Paleontology: Was the "Cambrian Explosion" a series of bangs?

The Atlantic said:

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.

“I think it’s a valuable reframing of the story,” says Phoebe Cohen, a paleontologist at Williams College. “The more we look at the Cambrian explosion, the less explosion-y it looks.”

The time before the explosion is known as the Ediacaran period. Running from 571 million to 541 million years ago, it marked the appearance of the first big, complex, living things. But what were those things? A weird miscellany of unfamiliar blobs, tall fronds, and ribbed mats, they were entirely unlike today’s animals, and some may not have been animals at all. Whatever they were, based on the fossil record, they seem to have suddenly disappeared when the Ediacaran gave way to the Cambrian period, and more recognizable animals arrived. That stark transition led some researchers to cast the Ediacaran biota as “failed evolutionary experiments” that were outcompeted by the ancestors of modern critters.

The divide between the Ediacaran and Cambrian has been so heavily mythologized that scientists who study the two periods became divided too. “You have people working on the Ediacaran and people working on the Cambrian, and they don’t really come together,” says Wood. But at a recent conference in the U.K, “a lot of us realized that those boundaries had started to become blurred.”

New fossils, she says, showed that some Cambrian-defining traits were actually pioneered in the Ediacaran. For example, fossilized tracks and burrows suggest that animals were already on the move about 25 million years before the Cambrian explosion. Hard shells and skeletons had also appeared pre-explosion, and some of these had boreholes, which hint that their owners were killed by drilling predators. Mobility, armor, hunting: These innovations were part of “a crescendo that started in the Ediacaran,” Wood says.

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