The Cambrian Period is the first geological time period of the Paleozoic Era (the “time of ancient life”). This period lasted about 53 million years and marked a dramatic burst of evolutionary changes in life on Earth, known as the "Cambrian Explosion." Among the animals that evolved during this period were the chordates, animals with a dorsal nerve cord; hard-bodied brachiopods, which resembled clams; and arthropods, ancestors of spiders, insects and crustaceans.Though there is some scientific debate about what fossil strata should mark the beginning of the period, the International Geological Congress places the lower boundary of the period at 543 million years ago with the first appearance in the fossil record of worms that made horizontal burrows. The end of the Cambrian Period is marked by evidence in the fossil record of a mass extinction event about 490 million years ago. The Cambrian Period was followed by the Ordovician Period.The period gets its name from Cambria, the Roman name for Wales, where Adam Sedgwick, one of the pioneers of geology, studied rock strata. Charles Darwin was one of his students. (Sedgwick, however, never accepted Darwin's theory of evolution and natural selection.)