Life explodes twice

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. Yet, the Ediacaran fauna were as varied as all animals in existence today and, more impressively, in the Cambrian, report Bing Shen and colleagues from Virginia Tech University in the Jan. 4 Science.

Earlier studies assessed diversity by counting fossil types. The apparent differences between three species of flies and a fly, whale, and mouse species weren’t distinguishable by this method. The new study mathematically analyzes differences in body types, regardless of name and rank. By doing so, the paleontologists revealed an unexpected trend. Rather than a steady increase in the range of body types as the period progressed, the paleontologists noticed an initial burst of variation that plateaued until fizzling out by the Cambrian.

Finding a trend that mirrors that of the Cambrian, in an independent group of organisms, gives scientists another reason to believe that macroevolution proceeds in spurts rather than only by gradual change.