A 500-million-year-old brain could help biologists understand the evolution of animal heads in the ancient past. The fossil provides rare evidence of a critical change in the evolution of brains, suggesting how heads were shaped by biology and the environment half a billion years ago.

Arthropods, a group of animals that includes modern-day spiders, insects and crustaceans, were studied by investigators examining ancient evolution. Specifically, the science team examined one soft-shelled group of animals known as trilobites as well as another creature that resembled a torpedo and which possessed a hard shell and limbs.

The anterior sclerite, a hard plate in the head, as well as simple photoreceptors, eye-like structures in the heads, were connected to the front part of the brain, similar to the structure of some arthropods in the modern world. The fossil record suggests some of these animals had the feature, while others did not, and the location varied.

Anomalocaridids, a common predator from the same period, was also found to have a similar plate, suggesting a common ancestor for the animals. These animals are usually considered to be ancestors of arthropods, although their body structures are quite distinct. Researchers in this study believe the anterior sclerite could be a "missing link" between the two groups of animals.

"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. 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," Javier Ortega-Hernández from the University of Cambridge said.

During the Cambrian Explosion, the diversity of life increased at a dramatic rate, producing many of the forms we still see around us today. Hard shells and limbs with joints were among the body adaptations first seen at this time, as previous life consisted primarily of forms such as algae and jellyfish. This crucial evolutionary transition from soft to hard shells, identified during an investigation of the ancient fossils, likely allowed to animals to capture prey and avoid predators, much as animals today use modern eyes.

"The question here is how do we go from something that looks nothing like an arthropod to something that looks completely like an arthropod?" Ortega-Hernández asked.

Fossils examined in the study were recovered from the Burgess Shale in western Canada, one of the most fossil-rich areas in the world. This is one of the few places on Earth were fossils are found with soft tissues, such as brains, preserved for study. Soft tissues are usually destroyed by bacteria, chemical and geological processes before the fossilization process is complete. Investigators in this study examined one of the most complete fossilized brains ever recovered.

Examination of the evolution of heads in ancient animals was profiled in the journal Current Biology.