By examining some of the oldest fossil brains ever discovered, researchers have a better idea of how heads (as we know it) first evolved in early animals. The 500-million-year-old preserved brains belonged to early ancestors of arthropods, the wildly diverse group that includes insects, spiders, and crustaceans. The findings, published in Current Biology this week, identifies a key moment in the shift from worm-like arthropod ancestors with soft bodies to those with a hard exterior like the ones we’re more familiar with today.
The Cambrian Explosion 500 million years ago was a wondrous time of evolutionary innovation. That was when most major animal groups first started showing up in the fossil record, including arthropods with hard exoskeletons and jointed limbs. Before this, most animals had bodies that were squishy and soft, similar to that of worms and jellyfish, and their heads seemed less defined.
Thanks to exceptionally preserved fossils unearthed from the Middle Cambrian Burgess Shale in Western Canada, Javier Ortega-Hernandez of Cambridge was able to closely examine the neurological remains of two arthropod ancestors: a soft-bodied trilobite called Helmetia expansa (pictured below) and a submarine-looking critter called Odaraia alata (pictured above).
Their bright, eye-like features -- simple photoreceptors -- were embedded in a hard plate called the anterior sclerite, seen in the white box in the image below. Their stalked eyes were connected to their anterior sclerite through nerve traces originating from the front part of the fossilized brain; in modern arthropods, this forebrain area corresponds to vision control. These ancient brains likely processed information like cockroaches and crabs today, and they were necessary for sensing the environment, locating food, and avoiding or escaping from predators.
“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,” Ortega-Hernández says in a news release. “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.”
Then he compared these fossils to anomalocaridids, a strange group of swimming predators during the Middle Cambrian. These are thought to be early arthropod ancestors, even though their bodies are so different. Turns out, the plate atop the anomalocaridid head shared similarities with the anterior sclerite -- suggesting a common origin. The anterior sclerite may be what bridges the head of anomalocaridids with those of more recognizable arthropods.
“Heads have become more complex over time,” Ortega-Hernández adds. “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.”
Images: Jean Bernard Caron/Royal Ontario Museum (top), J. Ortega-Hernández, Cell Press 2015 (middle)