Before the Cambrian period, 600 or so million years ago, life on earth consisted mostly of single-celled bacteria and algae--but after the Cambrian, multi-celled vertebrate and invertebrate animals dominated the world's oceans. The Cambrian was the first period of the Paleozoic Era (542-250 million years ago), followed by the Ordovician, Silurian, Devonian, Carboniferous and Permian periods; all of these periods, as well as the succeeding Mesozoic and Cenozoic Eras, were dominated by the vertebrates that first evolved during the Cambrian.
Climate and geography. Not a lot is known about the global climate during the Cambrian period, but the unusually high atmospheric carbon dioxide levels (about 15 times those of the present day) imply that the average temperature may have exceeded 120 degrees Fahrenheit. Eighty-five percent of the earth was covered with water (compared to 70 percent today), most of that area being taken up by the huge Panthalassic and Iapetus oceans; the average temperature of these vast seas may have been in the range of 100 to 110 degrees Fahrenheit. By the end of the Cambrian, the bulk of the planet's land mass was locked up in the southern continent of Gondwana, which had only recently broken off from the even bigger Pannotia of the preceding Proterozoic Era.
Marine Life During the Cambrian Period
Invertebrates. The major evolutionary event of the Cambrian period was the "Cambrian Explosion," a rapid burst of innovation in the body plans of invertebrate organisms. ("Rapid" in this context means over the course of tens of millions of years, not literally overnight!) For whatever reason, the Cambrian witnessed the appearance of some truly bizarre creatures, including the five-eyed Opabinia, the spiky Hallucigenia, and the three-foot-long Anomalocaris, which was almost certainly the largest animal ever to appear on earth up to that time. Most of these creatures left no living descendants, which has fueled speculation about what life in succeeding geologic epochs might have looked like if, say, the alien-looking Wiwaxia was an evolutionary success.
As striking as they were, though, these invertebrates were far from the only such life forms in the earth's oceans. The Cambrian period saw the worldwide spread of the earliest plankton, as well as trilobites, worms, tiny mollusks, and small, shelled protozoans. In fact, the abundance of these organisms is what made the lifestyle of Anomalocaris and its ilk possible; in the way of food chains throughout history, these larger invertebrates spent all their time feasting on the smaller invertebrates in their immediate vicinity.
Vertebrates. You wouldn't have known it to visit the earth's oceans 500 million years ago, but vertebrates, and not invertebrates, were destined to become the dominant animals on the planet. The Cambrian period marked the appearance of the earliest identified vertebrate organisms, including Pikaia (which possessed a flexible "notochord" rather than a true backbone) and the slightly more advanced Myllokunmingia and Haikouichthys. Essentially, these three genera count as the very first prehistoric fish, though there's still a chance that earlier candidates may be found dating from the late Proterozoic Era.
Plant Life During the Cambrian Period
There is still some controversy about whether any true plants existed as far back as the Cambrian period. If they did, they consisted of microscopic algae and lichens (which don't tend to fossilize well). It's a surer bet that macroscopic plants like seaweeds had yet to evolve during the Cambrian, giving their noticeable absence in the fossil record.
Next: the Ordovician Period