15 Oct 2018. 700 Million years old Fossil steroids!
The Zumberge et al. (2018) article published in Nature Ecology & Evolution consolidates the oldest current fossil evidence for animals. Fossil compounds further support that sponges, and therefore animals, were thriving in Neoproterozoic marine environments at least 100 million years prior to the Cambrian Explosion of animal phyla and body plans.
We often hear about organisms being fossilized but chemical compounds from these organisms can also be fossilized, and these can be used as evidence, or ‘biomarkers’ for the presence of particular organisms in ancient rocks. Steroids, such as the fossil products of cholesterol (found in most animals), are an ideal chemical biomarker because their structure can be preserved for hundreds of millions of years, especially in petroleums and bitumens. Several steroids are produced by specific groups of organisms whose presence can then be traced back in time to investigate the evolution of life.
Molecular data suggests that sponges are the first animals to appear on our planet in the Neoproterozoic (more than 600 Mya). However, fossils of sponges only appear with the Cambrian explosion (some 540 Mya) along with most other animal groups. Back in 2009, Gordon Love and collaborators published evidence of a steroid fossil biomarker (24-isopropylcholestanes) as old as 717-635 Myr, and still produced by demosponges today (Love et al., 2009). This constituted the earliest biomarker evidence for animals. This evidence was however challenged and failed to convince the whole scientific community.
Our latest finding published in Nature Ecology & Evolution of an additional and novel ancient steroid biomarker (26-methylstigmastanes) adds considerable weight and confidence to the view that these compounds are biomarkers from sponges. The new biomarker therefore provides a significantly more robust argument for the occurrence of animals in the Neoproterozoic.
Another important novel aspect of our paper is that we have identified the groups of modern sponges that are capable of producing the compounds, which would have yielded the corresponding fossil steroids in the rocks. Today, sponges that produce this new biomarker live in the North Atlantic (even off the Swedish West coast) and the Pacific, in the deep sea as well as in shallow tropical waters. This aspect of the work was achieved through collaboration with sponge experts from the Dept. of Medicinal Chemistry at Uppsala University, Sweden (Dr. Paco Cárdenas and Dr. Sunithi Gunasekera), who notably provided important advice and information about living sponges as well as specimens for analyses.