The earliest chapters in the history of life are in some ways the most interesting, but they’re also the hardest to read. The pages are badly stained and tattered, and the print was terribly small to begin with. You can occasionally trip on a dinosaur femur, but any evidence that remains from the earliest animals is incredibly subtle.
Despite the challenges, we’ve learned that the “Cambrian explosion” was far from the start of multicellular life. The title for “most ancient animal” currently belongs to the sponge. A recently described fossil just a millimeter across appears to be a 600 million year old sponge—that’s 60 million years before the start of the Cambrian period. But we can find chemical traces going back another 50 million years that have been interpreted as a calling card for sponges. Some have challenged that interpretation, however, on the grounds that this chemical “biomarker” is not unique to sponges and could instead have come from a type of algae.
The biomarker of interest is the remnant of a sterol (as in “cholesterol”), which is a key component in the cells of eukaryotes (as opposed to bacteria and archaea). With a small bit of the chemical structure lopped off, you get a sterane that can happily hang around in the rock record. A group of researchers led by MIT’s David Gold took a closer look at the sterol 24-isopropylcholesterol in sponges and other organisms to find out more about when the genes for it evolved—and what is most likely to have left it in 650 million year old rocks.
The researchers first examined the genomes of four organisms unrelated to either the sponges or the algae—selected to cover major branches of the tree of life—and found that none produce this particular sterol. They did, however, find a trend in these and other organisms: the more copies of a sterol-producing gene an organism has, the more complex the sterols it builds. The sterol in those Precambrian rocks is particularly complex, with more carbon atoms than your standard-issue sterol, so this suggested one possible way lifeforms may have evolved to make it.
That idea holds for the algae, too, which have enough copies of the gene to do the job. Sponges, however, break the rule. They don’t need additional copies of the gene to build this complex sterol. Instead, they may be getting extra rounds of sterol construction out of just two copies, the researchers say. Add it all up and it looks like these sponges and algae evolved independently to make 24-isopropylcholesterol rather than inheriting the ability from a common ancestor.
So when did they evolve that ability? The researchers analyzed the sterol-building gene in a number of organisms using estimates of mutation rates to come up with dates for gene duplication events. The results show that the sponges could have acquired the genetic equipment to build this sterol roughly 500 million to 700 million years ago, while the algae probably didn’t acquire it until at least 100 million years later. That would leave the sponges as the most likely source for the 650-million-year-old chemical calling card.
Given that this analysis places sponges 50 million years farther back in the early history of life than physical fossils can attest, the researchers note, “Such discoveries illustrate the significant incompleteness remaining in the early fossil record and reinforce the utility of biomarkers for understanding this critical period of biological evolution.”