Ancient Fossils Have Evolution’s First Shells

A series of spectacularly preserved, 750 million-year-old fossils represent the microscopic origins of biomineralization, or the ability to convert minerals into hard, physical structures. This process is what makes bones, shells, teeth and hair possible, literally shaping the  animal kingdom and even Earth itself.

The fossils were pried from ancient rock formations in Canada’s Yukon by earth scientists Francis Macdonald and Phoebe Cohen of Harvard University. In a June Geology paper, they describe their findings as providing “a unique window into the diversity of early eukaryotes.”

That window opens into an evolutionary period less celebrated than the kaleidoscopic radiations of the Cambrian, but in its own way no less impressive. The simple single-celled organisms that dominated life’s first few billion years were rapidly becoming more complex, building a store of innovations that sustained some through the so-called Snowball Earth period, when Earth’s climate turned so cold that the equator resembled Antarctica.

One such innovation was biomineralization, though evidence for its occurrence at this time was inconclusive. Using molecular clocks and genetic trees to reverse-engineer evolutionary histories, previous research placed the beginning of biomineralization at about 750 million years ago. Around that time, the fossil record gets suggestive, turning up vase-shaped amoebas with something like scales in their cell walls, algae with cell walls possibly made from calcium carbonate and sponge-like creatures with seemingly mineralized bodies. But in each of these examples, caveats abound. What appears to be biomineralization might be a fossil illusion produced as soft tissue turned to stone.

In the new study, Cohen and Macdonald examined hundreds of fossils under microscopes. They found three common species of algae, dubbed Archeoxybaphon, Bicorniculum and Characodictyon, bearing mineral traces suggestive of biological origins. Crucially, the shapes of these organisms didn’t vary between specimens. The fossils of soft-bodied creatures, by contrast, tend to be distorted by compaction.

Once identified, these standard-bearers for biomineralization raise a basic question: Why bones at all? After all, life did perfectly well for three billion years without them.

In a commentary accompanying the finding, paleontologist Susannah Porter of the University of California, Santa Barbara hazards an explanation: Bones evolved as a defense against predators. That’s the best guess for why, 200 million years later, skeletons evolved independently in at least two dozen separate animal clades. The same basic dynamics should apply to single-celled organisms, too.

Read more at Wired Science