The Cambrian period, part of the Paleozoic era, produced the most intense burst of evolution ever known. The Cambrian Explosion saw an incredible diversity of life emerge, including many major animal groups alive today. Among them were the chordates, to which vertebrates (animals with backbones) such as humans belong.
What sparked this biological bonanza isn't clear. It may be that oxygen in the atmosphere, thanks to emissions from photosynthesizing cyanobacteria and algae, were at levels needed to fuel the growth of more complex body structures and ways of living. The environment also became more hospitable, with a warming climate and rising sea levels flooding low-lying landmasses to create shallow, marine habitats ideal for spawning new life-forms.
Nevertheless, the scale of the Cambrian Explosion is likely exaggerated due to the proliferation of hard-bodied animals that fossilized much more readily than their soft-bodied precursors. These included brachiopods, which lived in shells resembling those of clams or cockles, and animals with jointed, external skeletons known as arthropods—the ancestors of insects, spiders, and crustaceans. These toughened-up creatures represented a crucial innovation: hard bodies offering animals both a defense against enemies and a framework for supporting bigger body sizes.
The iconic arthropods of the Cambrian were the trilobites, which left a huge number of fossils. Trilobites had flattened, segmented, plated bodies that helped to protect them in seas that were increasingly filled with predators. With many varieties and sizes—they ranged from a millimeter to more than 2 feet (0.6 meters) in length—trilobites proved among the most successful and enduring of all prehistoric animals. More than 17,000 species are known to have survived until the mega-extinction that ended the Permian period 251 million years ago.
A predator of the Cambrian was the giant, shrimplike Anomalocaris, which trapped its prey in fearsome mouthparts lined with hooks. Even stranger was the five-eyed Opabinia, which caught its victims using a flexible clawed arm attached to its head. These animals hunted along the seabed, where colonies of archaic sponges grew on organic, mineral structures formed by the activity of cyanobacteria. The sponges added to these reef habitats by building supporting skeletons from calcium carbonate, which they collected from the water.
The earliest known primitive chordate is Pikaia gracilens, a wormlike creature that swam in middle Cambrian seas. Fossils found in the Burgess Shale of British Columbia show traces of a notochord (a rodlike primitive backbone), a significant step in the evolution of vertebrates.
Cambrian sediments found in Canada, Greenland, and China have yielded rarely fossilized soft-bodied creatures such as marine worms buried during undersea mud avalanches. Representing the oldest known backboned animals with living relatives, the fossils showed that our vertebrate ancestors entered the evolutionary story some 50 million years earlier than previously thought.
The end of the Cambrian saw a series of mass extinctions during which many shell-dwelling brachiopods and other animals went extinct. The trilobites also suffered heavy losses.