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Brains of ancient jawless fish more advanced than thought

Previously, evolutionary biologists believed the brain's developmental path was cemented in jawed vertebrates.
By Brooks Hays   |   Feb. 15, 2016 at 5:20 PM

TOKYO, Feb. 15 (UPI) -- The complexities of the vertebrate were thought to be closely linked with the development of the jaw. Until now, researchers believed jawless fish brains lacked distinct brain components.

A re-examination of hagfish and lamprey brains suggests they are more like complex vertebrate brains than scientists previously thought.

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The brain of modern humans are considerably more complex than the brains found inside the skulls of Earth's earliest vertebrates, but the basic blueprint has remained largely the same for millions of years.

Evolutionary biologists believed the brain's developmental plan was cemented in jawed vertebrates during the Paleozoic era. Two specific design components are central to this development plan -- the cerebellum and the medial ganglionic eminence, or MGE.

In vertebrates, the expression of Nkx2.1 and Hedgehog genes triggers the formation of MGE from from a forward section of the neural tube. Genes called Pax6 trigger the development of the cerebellum.

Researchers found similar but not identical gene expression, as well as cerebellum and MGE formation in hagfish. The cerebellum-like region in jawless fish is called a rhombic lip. Scientists also found the expression of Nkx2.1 and Pax6 in lampreys, but no Hedgehog genes.

Patterns of cerebellum and MGE formation in hagfish brains were more vertebrate-like than expression and formation patterns in lamprey brains.

"We found that jawed-vertebrate patterning was more similar to the hagfish than to lampreys," lead researcher Shigeru Kuratani, of Japan's RIKEN University, said in a news release, "and the evidence indicates that this is likely due to secondary evolutionary changes in lamprey evolution, rather than changes unique to jawed vertebrates."

"This firmly places the development of these genoarchitectural patterns back to a common ancestor shared by jawless and jawed vertebrates," Kuratani said.

The new research was published in the journal Nature.

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