500 mln-year-old brain sheds light on how heads evolved

Scientists have identified one of the oldest fossil brains ever discovered - more than 500 million years old - and used it to help determine how heads first evolved in early animals.
The study from the University of Cambridge has identified a key point in the evolutionary transition from soft to hard bodies in early ancestors of arthropods, the group that contains modern insects, crustaceans and spiders.
The ancient brain belonged to a crustacean called Odaria alata, a bizarre creature resembling a submarine, whose remains were compared with another very ancient creature: a soft-bodied trilobite.
The study found that a hard plate, called the anterior sclerite, and eye-like features at the front of their bodies were connected through nerve traces originating from the front part of the brain, which corresponds with how vision is controlled in modern arthropods.

The new results also allowed comparisons with anomalocaridids, a group of large swimming predators of the period, and found key similarities between the anterior sclerite and a plate on the top of the anomalocaridid head, suggesting that they had a common origin.
"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 Dr Javier Ortega-Hernandez, a postdoctoral researcher from Cambridge's Department of Earth Sciences, who authored the study.

The fossils, from the collections of the Royal Ontario Museum in Toronto and the Smithsonian Institution in Washington DC, originated from the Burgess Shale in Western Canada.

"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," Ortega-Hernandez said.
Ortega-Hernandez observed that bright spots at the front of the bodies, which are in fact simple photoreceptors, are embedded into the anterior sclerite.
The photoreceptors are connected to the front part of the fossilised brain, very much like the arrangement in modern arthropods.
In all likelihood these ancient brains processed information like in today's arthropods, and were crucial for interacting with the environment, detecting food, and escaping from predators.

During the Cambrian Explosion, a period of rapid evolutionary innovation about 500 million years ago when most major animal groups emerge in the fossil record, arthropods with hard exoskeletons and jointed limbs first started to appear.
Prior to this period, most animal life on Earth consisted of enigmatic soft-bodied creatures that resembled algae or jellyfish.