Finding Turns Back Clock for Earth’s First Animals

The animal kingdom arose on Earth at least a billion years ago, twice as long ago as previously believed, researchers who used a molecular stopwatch to time life’s earliest evolution said Thursday.

Experts said the finding appears to undercut theories, based on fossils, that suggest that modern forms of life evolved more rapidly.

In particular, it renews a spirited debate over one of life’s most extraordinary episodes, a carnival of creation called the Cambrian explosion. It is widely believed that during this period, which began about 545 million years ago, almost all the main lines of the animal types known today--from burrowers, grazers and predators to the earliest vertebrate ancestors of humanity--suddenly appeared.

The new research, published today in Science, suggests that animal life started well before the Cambrian and that the pace of natural selection, in fact, is far more gradual, indicating that evolution acted no differently at the dawn of animal life than it does among species today, experts said.

“Our results cast doubt on the prevailing notion that the animal phyla [groups] diverged explosively during the Cambrian . . . and instead suggest that there was an extended period of divergence during the mid-Proterozoic, commencing about a billion years ago,” concluded researchers Gregory Wray, Jeffrey S. Levinton and Leo H. Shapiro, all of the State University of New York at Stony Brook.

If their finding holds up, the animal kingdom had hundreds of millions of years longer to develop than the fossils reveal.

Although life began on Earth about 3.8 billion years ago, there is little physical evidence to show how it evolved until the Cambrian period, when the remains of countless creatures were preserved as fossils in rock formations.

The Cambrian fossils record a burst of life unmatched in the history of evolution. Nearly all animal groups, or phyla, known today, as well as a collection of outlandish animals whose fossils cannot be assigned to any living group, appeared in about 10 million years--a split-second in the history of Earth--according to fossil evidence.

However, the scientists based their new finding not on fossils but on the molecular evidence of evolution found in genes drawn from living species of 16 animal groups. That allowed them to estimate when all animals last shared a common ancestor with other forms of life.

Evolutionary biologist Geerat Vermeij at UC Davis said the new research has “lent strong support to the idea that animals emerged much earlier than the Cambrian period.”

Bruce Runnegar, a UCLA scientist who helped pioneer the use of molecular genetics to calibrate early evolution, called the research “significant.”

“There is a clear paradox,” he said. “The molecular data are in clear conflict with the fossil record. Either the molecular data are not correct, or the sudden radiation of the animals is not correct. You can put some money on the molecular point of view, although I suspect there will be a flurry of critiques,” he said.

Indeed, at UC Berkeley, paleontologist James Valentine was openly skeptical.

“I don’t believe it,” he said. “I doubt their extrapolations are correct. They say this obviates the Cambrian explosion but that is not really right.” During the Cambrian period, he said, animals with skeletons and shells and other hard body parts show up for the first time. And, he noted, not a single new animal phylum has evolved since.

“The accuracy of the molecular clock is not something that is universally agreed on,” cautioned Harvard University paleontologist Andrew Knoll. “Any number tends to come with big error bars.” Nonetheless, he said, “the idea that animals should have originated much earlier than we see them in the fossil record is almost inescapable.”

To arrive at their conclusion, the researchers compared the differences between sets of seven genes drawn from rattlesnakes, blue whales, snails, hummingbirds, goldfish, tiger sharks and 200 other species. They used these to determine the rate at which the molecular sequences of the genes evolved, calibrating the timing of this “clock” against known fossils of these species.

Then they compared the same sequences to specimens drawn from outside the animal kingdom, to determine how long ago they may have shared a common ancestor.

“What we found,,” said Wray, “was that the genetic distances are so huge between different groups of animals that the most reasonable explanation is that they last shared a common ancestor long, long before the Cambrian.

“If we are right, what that means is that the Cambrian is not as special as it once was thought,” he said. “There was this concern that something qualitatively different was going on [in the Cambrian period]. There was a lot of concern about this among evolutionary biologists.”

Wray speculated that the relatively sudden appearance of fossils during the Cambrian period represents not the beginning of animal life, but the development of hard body parts, such as skeletons and external shells, which are more readily preserved. The shells and body armor could have evolved in response to the rise of large predators, he said.

“That seems like the most reasonable scenario,” Wray said. “They were squishy and tiny and then something acquired teeth and the neighborhood suddenly got tougher. Prior to that, animals were soft-bodied.”

* POPE’S VIEW ON EVOLUTION: Evolution is more than a theory, John Paul says. A14

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

The Dawn of Animals

Evolutionary biologists said on Thursday that the animal kingdom began about 1.2 billion years ago. That pushes back by hundreds of millions of years what is thought to be the era when the variety of creatures now living diverged from a common ancestor.

The Cambrian “Explosion”

Remarkably short period in which almost all forms of animals known today suddenly appeared.

*

Creatures of the Burgess Shale

A remarkable window into the life of the Cambrian period is offered by the Burgess Shale of the Canadian province of British Columbia, which contains remains of more than 70,000 tiny creatures that lived about 540 million years ago.

3.8 billion years ago: Earliest known life

1 billion years ago

600 million years ago

*

First invertebrates

*

550 million years ago

*

First trilobites

First fish

*

400 million years ago

*

First amphibians

300 million years ago: First reptiles

200 million years ago: First dinosaurs

*

First birds

100 million years ago: Dinosaur extinction

50 million years ago: Spread of mammals

2 million years ago: First hominids

Source: Science, “The Book of Life,” “The Rise of the Vertebrates”