by Erling Friis-Baastad
Once upon a time, say 1,000 million years ago or so, the Earth’s seas were probably reasonably safe places in which to drift, even if you were a microscopic, unicellular creature. Eventually, however, tiny predators appeared in the aquatic Eden and for their new food source it was time to get tough or die out.
During the Neoproterozoic, between 700 to 800 million years ago, members of the aquatic feast came upon a solution to their vulnerability: spiked armour. The evidence for this momentous development was recently discovered in limestone rocks found on the slopes of Mt. Slipper just over the Yukon border from Eagle, Alaska.
While working on her PhD thesis back in 2007, paleontologist Phoebe Cohen, along with her Harvard colleague, geologist Francis Macdonald, snatched hundreds of pounds of the fossiliferous limestone from under the nose of a proprietary grizzly who patrolled the Mt. Slipper region of what had long ago been the floor of a primordial sea.
After they packed their hard-won treasure back to Massachusetts, the scientists set to work analyzing samples in a laboratory. The results were spectacular – even if they required a scanning electron microscope to fully appreciate. “We found the oldest evidence of a living organism using a mineral to form a hard part,” says Cohen from her office at MIT. The process is called biomineralization, she adds.
Ancient fossils had been recovered from the Mt. Slipper area in the late 1970s and ‘80s by University of Alaska scientist Carol Allison, says Cohen. They’d been located in chert. “Chert is made up of silica. And silica is the same mineral that makes up glass, so when you find a piece of chert, you can make a very thin slice of it and look at it under a microscope. Chert, before it hardened into rock, hundreds of millions of years ago, trapped single-celled organisms.”
Looking through a standard light microscope, at powers as low as 40X, reveals the presence of the fossils but not the most startling details. But it turns out that the tiny Neoproterozoic era fossils are found not only in chert, but also in the limestone in which the chert is found, says Cohen. This makes all the difference. “We looked in the limestone, which had never been done before. Everyone always assumed that the fossils would only be found in chert, but we thought that because these little creatures were making little hard parts, they would be preserved in regular old limestone as well.”
Dissolving the bits of limestone in a weak acid (similar to vinegar) freed up the fossils. Suddenly scientists could employ the electron microscope and ratchet up the magnification.
Cohen says the change is like going from an aerial view of Manhattan to actually walking among the buildings. The view becomes three-dimensional. “You can see all sides of the fossils and really look into the nooks and crannies and see what is going on,” she says.
What was going on, at least 800 million years ago, was that some vulnerable little organisms were refusing to take it anymore, refusing to be gobbled up without a struggle. They covered their soft single-celled selves in about 10 to 20 hard plates and even developed spikes.
“Predation is actually a powerful force in evolution,” says Cohen. “It’s a powerful force in how creatures live and move while trying to protect themselves from being eaten. Meanwhile, the predators are trying to become better predators.” This process is often compared to modern-day arms races, she adds.
How long did it take from these single-celled organisms to come up with their spiked chain mail? Perhaps not very long, says the paleontologist. In fact, it may have happened too quickly to leave much of a trace in the fossil record. “Our resolution isn’t good enough,” she adds. When one considers that Yorkshire terriers likely evolved from wolves in a matter of thousands of years, one gets a sense of how quickly changes can occur.
As well, these particular fossil creatures have only been found on Mt. Slipper. If there were fossils from elsewhere it might be possible to piece together a more complete picture of the evolution of the armour.
Meanwhile, says Cohen, though she has been stressing predation protection as a function of the armour plates, she wants to draw attention to the beautiful, intricate shapes the armoured plates took. “Why does an organism make such intricate structures?” she asks, then adds: “In most structures in biology, there isn’t a single function for anything. Structures can play multiple roles.”
One of the other roles is that the plates may have helped organisms stay up on top of the ocean rather than sink down out of the sunlight. The sharp spikes may have improved buoyancy.
So are these beautiful creatures like snowflakes: no two quite alike? Cohen says that members of the same species share characteristic patterns, but that there “are always little variables.” It’s the miniscule variations among individuals of a species that make natural selection possible.
Cohen, who currently serves as an education and outreach worker for the MIT NASA Astrobiology Team, spends much of her time wandering around in the Neoproterozoic. The era attracted her for a variety of reasons, not the least of them is that “it immediately preceded the Cambrian radiation, when animals really took off and diversified,” she says.
“Just thinking about, ‘How do you go from a single-celled organism to an animal?’ It’s a little mind-blowing, all the changes in the environment and evolution that lead toward the world we see today.”
This column is co-ordinated by the Northern Research Institute at Yukon College with major financial support from Environment Yukon and Yukon College/ The articles are archived at www.taiga.net/yourYukon.