Tosches and her team propose that essentially all of the mammalian neocortex is an evolutionary innovation. So while at least part of the reptilian DVR was adapted from the brain region of an ancestral creature, the mammalian neocortex evolved as a new brain region burgeoning with novel cell types. Their answer to the decades of debate is that the mammalian neocortex and the reptile DVR are not homologous because they don’t have a common origin.
Georg Striedter, a neuroscience researcher at the University of California, Irvine who studies comparative neurobiology and animal behavior, hailed these findings as exciting and surprising. “I felt like it was providing really good evidence for something that I had only speculated about,” he said.
The new answer from Tosches’ team doesn’t mean that the neocortex in mammals evolved to sit neatly atop older brain regions, as the triune brain theory proposed. Instead, as the neocortex expanded and new types of pyramidal neurons were born within it, other brain regions kept evolving in concert with it. They didn’t just hang on as an ancient “lizard brain” underneath. It’s even possible that the complexity emerging in the neocortex pushed other brain regions to evolve — or vice versa.