Spiders and scorpions evolved in the sea, not on land.

A tiny, remarkably well-preserved fossil of a marine invertebrate that lived 500 million years ago has revealed that spiders, scorpions , and their relatives evolved in the ocean , and not after arriving on land as a widely held theory. This is according to a study published in the journal Current Biology, led by the University of Arizona, which discovered that the brain and nervous system of this small aquatic ancestor were already exactly the same as those of modern arachnids . According to the study's authors, the presence of these first spiders on land may have also contributed to the evolution of a fundamental defense mechanism for the insects they fed on: wings.

Spiders and scorpions have existed for approximately 400 million years and have since dominated the Earth as the most successful group of arthropod predators. Based on the fossils found so far, however, they appeared to have evolved exclusively on land, a belief that researchers led by Nicholas Strausfeld have refuted thanks to a detailed analysis of a fossil of a now-extinct marine animal called Mollisonia symmetrica .

Proof that Mollisonia was already an arachnid is its brain and nervous system. As in modern spiders and scorpions, the anterior part of the body contained radial nerves that controlled the movements of five pairs of appendages, and other small, shorter nerves that led to a pair of pincers reminiscent of the fangs of spiders and other arachnids. But what really convinced the study's authors was the brain 's unique organization : as in spiders, it is the exact reverse of that found in crustaceans, insects, and millipedes . "This is a fundamental step in evolution," says Frank Hirth of King's College London, co-author of the study, "which appears to be unique to arachnids."