Molecular fossil study sheds light on changing dietary strategy in ancient life

Paleontologists are gaining insight into life more than a billion years ago, thanks to chemical traces in ancient rocks and the genetics of living animals. Research published in Natural communications combines geology and genetics, showing how changes in early Earth led to a change in the way animals ate.

David Gold, associate professor in the Department of Earth and Planetary Sciences at the University of California, Davis, works in the new field of molecular paleontology, using tools from geology and biology to study the evolution of life . Thanks to new technologies, it is possible to recover chemical traces of life in ancient rocks, where animal fossils are rare.

Lipids, in particular, can survive in rocks for hundreds of millions of years. Traces of sterol lipids, originating from cell membranes, have been found in rocks 1.6 billion years old. Currently, most animals use cholesterol, sterols containing 27 carbon atoms (C₂₇)—in their cell membranes. In contrast, fungi generally use C₂₈ sterols, while plants and green algae produce C₂₉ sterols. The C₂₈ etc₂₉ sterols are also called phytosterols.

VS₂₇ sterols have been found in rocks 850 million years old, while C₂₈ etc₂₉ traces appear around 200 million years later. This is thought to reflect the increasing diversity of life at this time and the evolution of early fungi and green algae.

Without actual fossils, it’s difficult to say much about the animals or plants these sterols came from. However, a genetic analysis by Gold and colleagues sheds some light.

Don’t do it, eat it

Most animals are not able to make phytosterols themselves, but they can get them by eating plants or mushrooms. Recently, it was discovered that annelids (segmented worms, a group that includes the common earthworm) have a gene called smt, which is necessary for making longer-chain sterols. By examining the smt genes of different animals, Gold and his colleagues created a family tree of smt, first within annelids and then in animal life in general.

They found that the gene appeared very far in the evolution of early animals and then underwent rapid changes around the same time that phytosterols appeared in the rock record. Subsequently, most animal lineages lost the smt gene.

“Our interpretation is that these phytosterol molecular fossils record the rise of algae in ancient oceans and that animals abandoned phytosterol production when they could easily obtain it from this increasingly abundant food source ” Gold said. “If we are correct, then the history of the smt gene relates a change in the feeding strategies of animals early in their evolution.”