For most of the nearly 4 billion years that life has existed on Earth, evolution produced little beyond bacteria, plankton, and multi-celled algae. But beginning about 600 million years ago in the Precambrian, the fossil record speaks of more rapid change. First, there was the rise and fall of mysterious creatures of the Ediacaran fauna, named for the fossil site in Australia where they were first discovered. Some of these animals may have belonged to groups that survive today, but others don't seem at all related to animals we know.
Then, between about 570 and 530 million years ago, another burst of diversification occurred, with the eventual appearance of the lineages of almost all animals living today. This stunning and unique evolutionary flowering is termed the "Cambrian explosion," taking the name of the geological age in whose early part it occurred. But it was not as rapid as an explosion: the changes seems to have happened in a range of about 30 million years, and some stages took 5 to 10 million years.
It's important to remember that what we call "the fossil record" is only the available fossil record. In order to be available to us, the remains of ancient plants and animals have to be preserved first, and this means that they need to have fossilizable parts and to be buried in an environment that will not destroy them.
It has long been suspected that the sparseness of the pre-Cambrian fossil record reflects these two problems. First, organisms may not have sequestered and secreted much in the way of fossilizable hard parts; and second, the environments in which they lived may have characteristically dissolved those hard parts after death and recycled them. An exception was the mysterious "small shelly fauna" -- minute shelled animals that are hard to categorize -- that left abundant fossils in the early Cambrian. Recently, minute fossil embryos dating to 570 million years ago have also been discovered. Even organisms that hadn't evolved hard parts, and thus didn't leave fossils of their bodies, left fossils of the trails they made as they moved through the Precambrian mud. Life was flourishing long before the Cambrian "explosion".
The best record of the Cambrian diversification is the Burgess Shale in British Columbia. Laid down in the middle-Cambrian, when the "explosion" had already been underway for several million years, this formation contains the first appearance in the fossil record of brachiopods, with clamlike shells, as well as trilobites, mollusks, echinoderms, and many odd animals that probably belong to extinct lineages. They include Opabinia, with five eyes and a nose like a fire hose, and Wiwaxia, an armored slug with two rows of upright scales.
The question of how so many immense changes occurred in such a short time is one that stirs scientists. Why did many fundamentally different body plans evolve so early and in such profusion? Some point to the increase in oxygen that began around 700 million years ago, providing fuel for movement and the evolution of more complex body structures. Others propose that an extinction of life just before the Cambrian opened up ecological roles, or "adaptive space," that the new forms exploited. External, ecological factors like these were undoubtedly important in creating the opportunity for the Cambrian explosion to occur.
Internal, genetic factors were also crucial. Recent research suggests that the period prior to the Cambrian explosion saw the gradual evolution of a "genetic tool kit" of genes that govern developmental processes. Once assembled, this genetic tool kit enabled an unprecedented period of evolutionary experimentation -- and competition. Many forms seen in the fossil record of the Cambrian disappeared without trace. Once the body plans that proved most successful came to dominate the biosphere, evolution never had such a free hand again, and evolutionary change was limited to relatively minor tinkering with the body plans that already existed.
Interpretations of this critical period are subject of lively debate among scientists like Stephen Jay Gould of Harvard University and Simon Conway Morris of Cambridge University. Gould emphasizes the role of chance. He argues that if one could "rerun the tape" of that evolutionary event, a completely different path might have developed and would likely not have included a humanlike creature. Morris, on the other hand, contends that the environment of our planet would have created selection pressures that would likely have produced similar forms of life to those around us -- including humans.