MIT scientists have successfully identified what is likely the first animal to appear on our planet. Genetic analyses of molecular fossils has revealed that sea sponges have been residing on Earth for the last 640 million years, long before the Cambrian explosion took place. According to the new study, sea sponges seem to have predated the Cambrian explosion – basically the period during which most animal groups started appearing – by at least 100 million years.
As part of the research, recently published in the Proceedings of the National Academy of Sciences journal, the scientists discovered traces of a rather strange molecule in rocks that are over 640 million years old. Upon examination, the team found sea sponges to be the source of this particular molecule, which in turn suggests that these multi-cellular organisms might have been the first animals to inhabit our planet. Speaking about the find, David Gold, a postdoctoral student at MIT’s Department of Earth, Atmospheric and Planetary Sciences (EAPS), said:
We brought together paleontological and genetic evidence to make a pretty strong case that this really is a molecular fossil of sponges. This is some of the oldest evidence for animal life.
Over the years, paleontological excavations have resulted in the discovery of numerous fossils dating nearly 540 million years ago. Although a comparatively short event, the Cambrian explosion saw the birth and evolution of most animal groups, from single-celled beings to larger, more complex multi-cellular organisms. While a lot is known about this period, scientists are still unsure about what exactly happened before the Cambrian radiation.
Fossils belonging to the time before the Cambrian period are peculiar in many respects. For the current research, the team turned to molecular fossils, which basically refers to the miniscule amounts of molecules that have remained inside the cracks and crevices of ancient rocks long after the animal itself has withered away. Gold, who co-authored the paper along with EAPS professor Roger Summons, said:
There’s a feeling that animals should be much older than the Cambrian, because a lot of animals are showing up at the same time, but fossil evidence for animals before that has been contentious. So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates.
The scientists examined these rocks in search of 24-isopropylcholestane (or 24-ipc), a lipid compound or sterol that is in fact a slightly-modified version of cholesterol. During a 1994 study, of which Summons was a member, a team of MIT researchers first discovered 24-ipc in extremely high quantities in rocks dating back to the Cambrian period and earlier. At the time, they thought that sponges or their prehistoric predecessors might have been the source of this molecule.
Later in 2009, researchers from the University of California at Riverside undertook a detailed study of ancient rocks found in Oman, using incredibly advanced uranium-lead dating techniques. According to them, 24-ipc was found in sizable amounts in 640 million-year-old rocks, thus pointing toward what was likely the earliest evidence of animal life on Earth. Talking about that discovery, Summons stated:
This research topic has a 20-plus-year history intimately connected to MIT scientists. Now, in 2016 David Gold has been able to apply his skills and the new tools of the genomic era, to add a further layer of evidence supporting the ‘sponge biomarker hypothesis.
To confirm the findings, the current team of researchers set out to first identify the gene associated with 24-ipc. They went on to find the organisms possessing that particular gene, following which they traced the timeline of the gene’s evolution in these creatures. As part of the research, the scientists examined the genomes of nearly 30 different organisms, including fungi, sea sponges, plants as well as algae. Gold said:
What we found was this really interesting pattern across most of eukaryotic life.
During their research, the scientists were able to zero in on a single gene, whose task is to produce certain types of sterols in organisms that carry an additional copy of this gene. As the team points out, some species of algae and sea sponges possess an extra set of the SMT gene, unlike their relatives. To determine when the additional copy first appeared in the organisms, the scientists created a sort of evolutionary tree with the help of available fossil records.
As it turns out, the sea sponges evolved the extra SMT gene some 640 million years ago, which is same as the age of the ancient rocks in which 24-ipc was found. By comparison, the algae got around to it much later. This in turn indicates that the sea sponges were likely the first animals to have appeared on Earth. Gold was reported saying:
This brings up all these new questions: What did these organisms look like? What was the environment like? And why is there this big gap in the fossil record?. This goes to show how much we still don’t know about early animal life, how many discoveries there are left, and how useful, when done properly, these molecular fossils can be to help fill in those gaps.
The research was partially funded by the NASA Astrobiology Institute and the Agouron Institute.
The article was originally published in our sister-site HEXAPOLIS.