This Is the Way the Animals Arose: Not With a Bang, but With a Bunch of Bangs
The much-hyped Cambrian explosion may have been just one burst in a marathon evolutionary fireworks display.
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.
Recent finds have also reinforced the continuity between the pre- and post-explosion worlds. New fossils of a Cambrian animal called Stromatoveris showed that it’s related to an Ediacaran group called the petalonamids, named for their petal-like fronds. That connection shows that the Ediacarans were “alive and well over 20 million years into the Cambrian period,” wrote Jennifer Hoyal Cuthill from the University of Cambridge in The Conversation.
The Ediacaran biota wasn’t just a single set of organisms either. They were extremely variable, and appeared in distinct waves. First came the Avalon biota, characterized by stationary, fronded creatures that resembled kelp. Next up: the White Sea biota, which included mobile creatures such as Dickinsonia—a ribbed oval that was recently confirmed as an animal. Then the Nama biota—a so-called wormworld that ushered in tubular animals, including some with hard shells.
When the Cambrian fauna eventually arrived, Wood thinks it appeared in two pulses. The mollusks, for example, evolved just before the explosion, but most of the group’s early members died out in an extinction event 513 million years ago, in the early Cambrian. The lineages that gave rise to today’s mollusks—octopuses, clams, and more—flourished only after that point, in the late Cambrian.
That makes at least five distinct bursts of diversification that straddle the divide between the periods.
“There must have been some sort of biological continuity between the Ediacaran and the Cambrian, but the question is a matter of degree,” says Michael Lee from Flinders University. Did only a few Ediacaran lineages survive the transition to the Cambrian, or did many of them do so? Wood’s team is arguing for the latter; Lee still leans toward the former.
Lidya Tarhan from Yale University is more convinced. “There was considerable ecological overlap between the two communities,” she says. Rather than being distinct sets of losers and winners, Ediacaran and Cambrian animals were instead “part of an evolutionary continuum.”
Wood’s team also ties their biological narrative into a geological one. Others have argued that the Cambrian explosion was sparked by a rise in oxygen, which fueled the evolution of active behaviors and larger, gas-guzzling bodies. But the seas didn’t accumulate oxygen steadily or evenly. Instead, for much of the Ediacaran, levels of the vital gas fluctuated over time, and across different layers of the oceans. Wood and her colleagues suggest that these yo-yoing levels drove animal evolution: They created oxygen-rich oases that allowed new species to diversify in isolation, much as they do on today’s islands.
Those same dynamics might have repeated themselves at other pivotal moments. About 252 million years ago, at the end of the Permian period, the planet experienced its greatest mass extinction—a disaster that depleted much of the ocean’s oxygen and killed the majority of living species. But life rebounded immediately afterward, and that might have been driven by unsteady oxygen levels after the catastrophe. If this is right, Tarhan says, it suggests that “rather than being anomalous, the Ediacaran-Cambrian transition fits remarkably well within our understanding of other intervals of radiation and extinction.”
“This is an area of absolutely active debate,” Wood acknowledges. “Some people will agree, and others won’t, but I think that’s great. The diversification of animals is a profoundly important event, and we’re just adding a more holistic view of what happened.”