If lower limbs aren’t used, get rid of them

Scientists say they have discovered the earliest known example of an animal evolving to lose body parts it no longer needed.

Facivermis, a worm-like creature that lived approximately 518 million years ago in the Cambrian period, had a long body and five pairs of spiny arms near its head, leading to suggestions it might be a “missing link” between legless cycloneuralian worms and a group of fossil animals called lobopodians, which had paired limbs all along their bodies.

But a new study by the University of Exeter and the Natural History Museum in the UK and China’s Yunnan University reveals Facivermis was itself a lobopodian that lived a tube-dwelling lifestyle anchored on the sea floor, and so evolved to lose its lower limbs.

Lead author Richard Howard says it is the earliest known example of “secondary loss”, which is seen today in cases such as the loss of legs in snakes.

“A key piece of evidence was a fossil in which the lower portion of a Facivermis was surrounded by a tube,” he says. “We don’t know the nature of the tube itself, but it shows the lower portion of the worm was anchored inside by a swollen rear end.

“Living like this, its lower limbs would not have been useful, and over time the species ceased to have them.”

The findings are published in the journal Current Biology.

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