Around 520 million years ago, many major groups of animals appear in the fossil record for the first time. Trilobites belonged to the same lineage as today's crustaceans and insects.
By CARL ZIMMERThe name Myllokunmingia may not ring a bell, but it is worth knowing. This 520-million-year-old creature was the size of a guppy, with a tiny swordfish-like fin running high over its back. The fossils it has left behind preserve traces of a skull.
Humans have a skull, too. This and a number of other traits we share with Myllokunmingia reveal it to be one of the oldest, most primitive vertebrates yet found. It is, in other words, a hint of where we came from.
Myllokunmingia emerged during one of the most important phases in the history of life, an evolutionary boom known as the Cambrian explosion (named for the geological period when it took place). Over the course of about 20 million years, the oldest known fossils of most of the major groups of living animals appear, revealing a rapid diversification of life that led directly to humans.
“It’s rapid in geological terms, but it’s probably not rapid to anyone who’s not a geologist,” said Paul Smith, the director of the Oxford Museum of Natural History.
By some estimates, the first animals evolved about 750 million years ago. But it’s not until around 520 million years ago that many major groups of living animals left behind their first fossils. For decades, scientists have searched for the trigger that set in motion this riot of diversity in the animal kingdom.
Recently, Dr. Smith and his colleague David Harper of the University of Durham took a look at the hypotheses that have been offered about what caused the Cambrian explosion. “It became apparent just how many hypotheses there were out there,” Dr. Harper said. “Thirty-plus over the past 10 years.”
The scientists found that many of those explanations had boiled the cause down to just one trigger. Geologists suggested geological causes. Ecologists proposed ecological ones. Many of those ideas have merit, Dr. Smith and Dr. Harper argue in a commentary in this week’s Science, but it’s a mistake to search for a single cause. They propose that a tangled web of factors and feedbacks were responsible for evolution’s big bang.
Long before the Cambrian explosion, Dr. Smith and Dr. Harper argue, one lineage of animals had already evolved the genetic capacity for spectacular diversity. Known as the bilaterians, they probably looked at first like little crawling worms. They shared the Precambrian oceans with other animals, like sponges and jellyfish. During the Cambrian explosion, relatively modest changes to their genes gave rise to a spectacular range of bodies.
But those genes evolved in bilaterians tens of millions of years before the Cambrian explosion put them to the test, notes Dr. Smith. “They had the capacity,” he said, “but it hadn’t been expressed yet.”
It took a global flood to tap that capacity, Dr. Smith and Dr. Harper propose. They base their proposal on a study published last year by Shanan Peters of the University of Wisconsin and Robert Gaines of Pomona College. They offered evidence that the Cambrian Explosion was preceded by a rise in sea level that submerged vast swaths of land, eroding the drowned rocks.
“There’s a big kick that correlates with the sea level rise,” Dr. Smith said of the fossil record. He and Dr. Harper propose that this kick happened thanks to the new habitats created by the sea level rise. These shallow coastal habitats were bathed in sunlight and nourished with eroding nutrients like phosphates. Animals colonized these new fertile habitats, Dr. Smith and Dr. Harper argue, and evolved to take up new ecological niches.
But these great floods also poisoned the ocean. The erosion of the coastlines released calcium, which can be toxic to cells. In order to survive, animals had to evolve ways to rid themselves of the poison. One solution may have been to pack the calcium into crystals, which eventually evolved into shells, bones, and other hard tissues. Dr. Smith doesn’t think it’s a coincidence that several different lineages of bilaterians evolved hard tissues during the Cambrian explosion, and not sooner.
These shells and other hard tissues sped up animal evolution even more. Predators could grow hard claws and jaws for killing prey, and their prey could evolve hard shells and spines to defend themselves. Animals became locked in an evolutionary arms race.
This new ecological food web grew even more complex. Bigger predators evolved that could eat smaller predators. Meanwhile, some bilaterians burrowed into the sea floor for the first time, allowing oxygen-rich seawater to flow into the sediment. Those first burrowers profoundly transformed the world’s oceans, creating yet another habitat that other oxygen-breathing animals could also invade. “That drives the diversification onward,” said Dr. Smith.
Kevin Peterson, a biologist at Dartmouth, praised Dr. Smith and Dr. Harper for pointing to the right way to study the Cambrian explosion. “We are long past identifying single triggers for the event,” he said. Dr. Peters agreed that taking a holistic view of the Cambrian explosion would lead to a better understanding of it. “It’ll be a fun next decade,” he predicted.
But Philip Donoghue of the University of Bristol does not think the links Dr. Smith and Dr. Harper use in their hypothesis are tight enough yet. Questions still remain, for example, about how long vertebrates and other animals groups already existed before they left behind fossils like Myllokunmingia. If animals diversified earlier, then scientists will need to look at earlier causes.
“Timing,” said Dr. Donoghue, “is everything.”
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