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 in Nature, an international team lead by scientists from UCL (University College London) and the Universite de Montreal 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).

Professor Max Telford, from the UCL Department of Genetics, Evolution and Environment, and joint leader of the research said: "Because the simple Xenacoelomorpha are descended from the same ancestor that gave rise to complex groups such as vertebrates, echinoderms and hemichordates, these simple worms must have lost a lot of the complexity that they originally possessed."

Professor Telford said: "We can no longer consider the acoelomorphs as an intermediate between simple groups such as jelly fish and the rest of the animals. This means that we have no living representative of this stage of evolution: the missing link has gone missing!"

Professor Herve Philippe from the Universite de Montreal said: "This is the happy result of a more than ten year struggle with these highly unusual organisms that have proved very difficult to locate on the tree of life.

Improvements in DNA sequencing technology and in mathematical methods to infer evolutionary history were key to solving the conundrum of Xenoturbella and the acoelomorphs"

earlier related report
New Evolutionary Research Disproves Living Missing Link Theories
Montreal, Canada (SPX) Feb 14 - Evolution is not a steady march towards ever more sophisticated beings and therefore the search for the living "missing links" is pointless, according to findings published by a team of researchers led by Dr. Herve Philippe of the Universite de Montreal's Department of Biochemistry.

"Aristotle was the first to classify organisms - from the least to the most sophisticated. Darwin's theory of evolution continued this idea, with the concept of a hierarchy of evolution.

This way of thinking has led researchers and skeptics alike to look for less sophisticated ancestors in order to prove or disprove evolution," Philippe explained.

"What we now know is that evolution does not happen in a single direction - when people talk about a missing link, they're generally excluding the possibility of more sophisticated ancestors."

The researchers compared the genomes of two kinds of marine worms with simple morphology - Xenoturbellida and Acoelomorpha - with those of other animals.

They demonstrated that their previous position at the base of the bilateral symmetry animal group - that includes insects, mollusks and vertebrates - was inaccurate.

"Instead, we determined that Xenoturbellida and Acoelomorpha are closely related to the complex deuterostomes, which is a major lineage containing sea urchins, humans and sharks," Philippe said.

"I've put them in that order intentionally because it seems strange, which demonstrates our tendency to always put organisms in order of complexity." The findings mean that the worms had evolved from a more sophisticated ancestor through major simplifications.

"We did already know that most parasitic organisms had evolved to be less sophisticated than their ancestors - they lost certain abilities that they no longer needed. The independently living Xenoturbellida and Acoelomorpha do not fall in this category," Philippe said.

The research is a striking example for the important role of secondary simplification in evolution and is part of 20 year project that is nearing completion. The findings were published in Nature on February 10, 2011.

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