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Tiny fossil offers big clues to brain evolution

By Nicholas Gerbis
Published: Friday, November 25, 2022 - 3:09pm

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velvet worm
Getty Images
A modern day velvet worm.

Settling debates over the evolutionary roots of shared features takes evidence that often does not survive the ravages of time.

But a University of Arizona-led paper in the journal Science on the oldest known fossilized brain offers clues to the brain’s origins in modern arthropods — and, by extension, humans.

Studying the noggins of the 500-million-year-old ancestors of shrimp, centipedes and insects was once limited by the belief that brains don’t fossilize.

Millimeter-sized brains preserved in special clays changed that view over the past decade.

One such brain, discovered in 1984, comes from Cardiodictyon catenulum, a distant and ancient relative of the modern velvet worms that dwell in the southern tropics.

A bit like a centipede crossed with a tardigrade, such lobopodians were plentiful during the Cambrian Explosion, the biological Big Bang more than 530 million years ago when the ancestors of almost every animal began showing up in the fossil record.

Examination of the preserved tissues reveals a pattern that supports the argument that the brains of pre-arthropods evolved separately from their bodies’ nervous systems.

The organ’s unified form also suggests that brain segmentation came later in arthropod evolution: Later species would have two specialized segments and a distinct rear element, not unlike the human pattern consisting of forebrain, midbrain and spinal cord.