For many years, scientists had considered two groups of simple marine worms — the Acoelomorpha and Xenoturbella — as representatives of a very early stage of evolution, perhaps as an intermediate species between jellyfish and more complex organisms. But surprising new findings about the two groups — whose members are so simple that they possess only one opening for both feeding and elimination — suggest that Acoelomorpha and Xenoturbella are genetically more closely related to the ancestral group that eventually gave rise to humans, and in fact are far closer to humans than humans are to mollusks or insects.
Worms Ancestral to Deuterostomes
After genetic analysis, researchers discovered that both Acoelomorpha and Xenoturbella were in a direct line of descent from the same ancestor that gave rise to the deuterostomes, complex vertebrates that include humans. Initial research had shown that the worms belonged to a much earlier, simpler branch on the evolutionary tree, but these results had been affected by a genetic quirk called long branch attraction, in which species appear to be closely related due to rapid evolution, but are actually much more distantly related than their genes would suggest.
Once the data were recalibrated to eliminate the bias, scientists were able to reclassify the two species of worms into a completely new phylum called the xenacoelomorpha. This phylum is the fourth that falls under the deuterostome umbrella, which also includes three other phyla that comprise the vertebrates, the echinoderms, and the hemichordates.
Micro RNAs
The data that most strongly suggested that the worms were due for reclassification was the presence of RNA molecule sections called micro RNAs. Two of these sections were found in each of the worm groups, and since it is known that micro RNAs only appear gradually during the evolution of a species, researchers are almost certain that the worms belong to a later, more complex evolutionary branch.
From Complex to Simple
Both Xenoturbella and Acoelomorpha are undoubtedly very simple creatures today, but the history recorded in their genes suggests that because they are more closely related to the complex deuterostomes, they may once have been more complex organisms themselves. Researchers are curious as to what caused the worms to simplify in such a manner, speculating on which features they may have lost over the course of their evolution, but far more study is needed to determine the answers to such questions.
Source:
Jeanna, Bryner. "Lowly Worms Get Their Place in the Tree of Life." LiveScience. 9 Feb. 2011. Web. 23 Feb. 2011. livescience.com
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
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