Spider Brains May Have Evolved in Ancient Oceans

The distinctive brains of spiders may have begun evolving in the oceans, well before their ancestors made the move onto land.

A new examination of a 500-million-year-old fossil has uncovered striking similarities between the brains of extinct marine arthropods and those of living arachnids. The research was carried out by scientists from the University of Arizona and Lycoming College in the US, and King’s College London.

The finding enters a contentious debate about the evolutionary origins of spiders and their relatives.

The origins of spiders and other arachnids

Spiders, scorpions, mites and ticks are now almost entirely terrestrial. The prevailing view holds that these arachnids evolved from a shared ancestor that lived on land.

Where that ancestor originated is a separate mystery. Land-dwelling arachnids are related to other ocean-dwelling ‘chelicerates’, including sea spiders and horseshoe crabs, but the fossil record is extremely incomplete.

“It is still vigorously debated where and when arachnids first appeared, and what kind of chelicerates were their ancestors, and whether these were marine or semi-aquatic like horseshoe crabs,” explains University of Arizona neuroscientist Nicholas Strausfeld.

For a small creature, moving from the sea to land is a major transition, regardless of how many legs it has.

The oldest remains accepted as belonging to an arachnid come from a 430-million-year-old scorpion that lived on land. However, the new evidence suggests that arachnids as a group may have begun diverging from other chelicerates much earlier.

A fossil reveals an arachnid-like brain

From the outside, Mollisonia symmetrica may not appear particularly spider-like. It looks rather like a woodlouse with many small legs, and was previously regarded as an ancestor of horseshoe crabs.

Researchers have now used light microscopy to image the fossil’s central nervous system, revealing something unexpected.

The nervous system of Mollisonia is unlike that of a horseshoe crab, crustacean or insect. Instead, its pattern of radiating neural centres was flipped backwards, as it is in an arachnid.

“The arachnid brain is unlike any other brain on this planet,” explains Strausfeld.

In the Mollisonia fossil, this distinctive nervous system appears to supply numerous legs and two pincer-like mouthparts, in the position where modern spiders have fangs.

“This is a major step in evolution, which appears to be exclusive to arachnids,” says Frank Hirth, an evolutionary neuroscientist at King’s College London.

“Yet already in Mollisonia, we identified brain domains that correspond to living species…”

That resemblance appears unlikely to be coincidental. Further statistical analysis by Hirth and his colleagues suggests arachnids probably inherited structures similar to those of Mollisonia, rather than evolving them independently by chance.

If the team is correct, Mollisonia sits at the base of the arachnid lineage, as a sister to horseshoe crabs and sea spiders.

From marine brains to life on land

Although the idea remains speculative, the distinctive brain structure in the Mollisonia lineage may have helped its later descendants survive on land.

Neural ‘shortcuts’ to the legs and pincers, for example, might have made complex movements such as walking or weaving webs easier to control and coordinate.

“We might imagine that a Mollisonia-like arachnid also became adapted to terrestrial life making early insects and millipedes their daily diet,” Strausfeld theorises.

The earliest arachnids on land may even have driven insects to evolve wings, and therefore flight. Airborne prey could then, in turn, have prompted the evolution of webs.

From the ocean floor to the treetops, arachnids have shown a truly enviable ability to adapt as conditions changed.

The study was published in Current Biology.