The position of the Acoelomorpha flatworms in the hierarchy of evolution—familiarly known as the “tree of life”—has played a crucial role in the understanding of animal evolution for decades. The tree of life currently locates less complex cnidarians, such as jellyfish and sponges, at the “trunk” and more complex deuterostomes, such as humans and fish, at its “branches.” For the last 15 years or so scientists have placed the Acoelomorpha flatworm somewhere in the middle—until now.
The change is suggested in a paper co-authored by Associate Professor of Biological Sciences Kevin J. Peterson, titled “Acoelomorpha Flatworms are Deuterostomes Related to Xenoturbella.” The paper, published in the February 10, 2011, issue of the journal Nature, introduces a new analysis, and in the process redefines the classification of acoelomorph flatworms and Xenoturbella in relation to the tree of life.
Peterson calls the findings “totally unexpected.” “We never thought that they would go where all of the molecular data say that they really do go.” He continues, “The research completely changes our understanding of the Precambrian fossil record of animals as we no longer have any potential morphological intermediates between jellyfish and more complex animals like insects and vertebrates.”
Acoelomorph flatworms are small, simple worms found in marine environments. Long considered to be a holdover of a primitive organization of animals, the organisms do not have a separate mouth or anus and lack a central nervous system, key featuresassociated with protostomes and deuterostomes, both placed at the highest branches of the evolutionary tree. However, Peterson and his collaborators argue that the acoelomorphs are actually deuterostomes, closely related to modern echinoderms such as sea urchins and sea stars, and vertebrates such as ourselves.
This result was based partly on elucidating their microRNA repertoire. MicroRNAs are small regulatory ribonucleic acid (RNA) molecules, and the team discovered that acoelomorphs have microRNAs that are only shared with deuterostomes suggesting that “humans are more closely related to acoel flatworms than we are to insects or clams or earthworms,” says Peterson,a molecular paleobiologist. “Because the acoelomorphs are nested high up within the tree, their simple morphology is one of secondary simplification rather than being primarily primitive.”
The findings are not without controversy. Many researchers and evolutionary biologists have objected to the group’s finding. In a News Feature published in the same issue of Nature, writer Amy Maxmen acknowledges the controversy surrounding the novel findings stating that the “vehemence of the debate shows just how important these worms have become to evolutionary biology.”
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