Ancestors of today’s animals can be traced back 540 million years to the relatively sudden appearance of most major animal groups during a time called the Cambrian explosion. Before then, nebulous organisms of no obvious relation to present-day life drifted in primordial seas.
Fossils of animals from the Ediacaran period—just before the Cambrian—look like bloated ferns and deflated balloons. Some paleontologists argue that the Ediacaran biota represent an entire kingdom unrelated to the modern Kingdom Animalia.
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