600-Million-Year-Old Embryos Found

By William J. Cromie

Gazette Staff

Embryos of relatively advanced animals, together with plants similar to modern seaweeds, have been found in rocks almost 600 million years old by Harvard scientists.

The discovery moves the bottom line of animal life tens of millions of years back in time and dramatically clarifies our view of the evolutionary history of Earth.

Most of today's animals can be traced back to fossils about 540 million years old. Before then, signs of life are so limited that it is difficult to determine whether or not those fossils have any relationship to modern creatures. Such a sparse record has led many scientists to believe that no animals who may have lived earlier have been preserved, probably because their soft, fragile bodies could not be fossilized.

But now, beautifully preserved three-dimensional embryos and seaweeds have been dug from a phosphate mine in southern China by Andrew Knoll, professor of biology, and graduate student Shuhai Xiao.

"It's amazing that the embryos are so similar to modern ones," Knoll comments. "They're either the oldest or among the oldest animals ever found. We believe they belong to bilateral animals. If so, they are the oldest known bilaterians. Unfortunately we don't know what kind of animals they would have become."

Bilateral animals have bodies in which the left and right sides are mirror images of each other. All animals from worms to humans are bilateral.

"The algae are similar to red, brown, and green seaweeds you can find along shallow coastlines today, although they are smaller in size," notes Xiao. "The detail is so well preserved, you can see individual cells."

These plants and the parents of the embryos lived in warm, shallow coastal waters, probably in the tropics or subtropics, the fossil finders believe. In the same rocks, they found the remains of more primitive one-celled floating plants and animals.

"It's a wonderful snapshot of a very ancient ecosystem," Knoll notes.

Mining History

The embryos appear as clusters of round cells inside a membrane, or thin covering. "We found a whole sequence from one or two cells to 32 and more," Xiao says. "Such embryos are very fragile, so they must have been buried by sediments or volcanic ash very quickly."

The fossils were discovered in the largest phosphate mine in Asia, located near the border of China and Vietnam, just outside the Chinese village of Weng'an.

This rich site of ancient life was first located in 1989 by Yun Zhang, one of Xiao's former professors at Beijing University. In 1990, Zhang and others invited Knoll to join a collaborative exploration of this so-called Doushantuo Formation.

On a trip there in 1994, Xiao, working with Knoll and Zhang, found the fossils. It took Xiao more than three years to realize what he had discovered.

The fossils, cemented into rocks of calcium phosphate, are exceedingly small. The embryos measure about a half-millimeter, or 1/50 of an inch across. Xiao did the long, tedious job of using acid to digest the rock and free the fossils at Harvard's Botanical Museum.

"I did not work on them steadily because I didn't know what we had," Xiao recalls. "Even when I found a few embryos, I was not convinced of what I had."

Xiao and Knoll showed the fossils to a number of other biologists and paleontologists before they agreed that history's oldest embryos had been discovered.

"When we displayed them at a meeting of world experts, Knoll says, people immediately recognized them as embryos. But they said it was hard for them to believe they are 570 million years old. It would be easier to believe they are 57 million years old or even younger."

The age of the embryos and algae was determined from radioactive dating of ash layers above and below the fossils. The uranium in the ash decays into lead at a steady rate that is like the ticking of a natural clock. These rock clocks, together with single-celled fossils and organic chemicals, dated the rock layer above the embryos at 550 million years and the layer below at 590 million years.

"One of our goals is to try to establish a more accurate date," Knoll says.

Xiao, Zhang, and Knoll published a report and photographs of their discoveries in today's issue of the journal Nature.

Reading the Rocks

The fossils "support the argument that the history of life on Earth can be read from its rocks," Knoll maintains. "The record of animal evolution does not end at 540 million years."

Traces of algae go back almost 2 billion years, but these consist of only dark smears of carbon on ancient rocks. Nicholas Butterfield, a former Ph.D. student of Knoll's, found actual fossils of red algae in 1.2 billion-year-old rocks in northern Canada. Fossils of the oldest animals have been more difficult to find because many of them lacked shells and other hard parts that can be preserved.

The new finds convince Knoll that the gap will eventually be filled. "We're looking all over the world, including the Grand Canyon, for 700-million, 800-million, even 900-million-year-old animal fossils. I think the book of rocks will show that the enormous variety of animals did not appear suddenly about 540 years ago, but evolved gradually over a longer time period. Perhaps we're already near the floor of animal life at 600 million years, but it's also possible that we're 100 to 200 million years away. This is what we have yet to establish."