Two groups of lowly marine worms are related to complex species including humans and starfish, according to new research. Previously thought to be an evolutionary link between simple animals such as jellyfish and the rest of animal life - the worms' surprising promotion implies that they have not always been as simple as they now appear.
Although the marine worms Xenoturbella and Acoelomorpha are very simple animals - they lack a developed nervous system or gut - they have been a source of much debate among zoologists. Acoelomorphs were reclassified in the1990's as an early branch of evolution - the crucial link between the very simplest animals such as sponges and jellyfish and the rest of the animal kingdom including humans, starfish, insects and molluscs.
Now, in research published online today in Nature, an international team involving scientists at the WTCHG, UCL (University College London) and the Université de Montréal have shown that neither type of worm is an early branch of evolution. They show that both groups descended from the same ancestor that gave rise to the complex groups of animals that includes vertebrates and starfish. This implies that the worms have in effect ‘evolved backwards' into much simpler looking organisms.
Specimens of Xenoturbella were collected from the mud at the bottom of a Swedish fjord where it eats bivalve molluscs; the acoelomorphs are found in various marine environments - one called Meara stichopi even makes its home in the throat of a sea cucumber. Scientists compared hundreds of genes from both Xenoturbella and the Acoelomorpha with their counterparts from a whole range of animal species to determine their evolutionary relationships.
The results show that the two groups constitute a newly classified phylum (a major division of life), which the authors name the 'Xenacoelomorpha'. The xenacoelomorph phylum joins the three known phyla of deuterostomes: vertebrates (including humans), echinoderms (e.g. starfish) and hemichordates (acorn worms).
For more information on Dr Copley's research, click here.