The brain structures of jawless fish may not be as different from jawed vertebrates – including humans – as earlier thought, Japanese researchers have discovered.
While jawed vertebrates share many developmental characteristics that have remained unchanged for millennia, the findings suggest that complex divisions in the vertebrate brain first appeared before the evolution of jaws, more than 500 million years ago.
Published in the journal Nature, the study showed that two elements of brain genoarchitecture – or gene-expression patterns – thought to be unique to jawed vertebrates are actually present in two jawless fish – the hagfish and lamprey, the only jawless fish alive today.
“With these new findings from hagfish and lampreys, we have shown that both of the extant jawless-fish species have a rhombic lip and an MGE (medial ganglionic eminence) –the sources of the cerebellum, pallidum, and GABAergic interneurons in jawed vertebrates,” said one of the researchers Shigeru Kuratani from RIKEN Evolutionary Morphology laboratory in Saitama, Japan.
The brain’s basic developmental plan was thought by many scientists to have reached completion in jawed vertebrates because the brains of lampreys and hagfish seemed to lack these two key domains.
“We found that jawed-vertebrate patterning was more similar to the hagfish than to lampreys, and the evidence indicates that this is likely due to secondary evolutionary changes in lamprey evolution, rather than changes unique to jawed vertebrates,” Kuratani said.
“This firmly places the development of these genoarchitectural patterns back to a common ancestor shared by jawless and jawed vertebrates,” Kuratani pointed out.