Animals with skeletons did not exist before about 550 million years ago. Then, scientists have proposed, atmospheric oxygen levels rose and the chemistry of the oceans changed in such a way that animals could harness the minerals required to build hard structural parts. A new analysis of ancient rock layers in Siberia provides support for this idea, showing that the oceans became rich in skeletal building blocks around the same time the first fossils of animals with skeletons start to appear.
“This paper goes a lot of the way toward answering the question of why animals first grew skeletons, and evolved into the animals that we have today,” says Ashleigh Hood, a sedimentologist at Yale University who was not involved in the study.
The atmosphere today is about 20% oxygen, but that was not always the case. Before about 800 million years ago, it was as low as 0.1% of present day levels. Then, for reasons not fully understood by geologists, oxygen levels started climbing.
To understand the link between rising oxygen levels and the evolution of skeletons, Rachel Wood, a geobiologist at the University of Edinburgh, and her team studied ancient rock layers found deep in Siberia’s wilderness near the Yudoma River. The rocks, formed from layers of sediment deposited in ancient oceans, contain not only fossils but also sedimentary clues to how the oceans’ chemistry shifted during the time when skeletons are thought to have arisen. Wood spotted a series of ocean chemistry shifts during the Ediacaran and Cambrian periods, which together stretch from about 635 to 485 million years ago. Until roughly 545 million years ago, the rocks are rich in the mineral dolomite, which is believed to have formed in the oceans when oxygen levels were low, Hood says. After that, as levels of atmospheric oxygen increased, limestone rock predominates.
The limestone at the field site contains the minerals aragonite and calcite, which animals need to build skeletons; aragonite and calcite crystals form much faster and with less energy than dolomite, allowing animals to harness them for skeleton building in a way they can’t with dolomite. Also, on land, agents of erosion like wind and rain bombarded the continents at unusually high rates. This caused one particular nutrient needed for the formation of calcite and aragonite—calcium—to flood the oceans, which further fueled the evolution of skeletons, the team reports this month in Geology.
This skeletal revolution is reflected in the rocks themselves. In the dolomite-rich layers, the fossils of soft-bodied organisms predominate—like Aspidella, a soft, frond-shaped creature that anchored itself to the seafloor. Then, in the limestone, one of the first known skeletonized animals appears—Cloudina, a millimeter-scale creature made of a calcified shell that looks like a stack of ice cream cones. From such beginnings, skeletonized animals would go on to evolve into such familiar forms as fish, shellfish, dinosaurs, and, eventually, humans.
Hood praises the authors for showing how rising oxygen levels, ocean chemistry, and life changed in a coordinated way. The team is “linking three separate paths,” she says. Both Hood and Wood agree that, now that this story is known from Siberia, the next thing to do is to see if the tale holds true at other sites around the world.