The Cambrian was the first period of the Paleozoic era and the Phanerozoic Eon It lasted from 541 million years ago to 485 million years ago, (give or take a million year margin of error). The Cambrian marked a supposed explosion of life where almost all of today's phylum evolving during the Cambrian period. This fact compounded with the comparatively poor record of Precambrian life has given many misinformed people the idea that life sprung out of nothing. Despite that the Cambrian holds an important place in the history of Earth.
The Cambrian opened with the breakup of the super-continent Pannotia, the 8th super-continent in the history of planet Earth. It was a relatively short lived super-continent that was centered in the southern hemisphere. Over the course of the Cambrian period Pannotia broke up. These were the subcontinents of Laurentia (Most of modern North America), Baltica (modern day Northwestern Eurasian), and Siberia (Guess what that is today). These continents were slowly drifting north.
The sea levels were higher than they are today, which provided large amounts of warm, shallow seas which were the perfect cradle for the Cambrian explosion to take place. The climate was warmer than it was today, however fluctuating sea levels indicate the possibility of "ice ages".
The Cambrian saw the radiation of most modern phylum though in primitive and alien forms. The Arthropods were the most diverse and for many years Trilobites were considered the king among them in terms of numbers. The Cambrian saw their peak diversity in form and lifestyle with species ranging from tiny blind burrowers, to massive predators. They coexisted with many other arthropods including the first large predator Anomalocaris which was a terrifying 1 meter in size.
The earliest Chordates such as Pikaia, Haikouichthys, and Myllokunmingia were also present. The diversity indicates that chordates are an ancient group. Their small stature at the time limited them to minor roles in the Cambrian.
The unique Cambrian fauna is believed to have been the direct cause of the disappearance of Ediacaran life. Ironically it was short lived in of itself being replaced in the Ordovician with the marine fauna that would dominate for the rest of the Paleozoic. This turnover event is called the Cambrian-Ordovician extinction event and is the divide between it and the next period the Ordovician.
Common Cambrian misconceptions
Many misconceptions surround the Cambrian time period. One of the major misconceptions is that everything just spontaneously generated in the Cambrian explosion. This is not true on several fronts. Unicellular bacteria and Archaea both emerged long before the Cambrian, emerging at least 4 billion years ago. There is also the discovery of various Precambrian rocks which have strange multi-cellular organisms. Although strata this old is rare it has been found in Canada, South Australia, and Namibia. Even if the focus is just on animals several animal phylum emerged prior to the Cambrian including sponges, Jellyfish, and Comb-jellies. There is also evidence of other phylum emerging prior to the Cambrian but the evidence is inconclusive.
Now to be fair to both sides scientists themselves have made many mistakes regarding Cambrian life. The most famous of these blunders is the former belief that Anamalocaris was in fact two separate animals, a shrimp tail and a jellyfish