This molecular clock centered on SMT was created using BEAST, with white diamonds denoting fossil calibrations detailed in the Supplementary Methods. Putative gene duplication events in animals are indicated at relevant nodes with colored shapes: circles represent species-specific gene duplications in the study, while triangles signify events involving multiple species. Fossil calibrations are also represented by white diamonds. Phan. = Phanerozoic; Neo. = Neoproterozoic; Meso. = Mesoproterozoic; Paleo. = Paleoproterozoic. The source data can be found in a Source Data file. — Nature Communications
Paleontologists are gaining insight into life over a billion years ago through chemical traces in ancient rocks and the genetics of extant animals. A study published on Dec. 1 in Nature Communications integrates geology and genetics to illustrate how early Earth transformations influenced changes in animal dietary habits.
David Gold, an associate professor in the Department of Earth and Planetary Sciences at the University of California, Davis, delves into molecular paleontology, a nascent field that combines geological and biological tools to explore the evolution of life. Advanced technologies now enable the retrieval of ancient chemical remnants from rocks, especially in the absence of abundant animal fossils.
Among these remnants, lipids have remarkable preservation potential lasting hundreds of millions of years. Sterol lipids, derived from cell membranes, have been detected in rocks dating back 1.6 billion years. Presently, most animals utilize cholesterol—C27 sterols—in their cell membranes. In contrast, fungi typically employ C28 sterols, while plants and green algae produce C29 sterols, also known as phytosterols.
C27 sterols have been identified in rocks dating 850 million years, followed by the appearance of C28 and C29 traces approximately 200 million years later. This sequence likely mirrors the burgeoning diversity of life during that period, including the emergence of the first fungi and green algae.
In the absence of physical fossils, discerning the specific animals or plants linked to these sterols remains challenging. However, a genetic analysis conducted by Gold and his team offers some insights.
Feeding Habits of Early Animals
Most animals lack the ability to synthesize phytosterols but can acquire them through plant or fungi consumption. Recent findings unveiled a gene named smt in annelids (segmented worms, including common earthworms) essential for producing longer-chain sterols. By examining smt genes across different animal species, Gold and collaborators constructed a genealogy for smt within annelids and extended it to encompass animal life in general.
Their investigation revealed that the gene originated deep in the evolutionary history of early animals, undergoing rapid modifications around the period coinciding with the appearance of phytosterols in ancient rocks. Subsequently, the majority of animal lineages relinquished the smt gene.
“We interpret these phytosterol molecular remnants as evidence of the proliferation of algae in ancient oceans, prompting animals to forgo phytosterol synthesis as they could readily acquire it from this increasingly abundant food source,” explained Gold. “If our hypothesis holds true, the history of the smt gene documents a shift in animal feeding behaviors early in their evolution.”
Co-authors of the study include Tessa Brunoir and Chris Mulligan from UC Davis; Ainara Sistiaga from the University of Copenhagen; K.M. Vuu and Patrick Shih from the Joint Bioenergy Institute, Lawrence Berkeley National Laboratory; Shane O’Reilly from Atlantic Technological University, Sligo, Ireland; and Roger Summons from the Massachusetts Institute of Technology. The research received partial funding from the National Science Foundation.