From slime you came, and to slime you may return

Acoelomorpha

Forget about missing links; evolution does not necessarily imply a forward march to ever more sophisticated organisms. Evolution can also go backwards, with organisms losing characteristics that were common to their ancestors.

Biologists led by Hervé Philippe at the Université de Montréal compared the genomes of two types of marine flatworms – Xenoturbella and Acoelomorpha – and demonstrated that the classification of Acoelomorpha at the base of the animal group which includes insects, mollusks and vertebrates is wrong. The worms are in fact closely related to the lineage that contains sea urchins, humans and sharks.

“I’ve put them in that order intentionally because it seems strange, which demonstrates our tendency to always put organisms in order of complexity,” says Philippe.

Philippe’s research shows that Acoelomorph worms have evolved from a more sophisticated ancestor through major simplifications. Note that we are not talking about the loss of abilities which are no longer needed. You can think of it instead as a form of evolutionary regression.

Further reading

Philippe et al., “Acoelomorph flatworms are deuterostomes related to Xenoturbella”, Nature 470, 255 (2011)