Rock reader: Shuhai Xiao deciphers the geologic past to prepare for the future
Xiao studies the interactions between life and Earth, especially at a critical point in geologic history when animal life became much more abundant. For this original scientific research, he will be inducted into the National Academy of Sciences.
One day, 320 million years before the first dinosaur, a creature dragged its tail across a sandy stretch of ocean floor and died.
Fast forward to 2017, Virginia Tech geobiologist Shuhai Xiao and his collaborators marveled at what was left of that creature entombed along with the track it left behind — and clocked it as the oldest convincing evidence of a moving animal.
“The animal’s tail and trail were preserved together, and that’s critical because usually you only get one or the other,” Xiao said. “This fossil shows us who did it and what they did.”
The finding is one of Xiao’s many discoveries that contributed to excellence in original scientific research, for which he will be inducted into the National Academy of Sciences on April 26. Membership in the academy is one of the highest honors given to a scientist in the United States.
A geobiologist with a geological history focus, Xiao looks to the rocks to understand the interaction between life on Earth and the Earth itself — specifically at a time period that took place 500 million to 600 million years ago, when animals emerged explosively onto the scene.
What drove this rapid expansion of animal life? There are several hypotheses, but a leading theory holds simply that oxygen became more abundant. Xiao has spent the last decade testing this hypothesis by inventorying how oxygen moved through the environment. To gauge the amount of oxygen present in ancient environments, Xiao collaborates with Virginia Tech geochemists Ben Gill and Rachel Reid.
“Scientists can’t take an oximeter to a fossil to directly measure the oxygen, but oxygen leaves traces in the rock record,” said Xiao, who is a faculty member in the Virginia Tech College of Science. “The more oxygen present, the more we see certain types of elements concentrated in the rock.”
As Xiao and his team ponder the level of oxygen from their study of rocks, they are plumbing the fossil record to validate that the increase of oxygen levels and the expansion of animals happened at the same time.
“It’s kind of like in forensic science when you establish an alibi,” Xiao said. “They have to be there at the same time to be connected.”
Another factor that could have led to the growth of animal populations is what Xiao refers to as the predator-and-prey arms race. A predator, for example, runs faster to catch its prey while the prey develops a new way to hide or escape, spinning a feedback loop that leads to more and different kinds of animals.
It’s the interactions — the give and take — between life and Earth that has most engaged Xiao throughout his career. For instance, the relationship between animals and oxygen is a two-way street. Yes, animals consume oxygen and exude carbon dioxide, Xiao said, but it’s more complex because animals siphon and shuffle oxygen all over their environment. Sediment-dwellers will pump oxygen-rich water into their burrows for irrigation while coral reefs bring massive bioproductivity through oxygen distribution in seas around the globe.
“We think a lot about how humans change the Earth. We don’t think as much about how other organisms changed it in much bigger ways billions of years before,” Xiao said. “But what’s different with humans is the pace at which we are changing our planet. It’s unprecedented and startling.”
Xiao is drawing lessons from the geologic past, not only to provide context and guidance in the face of climate change, but to enhance the search for life on other planets.
“People in the business of looking for life on other planets really need to know what we are finding on Earth,” Xiao said. “If we were to find evidence of life out there, it’s much more likely that we would find fossils of simple organisms preserved in rocks than animals walking around on the surface.”
With support from the National Science Foundation, Xiao is sharpening his life-finding tools. Building on an extensive foundation of expert field work, Xiao’s collaboration with geochemists in the Geoscience Biogeochemistry Laboratory at Virginia Tech has brought a new perspective to his research in the past few decades.
Thus equipped, Xiao is flipping the pages back before early animals to shed light on the origin and diversification of all eukaryotes — organisms whose cells have a nucleus — which evolved into multicellular organisms and are credited for ushering in a whole new era for life on Earth, including animals, plants, and fungi.
“Rocks record stories that happened in the geologic past,” Xiao said. “They’re basically a book of history. But a rock cannot speak for itself, so we have to decode it.”
With each new chapter, Xiao learns more about how organisms interacted with each other, how they changed the Earth, and how the Earth changed them in turn.
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