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‹‹‹ Contents page for this issue     |     Winter 2010

Unearthing a Prehistoric Time Capsule

Scientist Searches for Clues About the Beginnings of Life

By Catherine Doss, College of Science

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 waaaaay 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 and Phanerozoic (542 million years ago to present). 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 woolly 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, as huge geological and biological changes have occurred during the Earth’s history.

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.

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

What the researchers think are the intermediate-stage embryos have a coiled, tubular embryo imbedded 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. The tubular embryo is also coiled, with three clockwise coils. In some specimens, the scientists found signs of uncoiling.

“We think it’s safe to say these are juvenile- or adult-stage organisms,” Xiao says. “Uncoiling indicates these embryos would have grown into the tubular organisms that we discovered earlier in our research.”

The group interpreted the embryos as something similar to corals.

Xiao says 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 what is special about calcium phosphate that the fossils were preserved in such great detail?

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

“Our best interpretation of this is that it represents dividing nuclei,” Xiao says. “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 cells that were incomprehensibly minute at 1 micrometer [10 to the minus 9 meters]. They surmised that these so-called nanocrystals started to nucleate and grow on the cell membrane before it 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, were preserved in rocks that predate the Cambrian period.

“This supports the existence of animals in the Precambrian,” he says. “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 9 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 animals 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.

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 that the atmospheric oxygen level rose to a fraction of modern levels.

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 that the atmospheric oxygen level rose to a fraction of modern levels.

“One of the reasons the numbers of animals exploded during the early Cambrian may have been oxygen,” Xiao says. “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.

“We went through road cuts, bed by bed, measuring and describing the exposed rock, then took small rock samples every few feet,” McFadden says. She collected about 200 samples.

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 says.

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

Where Next?

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

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

Where to next? 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.

Two of his graduate students, Jim Schiffbeauer and Bing Shen, are working on a research effort known as the Cambrian Project, which is partly supported by the National Science Foundation. This project studies the ecologic roles of early Cambrian animals and phytoplankton in South China and the Tarim Basin in far west China.

Future travels will take Xiao to the Himalayas, among other places.

Xiao’s fascination with geobiology started early in life. He studied under several internationally known geoscientists while working on his Ph.D. at Harvard. He earned his doctorate in 1998 and joined the Virginia Tech faculty in 2003 as an assistant professor. He was made full professor in 2008. Xiao’s research has been featured in prestigious scientific journals, such as Nature, Science, and PNAS.

Accolades aside, Xiao considers the discovery of fossilized Precambrian embryos his most surprising and rewarding professional accomplishment.

“Personally, I get the most satisfaction from looking at some of the earliest fossils,” he says. “Trying to find the beginning of ANYTHING is psychologically rewarding.”

 

Illustration by Natalie Goldsmith.

"One of the reasons the numbers of animals exploded during the base of the Cambrian may have been oxygen," says Shuhai Xiao. Photo by Jim Stroup.

Scanning electron photomicrographs of two fossil embryo specimens from the 600-million-year-old Doushantuo Formation in South China. The soccer-ball-shaped specimen is interpreted as an early stage (blastula) embryo and the baseball-shaped specimen is interpreted as an intermediate-stage helical embryo consisting of three clockwise coils. Each embryo used to be enclosed in an envelope, which was removed (a piece still remains in the soccer-ball-shaped specimen) so that the embryo itself is exposed. Embryos are about 0.55 to 0.75 millimeter in diameter. The background shows the Doushantuo rocks from which the embryos were extracted. Photo by Shuhai Xiao.

“It is not the strongest of the species that survives, nor the most intelligent that survives. It is the one that is the most adaptable to change.”
– Charles Darwin

Shuhai Xiao (left) and field assistant Guwei Xie (right) collected samples from the upper Doushantuo Formation. Photo by Kathleen McFadden.

This photo (field of view about 0.15 millimeter in width) is of an exceptionally preserved eukaryotic fossil from the Doushantuo Formation (635 to 551 million years old) in South China. High-resolution geochemical data from the formation indicate that the early diversification of eukaryotes may have coupled with episodic oxygenation of Ediacaran oceans. Photo by Shuhai Xiao.

Kathleen McFadden looked for evidence of oxidation events in the ancient rock along the Yangtze Gorges. Photo by Ganqing Jiang.

A 600-million-year-old four-cell embryo is magnified using a scanning electron microscope. The specimen is about 0.65 millimeter in diameter. Photo by Shuhai Xiao.

Shuhai Xiao examines a paper-thin slice of Doushantuo rock under a light microscope in the lab. Photo by Jim Stroup.

Yangtze Gorges and Yangtze River. Photo by Kathleen McFadden.