The forensic route to ancestors
"In chordates versus vertebrate, modern chordates are a broad phylum of animals that includes three subphyla: cephalochordates such as the fish-like lancelets, or amphioxus; urochordates such as sea squirts; and vertebrates such as lamprey and humans.

The common ancestors of these three modern groups are known as stem chordates. From these animals evolved the ancestors of the three modern groups, respectively stem cephalochordates, stem urochordates and stem vertebrates.

The Leicester team found that as these modern animals decay, the features that evolved more recently, and therefore distinguished them from their ancestors, rotted first. The last things to decay were features such the notochord.

This means that as they rot, the lamprey and the amphioxus seem to slip from 'crown' positions at the top of the phylogenetic tree back down to 'stem' versions of their actual species and eventually look like the common ancestor of them all.

Dr Rob Sansom, lead author (right) says: “Interpreting fossils is in some ways similar to forensic analysis – we gather all the available clues to put together a scientific reconstruction of something that happened in the past. However, we are dealing with life from millions of years ago, and… what we want to get at is what an animal was like before it died and, as with forensic analysis, knowing how the decomposition that took place after death altered the body, provides important clues to its original anatomy.”
 
Palaeontologists sometimes overlook decay, according to Sansom, “probably because spending hundreds of hours studying the stinking carcasses of rotting fish is not something that appeals to everyone.” But the rewards are worth the discomfort.
 
“These fossils provide our only direct record of when and how our earliest vertebrate ancestors evolved” adds (left) Dr Mark Purnell, one of the study leaders.

“Did they appear suddenly, in evolutionary explosion of complexity, or gradually over millions of years? What did they look like? – in what ways did they differ from their worm-like relatives and how did this set the stage for later evolutionary events? Answers to these fundamental questions - the how, when and why of our own origins - remain elusive because reading earliest vertebrate fossil record is difficult.”
  
Dr Sarah Gabbott, (right) who with Purnell conceived the Leicester study, is an expert. “Only in the most exceptional circumstances do soft-tissues, such as eyes, muscles and guts, become fossilised, yet it is precisely such remains that we rely on for understanding our earliest evolutionary relatives: half-a-billion years ago it’s pretty much all our ancestors had.”

The team says that their findings also apply to key fossils that have been described as early chordates — such as Metaspriggina and Cathaymyrus. These fossils cannot be reliably labelled as either chordate or vertebrate 'stem' animals, they say, as they could actually be the decayed remains of either.

Beyond fossils the MicroRNA route
The last ancestor we shared with worms, which roamed the seas around 600 million years ago, may already have had a sophisticated brain that released hormones into the blood and was connected to various sensory organs. The evidence comes from the study of (left) microRNAs – small RNA molecules that regulate gene expression – in animals alive today.

Scientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany, have discovered that these molecules are found in the exact same tissues in animals as diverse as sea anemones, worms, and humans, hinting at an early origin of these tissues in animal evolution. Their findings, published in Nature, also open new avenues for studying the current functions of specific microRNAs.

Animals from different branches of the evolutionary tree possess specific microRNAs that evolved only in their lineage. But they also have microRNAs in common: ones which they inherited from their last common ancestor, and which have been conserved throughout animal evolution.

The EMBL scientists looked at the marine annelid Platynereis dumerilii (right), which is thought to have changed little over the past 600 million years. They visualised where these conserved microRNAs are expressed, and compared Platynereis with other animals.

They found that in Platynereis microRNAs are highly specific for certain tissues and cell types and discovered that tissue specificity was conserved over hundreds of millions of years of evolutionary time.

The scientists reason 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.  So they were able to glean a glimpse of the past, an idea of some of the traits of the last common ancestor of worms and humans.

“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, (left) who headed the study:

“But uncovering where these ancient microRNAs are expressed in animals from different branches of the evolutionary tree has so far been very challenging.”

“We found that annelids such as Platynereis and vertebrates such as ourselves share some microRNAs that are specific to the parts of the central nervous system that secrete hormones into the blood, others that are restricted to other parts of the central or peripheral nervous systems, or to gut or musculature”, says Foteini Christodoulou, (right) who carried out most of the experimental work.  “This means that our last common ancestor already had all these structures.”

 Knowing where microRNAs were expressed in our ancestors could also help scientists understand the role of specific microRNA molecules today, as it gives them a clue of where to look.

“If a certain microRNA is known to have evolved in the gut, for instance, it is likely to still carry out a function there”, explains EMBL scientist Peer Bork, (right) who also contributed to the study.

Arendt and colleagues want next to investigate the exact function of each conserved microRNAs – what genes they regulated, and what processes those genes were involved in – in an attempt to determine what their role might have been in the ancient past.

Serendipitous how different approaches are closing in on our earliest ancestors.