Some Evolutionary Experiments

During the Cambrian period, about 535 to 530 million years ago, a most fertile time occurred in evolutionary history. For over 3 billion years before this time, evolution had forged little more than prokaryotes and unicellular eukaryotes. Then, within the space of a few million years, all of the major phyla of macroscopic invertebrates, and probably all of the smaller phyla, became established. This was the Cambrian explosion, the greatest evolutionary “bang” the world has known. In fact, the fossil record suggests that more phyla existed in the Paleozoic Era than exist now, but some disappeared during major extinction events that punctuated the evolution of life on earth since that time. Greatest of these disruptions was the Permian extinction about 230 million years ago. Thus evolution has led to many “experimental models.” Some of these models failed because they were unable to survive in changing conditions. Others gave rise to abundant and dominant species and individuals that inhabit the world today. Still others produced a small number of species, some of which persist, while others were formerly more abundant but are now in decline.

The great evolutionary flow that began with the appearance of a coelom and led to the three huge phyla of molluscs, annelids, and arthropods produced other lines as well. Most of those that have survived are small and lack great economic and ecological importance. The relationships among these groups have been and continue to be the subject of considerable controversy.