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Unearthing a Prehistoric Time Capsule

By Catherine Doss

The Cambrian era has long been identified as the age when life as we know it came into existence — the time when trilobites, mollusks, and arthropods began existing on the ocean floor. Long before the Mesozoic era when dinosaurs roamed the Earth. We’re talking way back…542 to 488 million years ago.  But more
recently, scientists have uncovered evidence that animals actually existed before the Cambrian era in the Precambrian eon, which spans from 4.5 billion years ago to the beginning of the Cambrian period.

EARTH'S AGE – A BROAD PERSPECTIVE

The Earth’s age is divided into two eons: Precambrian (more than 542 million years ago) and Phanerozoic. The latter eon, in which the Cambrian period falls, is most frequently associated with living species, from the basic single-cell organisms, reptiles, sharks, and some plants (488 million years ago) to dinosaurs (250 million years ago) to large mammals (60 million years ago) to more modern-day animals, such as wooly mammoths and saber-toothed tigers (5 million years ago), to present-day man. The origin of man is only a tiny speck on the timeline of the Earth.

If the history of the Earth is scaled to a 24-hour day, man would have evolved at about 11:59 p.m. of that day. The Cambrian period, then, would be sometime between 9:12 p.m. and 9:29 p.m. of that day; any time before 9:12 p.m. would be the Precambrian eon.

Nearly all branches of natural science have contributed to the understanding of the main events of the Earth’s past, which includes huge geological and biological changes.

EVIDENCE OF PRECAMBRIAN LIFE

Shuhai Xiao, professor of geobiology at Virginia Tech, has devoted much of his career to researching life in the Precambrian eon. In 1997, Xiao and his colleagues discovered thousands of 600-million-year-old embryo microfossils in the Doushantuo Formation, a fossil site near Weng’an, South China. In 2000, Xiao’s team reported the discovery of a tubular coral-like animal that might be, for lack of a better word, a parent of these embryos.

“We’re trying to learn more about development of these animals between the embryonic and adult stages,” Xiao said. “While there are thousands of early-stage embryos, only 80 have been recovered that have advanced to an intermediary stage of development.”

The researchers identified what they think are intermediate-stage embryos that possess a coiled, tubular shape embedded in their egg cases. The egg cases have a groove on the surface, consisting of three clockwise coils. Using microfocus X-ray computed tomography imaging, the scientists virtually peeled off the egg case and exposed the embryo inside. In some specimens, the scientists found signs of the embryos uncoiling.

“Uncoiling indicates these embryos would have grown into the tubular organisms that we discovered earlier in our research,” Xiao said.

The group interpreted the embryos as something similar to corals.

Xiao said these discoveries were significant for two reasons. First, they confirmed what had long been suspected that, at half a billion years old, corals were the closest relatives to living animals. Second, the embryos were exceptionally well preserved.

These findings opened up even more avenues of exploration with fascinating questions, such as how did these early animals feed and how did they develop from a single cell?

And perhaps most importantly, how did these fossils get preserved and why are the fossils in calcium phosphate preserved in such great detail?

Using X-ray imaging technology, the researchers studied multiple-celled fossilized embryos and found each cell had a brownish mass near the center.

“Our best interpretation of this is that it represents dividing nuclei,” Xiao said. “That’s what is captured in fossil records. This is an amazing preservation.”

The group has some ideas about how these fossils were preserved. By using high-powered imaging equipment, researchers observed particles that were incomprehensibly small at 1 micrometer [10 to the minus 6 meters]. They surmised these so-called nanocrystals started to nucleate and grow on the cell membranes of the embryos before they degraded. A major part of Xiao’s research over the past decade has been to determine under what conditions the crystals grew and duplicated cells before they degraded.

GONE WITH A TRACE

The organisms Xiao and his colleagues discovered were soft-bodied without skeletons, and yet curiously, they were preserved in rocks that predate the
Cambrian period.

“This supports the existence of animals in the Precambrian,” he said. “We didn’t see the animal fossil, but we saw the trace of the animal.” Xiao and his colleagues estimated this animal to be at least 550 million years old (or about 8 million years before the Cambrian period).

Another question the researchers would like to answer is why a few Precambrian animals had skeletons.  Evidence of tunnels near soft-bodied fossils indicated that some of the species may have developed skeletons to serve as protection against predators. After the Cambrian period, when life became more prevalent and diverse, hundreds of animal species had skeletons.

THE ROLE OF OXYGEN-RELATED EVENTS

The rate of evolution of life accelerated in the Cambrian period. The sudden origin of many new species in this period is referred to as the Cambrian Explosion.
Oxygen is believed to have played a significant role in the rapid evolution of animals at the beginning of the Cambrian period. Xiao’s research has also taken him into the area of geochemistry. He has worked to answer a number of geochemical questions, such as determining how much oxygen was in the ocean water during these primitive ages.

Today we take oxygen for granted, but the atmosphere had almost no oxygen until 2.5 billion years ago, and it was not until about 600 million years ago when the atmospheric oxygen level rose to a fraction of modern levels.

“One of the reasons animals exploded during the base of the Cambrian may have been oxygen,” he said. “Suddenly there was more oxygen, and we know that almost all animals need oxygen to survive.”

Evidence of several oxidation events during the time period suggests a connection between oxidation and evolution.

Xiao credits the work of his former Ph.D. student Katherine McFadden, who, in a collaborative research project with other universities around the world, looked for evidence of oxidation events in ancient rock. McFadden studied the ratio of carbon and sulfur in these million-year-old rocks and determined from the carbon/sulfur cycles in the rock that several significant oxidation events occurred as life was forming. McFadden and her colleagues painstakingly analyzed layers of rocks at the site of an ancient sea in Yangtze Gorges of South China. The layers of sediment represent millions of years of deposits.

The triggers for the oxidation events remain elusive. “These events recorded in the ocean were probably related to oxygen in the atmosphere reacting with sediments on land,” McFadden said.

“After each oxidation event, we see more diversification of species,” Xiao said.  McFadden’s research was featured in notable scientific journals, including the Proceedings of the National Academy of Science.

WHERE NEXT?

Xiao’s research and groundbreaking discoveries have made him an internationally known and highly esteemed paleontologist and geobiologist. He traveled to
Siberia last year to conduct geographic surveys of prehistoric fossils.

“Even deep in Siberia, someone has read my papers or has heard my name,” Xiao said. “That is very satisfying to know that I have made an impact in the field of geosciences.”

Xiao’s future research interests are exploring how these Precambrian animals were preserved and determining if there are descendant animals in the Cambrian
period that can be linked to the Precambrian eon.