A 500 million-year-old fossilized brain could reveal how heads first evolved.

One of the oldest fossilized brains ever discovered could also pinpoint a key era in the amazing evolutionary transition from soft to hard-bodied arthropods, a group containing most modern insects and spiders, the University of Cambridge reported.  

The recent study looked at a soft-bodied trilobite and a strange submarine-like creature, both of which were in the arthropod family. They discovered the hard plate (anterior sclerite) and eye-like features at the front of their bodies were connected through the nerve traces linked to the front part of the brain that controls vision.

The findings allowed the scientists to compare a group of swimming predators (anomalocaridids) to reveal important similarities between the anterior sclerite and a head plate, suggesting they both had a common origin. The preserved brains in these ancient fossils demonstrated anterior sclerite as a bridge between the head of anomalocaridids and more modern arthropods.

"The anterior sclerite has been lost in modern arthropods, as it most likely fused with other parts of the head during the evolutionary history of the group," said Javier Ortega-Hernández, a postdoctoral researcher from Cambridge's Department of Earth Sciences, who authored the study. "What we're seeing in these fossils is one of the major transitional steps between soft-bodied worm-like creatures and arthropods with hard exoskeletons and jointed limbs - this is a period of crucial transformation."

The researchers spotted bright spots at the front of the fossilized bodies, which are believed to have been photoreceptors embedded in the anterior sclerite. These receptors were connected to the front part of the brain in a similar way to what can be seen in today's arthropods, which first appeared during the Cambrian explosion 500 million years ago. Before this "explosion" of new species, most creatures on Earth were soft-bodied.

"Heads have become more complex over time," Ortega-Hernández said. "But what we're seeing here is an answer to the question of how arthropods changed their bodies from soft to hard. It gives us an improved understanding of the origins and complex evolutionary history of this highly successful group."

The findings were published in a recent edition of the journal Current Biology.