Most living vertebrate species have jaws, a development thought to have occurred sometime in the Paleozoic era. Jawed vertebrates–including humans–share many developmental characteristics that have remained unchanged for millennia. 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–the only jawless fish that remain alive today–seem to lack two key domains.
However, it turned out that hagfish [jawless fish] do have the required equipment.
“The problem was that lampreys had not yet been shown to have a similar patterning,” explains Kuratani. “The shared pattern of brain development between hagfish and jawed vertebrates raised the possibility that the apparently primitive brain of the lamprey is simply a lamprey-unique characteristic.”
“We found that jawed-vertebrate patterning was more similar to the hagfish than to lampreys,” says Kuratani, “and the evidence indicates that this is likely due to secondary evolutionary changes in lamprey evolution, rather than changes unique to jawed vertebrates.”
“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 –the sources of the cerebellum, pallidum, and GABAergic interneurons in jawed vertebrates. This firmly places the development of these genoarchitectural patterns back to a common ancestor shared by jawless and jawed vertebrates.” (paywall) More.
That common ancestor lived about half a billion years ago. All just sort of happened. Funny that.
See also: Does intelligence depend on a specific type of brain?
Devolution: Getting back to the simple life
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Here’s the abstract:
The vertebrate brain is highly complex, but its evolutionary origin remains elusive. Because of the absence of certain developmental domains generally marked by the expression of regulatory genes, the embryonic brain of the lamprey, a jawless vertebrate, had been regarded as representing a less complex, ancestral state of the vertebrate brain. Specifically, the absence of a Hedgehog- and Nkx2.1-positive domain in the lamprey subpallium was thought to be similar to mouse mutants in which the suppression of Nkx2-1 leads to a loss of the medial ganglionic eminence1, 2. Here we show that the brain of the inshore hagfish (Eptatretus burgeri), another cyclostome group, develops domains equivalent to the medial ganglionic eminence and rhombic lip, resembling the gnathostome brain. Moreover, further investigation of lamprey larvae revealed that these domains are also present, ruling out the possibility of convergent evolution between hagfish and gnathostomes. Thus, brain regionalization as seen in crown gnathostomes is not an evolutionary innovation of this group, but dates back to the latest vertebrate ancestor before the divergence of cyclostomes and gnathostomes more than 500 million years ago. – Fumiaki Sugahara, Juan Pascual-Anaya, Yasuhiro Oisi, Shigehiro Kuraku, Shin-ichi Aota, Noritaka Adachi, Wataru Takagi, Tamami Hirai, Noboru Sato, Yasunori Murakami, Shigeru Kuratani. Evidence from cyclostomes for complex regionalization of the ancestral vertebrate brain. Nature, 2016; DOI: 10.1038/nature16518