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Scientists discover world's earliest known brain

The 520-million-year-old fossil of an extinct marine animal sports the oldest central nervous system to ever be found intact.

By Contributor / October 16, 2013

This illustration shows the nervous systems of the Alalcomenaeus fossil (left), a larval horseshoe crab (middle) and a scorpion (right). A new paper published in Nature reveals that the Alalcomenaeus is an ancestor to the group that includes both scorpions and horseshoe crabs.

N. Strausfeld/University of Arizona


Scientists have described the oldest complete central nervous system to ever be found: the brain of a 520-million-year-old fossil of an extinct marine animal.

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The find, published this week in the scientific journal Nature, would be remarkable just as a simple superlative. But what makes the report all the more stunning is that this hundreds of millions years old brain looks, well, like a brain that is much more evolved than that of something hundreds of millions of years old.

In fact, the layout of this ancient fossil’s central nervous system resembles the organization of the brain in a modern scorpion, or spider, or horseshoe crab.

“This was a very big surprise for us,” says Nicholas Strausfeld, a neuroscience professor at the University of Arizona and an author on the paper, “that such an ancient animal had such a sophisticated brain.”

The described fossil is a linchpin in scientists’ effort to piece together the evolutionary tree of the arthropods, the broad taxonomic group that includes modern insects, arachnids, and crustaceans and encompasses about four fifths of all known animal species.

It is a tree as complicated and nuanced as the veins in an insect’s wing, but one that, over the last year, is becoming ever clearer. Now, the new find, coupled with a similar find made last year, suggests that the two major branches that form the arthropod tree split from each other to develop their own complex brain systems as early as the early Cambrian period, or even earlier.

In other words, the brains in both groups of modern arthropods have obvious roots in the neural layout of organisms from half a billion years ago.

This week’s paper comes just one year after the same team published in Nature a description of another 520-million-year-old fossil with a complex brain. The fossil, Fuxianhuia protensa, had the primitive body plan expected of something dating to the Cambrian period, some 233 million years before the Triassic period when dinosaurs appeared. But it had a brain much more complex than the one that scientists had expected to find in something so old – a brain, in fact, like that of a mandibulate, the branch of the arthropod tree that includes modern shrimp and insects.

This meant that animals with mandibulate-like brains split from the arthropods to form a separate tree branch at least 520 million years ago. But what about the other, major group of arthropods, the chelicerates, which include modern spiders, ticks, horseshoe crabs, and scorpions? Might the forerunners to this big group of modern crawlers have also existed some 520 million years ago?


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