Molecular fossils shed light on ancient life

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 the tools of both geology and biology to study the evolution of life. With new technology, it's possible to recover chemical traces of life from ancient rocks, where animal fossils are scarce.

Lipids in particular can survive in rocks for hundreds of millions of years. Traces of sterol lipids, which come from cell membranes, have been found in rocks up to 1.6 billion years old. In the present day, most animals use cholesterol -- sterols with 27 carbon atoms (C27) -- in their cell membranes. In contrast, fungi typically use C28 sterols, while plants and green algae produce C29 sterols. The C28 and C29 sterols are also known as phytosterols.

C27 sterols have been found in rocks 850 million years old, while C28 and C29 traces appear about 200 million years later. This is thought to reflect the increasing diversity of life at this time and the evolution of the first fungi and green algae.

Without actual fossils, it's hard to say much about the animals or plants these sterols came from. But a genetic analysis by Gold and colleagues is shedding some light.

Don't make it, eat it

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

They found that the gene originated very far back in the evolution of the first animals, and then went through rapid changes around the same time that phytosterols appeared in the rock record. Subsequently, most lineages of animals 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're right, then the history of the smt gene chronicles a change in animal feeding strategies early in their evolution."

Co-authors on the paper are: at UC Davis, Tessa Brunoir and Chris Mulligan; Ainara Sistiaga, University of Copenhagen; K.M. Vuu and Patrick Shih, Joint Bioenergy Institute, Lawrence Berkeley National Laboratory; Shane O'Reilly, Atlantic Technological University, Sligo, Ireland; Roger Summons, Massachusetts Institute of Technology. The work was supported in part by a grant from the National Science Foundation.