It appears that vertebrates have some distant relatives they didn’t previously know about. New research says that complex species such as humans and starfish are related to two species of lowly marine worms.
According to a University College London press release, the worms were previously thought to be an evolutionary link between simple animals like jellyfish and the rest of animal life. The worms’ recent promotion to a higher form of life suggests that they were not always as simple as they now appear.
Even though the marine worms Xenoturbella and Acoelomorpha are extremely simple animals – they don’t have a developed nervous system or gut – they have been debated heavily among zoologists. Acoelomorphs were recategorized in the1990’s as an early part of evolution - the important connection between the simplest animals such as sponges and jellyfish and the rest of the animal world including humans, starfish, insects and mollusks.
Currently, in research that was published online in Nature, an international team of scientists from UCL (University College London) and the Université de Montréal have shown that neither class of worm is an early stage of evolution.
They have shown that both types of worms descended from the same relative that gave rise to the complex species of animals that includes vertebrates and starfish. This means that the worms have basically “evolved backwards” into organisms that have a much simpler appearance.
The conclusions show that the two types of worms make up a newly classified phylum (a major division of life), in which the authors name the ’Xenacoelomorpha’. The xenacoelomorph phylum joins the three known phyla of deuterostomes: vertebrates (including humans), echinoderms (like 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.”
He also 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 Hervé Philippe from the Université de Montréal 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".