theory of evolution. Today, we know that animal life did appear comparatively rapidly in the fossil record, and new dating techniques now put the time of the first complex fossils at slightly older than 540 million years ago, with the first trilobites appearing in the record some single-digit millions of years after that.
In contrast to this slightly more than half-billion-year-old age for the first animals, molecular studies of extant animals looking at the age of divergence of basic animal lineages suggested a far older origin of animals. One influential study by Charles Wray and his associates at the American Museum of Natural History dated the divergence of animals from protozoan (single-celled organisms, as compared to animals, which are all composed of many cells) ancestors at about a billion years ago. In this latter view, animal phyla diverged early but remained at a very small size and invisible to the fossil record for a half billion years. Both views agreed that the appearance of animals in the fossil record was a significant event, which has been called the Cambrian Explosion. To paleontologists, the Cambrian Explosion marked the first evolution of animals. To molecular geneticists, it marked the first evolution of animals large enough to leave remains in the rock record. The controversy raged through the 1990s, to be resolved in the early years of the twenty-first century when new molecular studies, using more sophisticated analyses, essentially confirmed the younger date for the origin of animals that had been championed by paleontologists. There is now agreement that animal life on Earth did not predate 600 million years ago and might be closer to 550 million years in age.
The Cambrian Period is now dated from 544 million to about 495 million years ago. Over those roughly 50 million years, the vast majority of animal phyla first appeared. All specialists agree that this is the most important event in the entire history of animal life, superceded in importance only by the first appearance of life on Earth, perhaps, in the context of the entire history of life on our planet.
The Cambrian Explosion has thus been a “hot” topic in science. A vast library of books, technical articles, and popular accounts of the Cambrian Explosion exists. The paper devoted to these many pages has probably destroyed whole forests, which is a bit ironic, since the Cambrian period occurred long before vascular plants colonized the