Tiny oganisms a giant discovery for Williams College professor
WILLIAMSTOWN -- Phoebe Cohen is a detective, of sorts.
She's examining clues from the scene of a series of attacks by predators. Problem is, the clues are getting old -- more than 750 million years old.
Cohen, 32, an assistant professor of geosciences at Williams College, is a paleontologist and educator. In 2007, while working towards her PhD at Harvard University, she and her research team discovered evidence of predatory behavior in a Yukon field of geological samples from the Neoproterozoic era that are between 717 to 812 million years old -- which is even before the evolution of animal life.
The samples contained the fossilized remains of single-cell organisms that populated the oceans of the time.
Using a new take on a scientific technique that frees larger fossils from rocks, Cohen's team used mild acid to dissolve the carbonate rock material to uncover the microscopic fossils of hundreds of ancient organisms less than 10 microns across, or one-tenth the diameter of a human hair.
Until she looked in the microscope she had no idea what she would see.
"There's nothing cooler than looking in a microscope and seeing a fossil no one else has ever seen before," she said.
The samples showed that these organisms were able to form shield plating through biomineralization -- somewhat like exoskeletons -- that survived the eons long after the single-cell host had dissolved. The plate-armor weighed down the hosts, which sank to the bottom of the ocean to be trapped in sediment. Tectonic activity eventually propelled some of that sedimentary rock to the planet's surface near the peaks of a mountain range along the western border of the Yukon, a province in northwestern Canada.
Cohen said the development of the tiny armor plating is likely in response to external threats, such as predatory danger. She noted that this is the earliest evidence of predatory and defensive behavior in the single cell organisms -- which likely had an early influence in the evolutionary cycle of life on Earth.
"As far as we can tell, these are the first organisms to show evidence of using minerals to make hard parts, a behavior that is very common in today's environment," she said. "It's also probably the first evidence of predation, which has played a very important role in the evolution of animal life."
According to a colleague, Sara Pruss, an assistant professor of geosciences at Smith College in Northampton, Cohen's work "changes the way we think about ecology of the Neoproterozoic, because it is possible that skeleton-building indicates some need for protection, either from predators or the environment. This is a dramatic shift in how we have thought about biomineralization and has opened up new windows of exploration."
Another associate, Susannah M. Porter, associate director of the Earth Research Institute at the University of California at Santa Barbara, said Cohen's conclusions were surprising.
"These fossils represent not only the first good evidence for mineralized skeletons in eukaryotes, they also represent a rather unusual choice of mineral for a protist," she said.
Cohen's work had the additional value of bringing a new technique to the field of Precambrian paleontology that will allow scientists to discover microscopic relics in new areas that may be encased in different kinds of rock.
"Some of Phoebe's fossils were found in acid-resistant residues of limestone, which is a common sedimentary rock formed in tropical environments," Pruss said. "This type of rock has only rarely been examined for fossils in the Neoproterozoic, so Phoebe's work also broadened the way paleontologists search for fossils."
"In terms of new avenues of research," Porter noted, "this work has highlighted the potential of looking in carbonate rocks for fossils of early life. In the past, Precambrian micropaleontologists have focused on [shales and fine-grained silica] rock types and have pretty much ignored carbonates. We won't do that any more."
With part of Cohen's case solved, there is much more to do to establish the vast timeline of evolution and its early connections to modern life forms.
Last summer, Cohen and a research team went back to the Yukon to a spot a few miles east of the 2007 site. They were able to establish that the fossils were not present in that area.
This summer, Cohen said, she hopes to return to the original site to re-establish the presence of the fossils and to expand on the research and on the variety of samples gathered. It may not be action-packed -- there will not likely be a TV show called CSI: Neoproterozoic -- but for Cohen, it will be thrilling and could certainly be illuminating.
"It's still quite a mystery," she said. "For example, what was so special about that one spot? We haven't stopped looking."
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