Almost all animal species on Earth emerged simultaneously and suddenly, or they evolved for many millions of years, but we simply cannot find traces of this evolution - the issue known as the "Darwin's dilemma" is relevant today. Scientists still find it difficult to explain the reasons for the explosive development of organisms that happened half a billion years ago in the Cambrian period.
Window to the past
More than a hundred years ago, in the Southwest of Canada in the Rocky Mountains, in the Burgess shale, paleontologists discovered many well-preserved fossil prints. The creatures that had left them were something strange. Gradually it became clear that sea animals were buried there, five hundred million years old, which gave rise to modern fauna.
The Burgess Shale is the most famous and well-studied evidence of the Cambrian explosion. There are prints of tens of thousands of soft-bodied creatures in which you can see the outer covers, muscles, internal organs. In total, scientists have described about 280 species of ancient animals, and dozens of new products are added to the fossil record every year.
Such a mass grave of the oldest fauna in science is called the German term "lagerstett". Only they allow us to trace how modern life was born.
Cambrian fossil of Eldonia from the Burgess Shale, Canada. Initially, it was mistaken for a sea cucumber, but later it was found out that it combines the features of several groups of animals at once.
One of the largest Lagerstett, where the Cambrian explosion was captured, was discovered in the late 1980s in the north of Greenland, in the town of Sirius Passet, 800 kilometers from the North Pole.
Promotional video:
Chalkieria evangelist - mysterious ancient animal from Greenland (Sirius Passet).
Two worlds
According to modern concepts, living organisms arose on Earth more than three billion years ago. They remained unicellular for a very long time, which allowed them to survive the worst catastrophes that have happened more than once on the planet. Microorganisms have successfully adapted first to an oxygen-free atmosphere, a strong greenhouse effect, and then to oxygen, which, although it was a poison for them, helped to use solar energy more efficiently.
Single-celled organisms survived several severe cold snaps, as a result of which the planet, as scientists believe, completely turned into an ice ball. The last glaciation ended about six hundred million years ago and was replaced by a rapid flourishing of marine life. The inhabitants of the reservoirs at that time were not like any of the living organisms. Mattresses, socks, feathers are the most suitable comparisons for Vendobionts (as these unusual creatures are called). Apparently, they were multicellular. They already had the rudiments of familiar tissues, the nervous system, and the intestines.
The ancient seas literally swarmed with these "socks" that lived peacefully, without any signs of predation. Several tens of millions of years have passed, and suddenly this whole strange world is dying out, and without signs of any global catastrophe. Almost immediately, the deserted seas are filled with a completely different fauna - the Cambrian (according to the name of the geological period, which marks the beginning of a new era of multicellular animals).
For 20 million years, more than one and a half thousand genera of animals appeared on Earth, among which are the ancestors of almost all modern groups.
In search of transitional forms
Scientists still cannot find any evolutionary connection between the strange Vendobionts and the more modern Cambrian who replaced them. This abrupt transition in the last century has served as an argument in favor of creationists demanding evidence of smooth evolution of species and "transitional forms." They themselves explained the Cambrian explosion by the intervention of the creator.
Thanks to the work of paleontologists and the development of accurate research methods, the evolutionary view finally prevailed. Scientists decided that modern organisms originated among the Vendobionts, gradually evolved, but due to their tiny size they did not survive, or we simply cannot find them.
However, British and Swiss scientists recently calculated by molecular methods that arthropods - the largest group of organisms on Earth, uniting eighty percent of all animal species - appeared no earlier than 550 million years ago. That is, after all, there was a Cambrian explosion, but it stretched out for 15-20 million years, which by geological standards, of course, moments.
Trilobites have become a symbol of the beginning of modern fauna. They appeared during the Cambrian explosion - about 540 million years ago.
During this time, more than one and a half thousand genera of animals occurred from one or two Vendobionts. These are the very transitional forms between ancient multicellular and modern fauna. Most of them share the features of several contemporary groups. For example, ecdizozoa (molting animals), from which many skins have survived. Or xenusia resembling large larvae with telescopic legs. Chalkieria is a spiny, multi-membered worm with scales. A crustacean-like orthrosanclus with several rows of soft spines and a sort of shell at one end.
The arthropod peytoya lived more than 500 million years ago. Burges Slates, Canada.
The Cambrian explosion gave birth to almost all biological types that exist on the planet today, including chordates, to which humans belong.
Many hypotheses have been put forward about the Cambrian explosion, but none of them fully explains this phenomenon. The most commonly referred to is an increase in the oxygen content in the atmosphere. By learning how to efficiently extract energy, the body was able to become multicellular. Perhaps, the ocean offensive, which began due to the expansion of the supercontinent Gondwana and small Laurentia (a modern fragment from it - North America), played some role.
But why was the variety of animals so great? One of the possible reasons is the competition of species, which provoked the emergence of predators.
Six hundred to five hundred million years ago, most of the modern continents were part of Gondwana. An ocean began to form between her and Laurentia.
Tatiana Pichugina