We as a bird
Almost 50 years ago, UC San Diego neuro-psychiatrist Harvey J. Karten, MD, then 30 years old and working at Massachusetts Institute of Technology, and colleagues discovered that a region of the avian brain (he was studying auditory and visual pathways, in particular) was surprisingly similar to a distinctly different looking region of the mammalian brain, including humans.
Notably, they reported that neural inputs and outputs into the dorsal ventricular ridge (DVR), a cluster of neurons found in bird (and reptile) brains, was strikingly similar to the functioning of the neocortex in mammalian brains.
Here’s the kicker: The neocortex is a part of the brain’s outer layer where higher-order processing is thought to occur. Research at the time posited that the neocortex was unique to mammals, and perhaps responsible for their presumed greater cognitive powers. Karten’s findings threatened to upend that notion of singularity. More importantly, it suggested a shared evolutionary history with mammalian cortex, one that predated the evolutionary separation of mammals and birds. And it proposed a mechanism of cortical development in mammals that was clearly at odds with the prevailing notions.
Ever since, the debate over similarities (and their significance) between the brains of mammalian and non-mammalian vertebrates has rumbled along. Karten has steadfastly pursued it, most recently in a 2010 PNAS paper that demonstrated that the microcircuitry in the region of the avian brain that processes auditory signals (hearing) is similar to the region of the mammalian brain responsible for the same function.
Final confirmation that Karten and colleagues were right, however, may have come earlier this month with the report (also in PNAS) by researchers at the University of Chicago, who tested Karten’s 47 -year-old hypothesis by using new molecular markers capable of identifying specific neuron types in the mammalian cortex, then looking to see if the same marker genes were expressed in DVR nuclei in the brains of chickens and zebra finch.
They were. The neurons of the avian DVR are homologous to those of the mammalian neocortex.
“Here was a completely different line of evidence,” said Clifton Ragsdale, PhD, an associate professor of neurobiology and senior author of the study. “There were molecular makers that picked out specific layers of cortex; whereas the original Karten theory was based just on connections, and some people dismissed that. But in two very different birds, all of the gene expression fits together very nicely with the connections.
It’s welcome, if expected, news for Karten.
"I recall that my mentor, the distinguished neuroanatomist Walle J. H. Nauta, cautioned me about not hoping for much of an enthusiastic reception. He said that for really novel, iconoclastic ideas, it can take 40 years before they are accepted. This is closer to 50 years! The most exciting part of the story will be watching how this may serve as a foundation for exciting new research into the evolution and development of the mammalian cortex by the next generation of bright young scientists."
Confirming functional similarities between avian and mammalian brains does more than just end an old argument. It opens up new avenues of investigation for neuroscientists, who now have another animal model to study. They can compare developmental steps between more, diverse organisms. They can look at how neurons take different form to provide the same function and how their differences impact behaviors and abilities, notably communication skills.
More fundamentally, scientists are slightly closer to addressing the ultimate question of evolution: How did humans get from there to here?
Part of the answer, it now appears, lies in the part of your brain that behaves like a bird’s.