If you were to condense the planet’s 4.5-billion-year history into a single calendar year, then sometime from the 18th to the 20th of November, as conventional wisdom would have it, the animal kingdom would undergo a dramatic transformation. A world dominated by blobby, sedate creatures that sift seawater for food would suddenly give way to a new menagerie of active predators and prey, sporting innovations such as eyes, jaws, legs, and shells. The ancestors of all the major modern animal groups would appear, and seemingly take over from their predecessors.
This 20-million-year period is known as the Cambrian explosion, and few events in the history of the Earth have been so retrospectively hyped. It has been billed as “arguably the most important biological event after the origin of life,” “the most important geobiological revolution of the past billion years,” and “evolution’s ‘big bang.’” But a team of researchers led by Rachel Wood at the University of Edinburgh think the famed event wasn’t all that singular.
In a provocative new paper, they argue that the traditional explosion was bracketed by several equally important pulses of evolutionary innovation. In each of these, existing communities of species gradually bled into new ones, rather than being suddenly replaced. “It’s very difficult to pick out a discrete Cambrian explosion,” says Wood. “It’s more fruitful to think of it in terms of a very long narrative of change that started before, and continued long afterwards.” The Cambrian explosion, in other words, was just one burst in the middle of a protracted fireworks display.