Present-day annelid worms, like earthworms and this bearded fireworm, have retained a gene to make some types of lipids that most animals have lost.
Credit: Johnandersonphoto
via Getty Images
Fig. 6: Summary of smt gene loss in the animals.A simplified animal phylogeny, with the hypothesized presence of smt visualized with blue lines. This tree provides a conservative estimate of the number of smt losses, as it excludes many understudied animal phyla that may also lack the protein.
Early organisms, particularly from before animals with hard body parts like teeth, bones and hard exoskeletons had evolved, leave few traces in the fossil record, but that's not to say they leave no trace whatsoever. What they leave is a chemical signature in the rocks that can last for hundreds of millions, even billions of years.
Sterol lipids, for example are highly stable chemicals that come from cell membranes and can be found in rocks dated to 1.6 billion years old. Since they can only be produced by living organisms, they are compelling evidence for the existence of life when those rocks were laid down.
In the present day, most animals use cholesterol — sterols with 27 carbon atoms (C
27) — in their cell membranes. In contrast, fungi typically use C
28 sterols, while plants and green algae produce C
29 sterols. The C
28 and C
29 sterols are also known as phytosterols.
C
27 sterols have been found in rocks 850 million years old, while C
28 and C
29 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.
Early organisms needed to synthesise their own sterols and did so using a gene called smt, but, as more sources of sterols became available by eating fungi and algae, so this gene became redundant and was eventually lost from many evolutionary lines. When this gene disappeared from these lines shows when they began consuming these new sources of sterols.
By constructing a family tree for this gene using data from first annelids then across animal life in general, the UCDavid team were able to map when this gene was lost onto changes in the sterol record in rocks - and they mapped closely to the chemical record in the rocks.
According to a news release from the University of California Davis (UCDavis), where David Gold, associate professor in the Department of Earth and Planetary works in this new field of molecular paleontology, using the tools of both geology and biology to study the evolution of life:
