How the Animal got its Head

Ancient brain fossils show how early animals evolved heads.

Odaraia alata
This is Odaraia alata, an arthropod resembling a submarine from the middle Cambrian Burgess Shale. Jean Bernard Caron (Royal Ontario Museum)

The earliest animals were a bit on the squishy side, resembling jellyfish or algae more than the complex, jointed animals that evolved later. But while scientists know that the first segmented, hard-bodied arthropods appeared during the Cambrian Period about 500 million years ago, the transition from squishy to shelled can be hard to pinpoint. Now, a group of researchers from the University of Cambridge believe they’ve discovered how the head came to be.

A recent study published in the journal Current Biology points towards one of the oldest fossilized brains in existence as the culprit. Brain fossils are hard to come by, as squishy things don’t tend to preserve well over hundreds of millions of years. But by comparing the fossils from a soft-bodied trilobite and an arthropod some say resembles a submarine, lead author Dr. Javier Ortega-Hernández discovered that the two creatures had both optic nerves and a hard plate called the anterior sclerite in the front of their brains.

"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," Dr. Ortega-Hernández said in a statement.

The anterior sclerite has disappeared over millennia and likely fused with other parts of the head, Dr. Ortega-Hernández says. In fact, another species called Anomalocaridid that lived at the same time and shared a common ancestor with the submarine-like arthropod also had an anterior sclerite in the same part of the brain. Since that region of the brain controls the eyes in modern arthropods, Dr. Ortega-Hernández believes that the anterior sclerite was the first step towards a modern head structure.

"Heads have become more complex over time," said Dr. Ortega-Hernández. "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."

With this piece in place, researchers are one step closer solving the puzzle of how early animals became complex creatures.

h/t The Washington Post

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