RNA research leads to paleontology without fossils

As long as 600 million years ago, our tiny marine ancestors may have had a sophisticated brain that released hormones into the blood and was connected to various sensory organs.

The evidence comes not from a fossil, but from the study of microRNAs – small RNA molecules that regulate gene expression – in animals alive today.

Scientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg have discovered that these molecules are found in exactly the same tissues in animals as diverse as sea anemones, worms, and humans, hinting at an early origin of these tissues.

Animals from different branches of the evolutionary tree have certain microRNAs in common: those which they inherited from their last common ancestor.

The EMBL scientists looked at the marine annelid Platynereis dumerilii, which is thought to have changed little over the past 600 million years.

They found that in Platynereis these microRNAs are highly specific for certain tissues and cell types and, what is more, that tissue specificity was conserved over hundreds of millions of years of evolution.

The scientists reasoned that if an ancient microRNA is found in a specific part of the brain in one species and in a very similar location in another species, then this brain part probably already existed in the last common ancestor of those species.

“By looking at where in the body different microRNAs evolved, we can build a picture of ancestors for which we have no fossils, and uncover traits that fossils simply cannot show us,” says Detlev Arendt, who headed the study.

The team found that annelids such as Platynereis and vertebrates such as human beings share some microRNAs. Some are specific to the parts of the central nervous system that secrete hormones into the blood, and others are restricted to other parts of the central or peripheral nervous systems, or to gut or musculature.

The implication is that our common ancestor already had all these structures.

Next, Arendt and colleagues plan to investigate the exact function of each of these conserved microRNAs – what genes they regulated, and what processes those genes were involved in.

Their findings are published in Nature.