditions, gradually transforming beyond recognition, or died out, turning into dead-end branches.

And only a small part of the group, by the will of circumstances, became paleoendemic. She found herself in conditions that, firstly, practically did not change over millions of years, and therefore did not require radical adaptation, and secondly, isolated this population from natural enemies. In these evolutionary laboratories, the genetic clock passed at the same speed, however, natural selection had no choice but to fine-tune the once established morphology.

Bible and rock and roll

Several other paleontological phenomena are closely related to the phenomenon of "living fossils". The "Lazarus Effect" is named after a biblical character resurrected by Christ. We are talking about species that, once recorded in the fossil record, then seem to disappear for a long time, and then appear ("resurrect") again.

Most often this is simply due to the lack of paleontological data: after all, the formation of a fossil is not so much the norm as a rare case, and if for a given era the remains of any creature were not found, this does not mean that it was not. Perhaps he was simply "unlucky" to leave footprints in the fossils, or these footprints have not yet been found. The Lazarus effect also includes rare cases when an animal considered extinct suddenly appears among the living.

The Riddle of the Depths

Latimeria, due to its extremely "prehistoric" appearance, has long been considered a classic example of a "living fossil". However, over time, significant differences were identified between this inhabitant of the Indian Ocean and the ancient celacanths. In particular, some of the metabolic features indicate that the fossil relatives of the coelacanth lived in freshwater bodies, where, possibly, the muscular fins helped them to move, relying on the bottom of the shallow water. In addition, the modern coelacanth is larger than the ancient cross-finned fish.

A classic example of the Lazarus taxon is the flightless takahe bird on the South Island of New Zealand. The remains of the bird were discovered in the middle of the 19th century, and although its species is not particularly ancient, for 100 years the takahe was considered completely extinct. But the resurrection still followed. Roughly the same fate befell the Chak bakers, a woolly pig-like inhabitant of South America. In 1930, his bones were discovered, and not yet fossilized, indicating a relatively recent extinction of the species. And only 45 years later it turned out that there was no disappearance - just the animal hid well from prying eyes.

The "Elvis effect" also testifies to a kind of scientific delusion. As you know, after the untimely death of the king of rock and roll, there were many people who saw Elvis alive in different parts of America and the world. In the same way, creatures with very similar morphological characteristics, separated by large time intervals, were sometimes mistaken for the same biological species that had survived epochs. A typical example comes from the world of marine invertebrates known as brachiopods or brachiopods.

A species of brachiopod named Rhaetina gregaria has been recorded in the Late Triassic fossils. The Triassic, about 200 million years ago, was followed by an event known as the Triassic (or Triassic-Jurassic) extinction, which led to the extinction of many species of invertebrates.

However, fossils dating back to the Jurassic period also contain the remains of a creature very similar to Rhaetina gregaria. Nevertheless, further research showed that the Jurassic brachiopod is the same "resurrected Elvis", that is, a creature that is not a descendant of the Triassic shoulder-head, but a representative of another branch, which acquired similarities as a result of convergent evolution - a phenomenon that gave wings to birds and bats who have no close relationship.

The list of creatures that have survived, as it were, unchanged, entire geological eras is extensive and includes mammals, fish, birds, molluscs, as well as plants and bacteria. But, as the data of science show, none of these creatures can be evidence of "stopping evolution." It's just that we don't always know her path.