Mysterious Ancient Sea Bug: Did It Inspire Every Spider on Earth?

The garden variety spider you encounter might just have its origins in an ancient sea creature resembling a trilobite.

Mollisonia symmetrica, a marine arthropod dating back 500 million years, is now considered by scientists to possibly be the earliest form of arachnid. This discovery hints at the beginnings of the lineage that would evolve into today’s diverse array of spiders, at least in terms of their cranial structure.

In a recent study featured in Current Biology and discussed in Science Alert, a team from the University of Arizona, Lycoming College, and King’s College London reanalyzed a fossilized Mollisonia. They uncovered a brain structure strikingly similar to that of modern spiders.

This brain organization more closely resembles an altered version of a spider’s brain rather than that of a horseshoe crab, with which Mollisonia had previously been categorized.

Despite our extensive knowledge of arachnids, their exact origins have remained somewhat elusive. It has been widely believed that arachnids, including spiders, scorpions, mites, and ticks, evolved from a terrestrial ancestor. However, this new finding suggests their evolutionary journey might have started in the ocean, well before they were known to inhabit land around 430 million years ago.

The Blueprint for Spiders Provided by an Ancient Sea Insect

Neuroscientist Nicholas Strausfeld notes that arachnid brains are unique in the animal kingdom. The fossilized neural architecture of Mollisonia shows primitive versions of brain regions present in modern spiders, which I find quite unsettling (yes, I’m not a fan of spiders at all).

This resemblance in neural design isn’t just a random evolutionary fluke that just happened to persist through the ages. Rather, it represents a purposeful design emerging from an ancestor that challenges our traditional view of spiders.

This ancient creature, equipped with compound eyes, pincers, and a sophisticated neural network capable of complex tasks, likely passed down the foundational brain structure of arachnids. The intricate connections between its brain and appendages might explain the predatory prowess of arachnids on land.

Strausfeld suggests that the influence of Mollisonia goes beyond just spiders. He speculates that a creature resembling Mollisonia might have ventured onto land, feeding on insects, which in turn may have driven those insects to develop flight as a survival strategy.

This adaptation by the insects would have required Mollisonia to evolve as well, possibly leading to the development of web-spinning abilities to catch these now airborne meals. This interplay of evolutionary pressures showcases how the evolution of one species can drive the evolutionary trajectory of another, necessitating further adaptations for survival.

Though Mollisonia doesn’t look much like a spider, its internal structure suggests it could have been an early stage in the evolutionary path leading to today’s spiders, perhaps even responsible for the fright you experience upon spotting one in your bathroom.