New Fossils Push Back Earliest Single-Celled Skeletons 200 Million Years
These 810 million-year-old fossils suggest early marine life protected itself by making skeletons from phosphorus.
Imagine trying to eat food without teeth, or walk without a backbone.
While the single-celled organisms that dominated early Earth’s oceans didn’t have much need for teeth, they did find some evolutionary advantage in making their own minerals. According to research published last week, life has been making its own hard parts for at least 810 million years, about 200 million years longer than previously thought. It’s the first occurrence of what scientists call biomineralization, and it could give us deeper insight into both the evolution of living things and Earth’s early climate.
Previously, scientists hadn’t found evidence of organisms making their own skeletons before the rapid diversification of life around 490-550 million years ago known as the Cambrian explosion. But in the mid-2000s, Phoebe Cohen, a Williams College paleontologist and lead author of the recent study, re-examined fossil evidence that would end up setting the date back much further.
Cohen was in the Yukon studying microfossils from a 200-foot-thick section of lime, mudstone, and slate, and she suspected that they could have been formed by biological processes. These microfossils were originally discovered in the 1980s, when they were dated to the Neoproterozoic Era, about 250 million years prior to the Cambrian explosion. But scientists at the time were unable to determine the exact age of the fossils, nor were they able to discern whether they were crafted by geology or life itself.
To unlock the minerals’ secrets, Cohen and her co-author Nicholas Tosca borrowed a method of microscopic imaging from materials scientists that uses electrons instead of light. That imaging revealed highly organized, long, and slender crystals that were oriented and spaced in such a way that they could only have been made biologically, Cohen said.
The minerals possibly served as an armor-like casing constructed by now-extinct organisms, though Cohen and her team did not find conclusive evidence of their function, nor did they find remnants of the cells themselves, which rarely preserve for so long. Cohen said these parts could have been a defense against a predation, since some single-celled organisms today use hard parts for protection. Plus, evidence in the fossil record suggests that predatory single-celled organisms would have existed at the same time as Cohen’s fossils.
Shuhai Xiao, a paleobiologist at Virginia Tech who did not contribute to the paper, said it’s a reasonable hypothesis. “Organisms don’t make something just because there is material available,” Xiao said. “There must also be an ecological need.”
Cohen and Tosca’s research cleverly uses “the way a mineral is constructed” to determine its origin, said Sara Pruss, a Smith College geologist who did not contribute to this study. It’s the first study to use a detailed analysis of crystal structure to find evidence of biomineralization. The same technique could be used to find other biomineralization from the same era in other parts of the world.
The analysis also revealed that the minerals were composed of a particular flavor of phosphorus called hydroxylapatite, which is rarely found in oceanic single-celled organisms today. Cohen and her team found evidence in the rocks suggesting that the oceans at the time contained less oxygen than today. This provided a suitable environment for elevated phosphorus concentrations, which may have helped spur its use in biomineralization.
“Hydroxylapatite is not very stable, so the preservation of the mineral in rocks of this age is remarkable,” Xiao said.
The team was able to constrain the age of the fossils to an unusually narrow time-frame – 810 million years ago, give or take about 6 million years. As a result, Cohen thinks she’ll be able to find more fossils because she will only have to search a much narrower slice of the fossil record than before. “I was looking through a hundred million years of time before, and now I’m looking through ten million years,” she said.
The new study leaves a few questions unanswered, and poses a few new ones of its own. For one, biomineralization has evolved several different times. This study illustrates how one branch of life gained the ability to make skeletal parts, but there are still several others to puzzle out. And then it blasts open a period of more than 200 million years in which scientists so far have found no evidence of biomineralization, a range of time that Cohen points out is as wide as that between the present day and the dawn of the dinosaurs.
“We’re seeing a little bit of a mystery there,” Cohen said of the gap. They will now try to pinpoint when the shift from using phosphorus to using carbon occurred and find out what, if anything, organisms were making during that time. “That’s the big open question,” Cohen said.
Photo credit: Phoebe A. Cohen