Humans' new relative: a 25-centimeter-long worm

Researchers claim that the "Xenoturbella" has so far been mistakenly classified as a mollusk, while in fact it belongs to the same division from which humans evolved

Yuval Dror

A 25-centimeter-long worm found in the muddy bottom of a Swedish fjord is a close cousin of humans; This is what a group of zoologists from Great Britain and Denmark claims.

According to the group of researchers, "Xenoturbella bocki" - (Xenoturbella bocki) "strange flatworm" in Latin - which was discovered back in the late 40s, has been misclassified until now, while in fact it is a close relative of the human species.

In an article published in Nature magazine, the researchers claim that a genetic analysis of the worm showed that for years it was mistakenly classified in the category of molluscs, which includes clams and snails, while in fact it belongs to the same evolutionary division in the animal world from which humans and other vertebrates evolved.

If the researchers' determination is correct, then the worm and humans are descendants of the same biological parents, who resembled worms, and who lived on Earth 500 million years ago. Evolution caused these worms to evolve into reptiles, fish, birds and mammals.

According to Dr. Max Telford from the University of Cambridge, who was a partner in the research, "We can prove that of all the invertebrates that live on Earth, the Xenoturbella is the closest relative to humans. It's amazing to think that we evolved from something similar to this worm."
According to Dr. Micha Ilan, a zoologist from Tel Aviv University, the simplicity of the Xenoturbella is impressive. "She is missing genitals, a digestive system, various body cavities and more. In fact, it is not at all clear how they reproduce."

According to the researchers, a genetic analysis of the Xenoturbella in 1997 led to the incorrect conclusion that it belongs to the oyster category. The mistake was due to the fact that the worm usually eats a type of clam that lives near it, which confused the results of the analysis.

"At the very beginning of the study we had a hard time believing that Xenoturbella was really a close relative of the oyster, because they don't even look the same," Telford said.

The research team was able to locate the worm at a depth of 100 meters below sea level, in the muddy bottom of a Swedish gulf. From a re-examination of the worm's genetic sequence, it appears that it is the most primitive descendant of the "mouth gnats" group, from which vertebrates and humans evolved.

"Worms such as Xenoturbella do not have a skeleton at all, so we do not have many fossils that captured them. If it does indeed belong to the group of oral bivalves, then we have before us an animal that is very close to the ancestor of the whole group and now it will be possible to study it", says Ilan.

The researchers themselves wish to qualify the results of their research. According to them, the 500 million years of evolution also affected the Xenoturbella and many of the features that were in the ancestor of the worm degenerated and disappeared, and hence Xenoturbella should not be seen as a model that is completely identical to the ancestor from which the group that also gave birth to humans developed.

According to Telford, the next challenge in research will be to find out how these creatures reproduce.
For information in Nature