The Cambrian Period has been identified as the age when life as we know it came into existence. This is the time when trilobites, mollusks, and arthropods began existing on the ocean floor, as long as 542 million to 488 million years ago.
More recently, scientists have uncovered evidence that animals actually existed before the Cambrian Period in the Precambrian eon.
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 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.; 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 geological and biological changes have occurred during the Earth’s history.
Shuhai Xiao, professor of geobiology in the College of Science at Virginia Tech, has devoted much of his career to researching life in the Precambrian eon. In 1998, Xiao and his colleagues discovered thousands of 600-million-year-old embryo microfossils in the Doushantuo Formation, a fossil site near Weng’an, 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 the 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 structure 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 confirmed what had long been suspected that, at half a billion years old, corals and their relatives were some of the oldest animals. The preservation fossil embryos gave researchers an opportunity to study how early animals reproduced and developed.
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. “They left a trace in the fossil record.” Xiao and his colleagues estimated that the earliest animals are somewhere between 600 million and 550 million years old (or about 58 million to 8 million years before the Cambrian period).
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. “We know that almost all animals need oxygen to survive.”
Virginia Tech's Department of Geosciences focuses on research, education, and outreach dealing with the nature of the earth. Students and faculty investigate earth processes at scales that range from atomic to planetary.
The department operates the Museum of Geosciences at 2062 Derring Hall. Exhibits include gems, a full-scale model of an Allosaurus dinosaur skeleton found in Utah, and a collection of more than 13,000 minerals used for research, education, and display.
Take a look back at previous Spotlight features in our archive.