LIFE × EVOLUTION

The Cambrian Explosion

~540–518 million years ago

Phanerozoic · Paleozoic · Cambrian

The Cambrian explosion is the appearance of nearly all animal body plans, including those of arthropods, molluscs, annelids and the first chordates, within about twenty million years, from 540 to 518 million years ago.

The Cambrian explosion is the appearance of nearly all animal body plans within an interval of about twenty million years. A body plan is the fundamental arrangement of an organism: its symmetry, its segmentation and the positions of its organs. Each body plan defines a phylum, a major division of the animal kingdom. These plans include those of arthropods, such as insects, of molluscs, of annelids, or segmented worms, and of the first chordates, the group that contains the vertebrates. No genuinely new plan has appeared since. Paterson and colleagues (2019) used trilobites, extinct marine arthropods, to bound its duration. They estimated rates of morphological evolution, the speed at which trilobite forms change from one bed of rock to the next. These rates are no higher at the beginning of the Cambrian than at its end. This indicates that the diversification was largely complete before the ordinary Cambrian fossil record even begins, some 521 million years ago.

Why it matters

Every living animal, the human species included, is a variation on a body plan fixed within some twenty million years at the start of the Cambrian, the geological period in which the explosion took place.

Dating & uncertainty

The term explosion overstates the suddenness of the event. The diversification of animals begins some 571 million years ago with the Ediacaran macrofossils, fossils visible to the naked eye from the period before the Cambrian. It continues into the Ordovician, the period after the Cambrian. The environment was changing throughout. Only the appearance of body plans is compressed in time. The radiation as a whole, the multiplication of species from common ancestors, is not, and it is itself one radiation among others. Paterson and colleagues (2019) used trilobite evolutionary rates to bound the duration of that compression.

Earth history as one day

21:09:212 h 51 min before midnight

if Earth’s 4.54 billion years were compressed into a single day