“We are not this optimal solution to intelligence,” said Niklas Kempynck, who pursues graduate studies at KU Leuven, in a sweeping statement that tears up old dogmas about the cognitive superiority of humanity. His own research, and an accumulating trail of evidence, points to the fact that birds for all their absence of a mammalian neocortex have independently evolved advanced intelligence. This discovery is rewriting our understanding of brain evolution and the variety of cognitive strategies in vertebrates.
For much of the 20th century, birds were deemed simpletons for the simple reason that they do not have a neocortex the six-layered structure in mammals that is associated with more sophisticated processes such as reasoning and language. Most neuroanatomists believed that birds’ behavior was largely reflexive and driven largely by what Fernando García-Moreno, a neurobiologist at the Achucarro Basque Center for Neuroscience, characterized as “unspecified balls of neurons without landmarks or distinctions,” or, put simply, the structure of their brains. That was based on the assumption that birds could never perform the kind of cognitive tricks that mammals with a neocortex could more easily pull off.
But that narrative began to fray in the 1960s, when Harvey Karten, then a young neuroanatomist at M.I.T., mapped the brain circuits of pigeons and other birds. But what Karten discovered was a striking similarity between the neural circuitry of the avian dorsal ventricular ridge (DVR). A cluster of neurons in bird brains the avian DVR shares the same general form and function as the mammalian neocortex. “His work was really revolutionary,” said Maria Tosches, a researcher studying vertebrate brain development at Columbia University. Karten’s discoveries suggested that these circuits were derived from a common ancestor, and for decades the question of whether birds and mammals have homologous brain structures, or whether their cognitive portfolios evolved independently, has been an active area of contention.
Adding complexity to the issue, in the 1990s an anatomist, Luis Puelles of the University of Murcia, proposed an alternative view. Puelles concluded that the mammalian neocortex and avian DVR develop from two different domains of the pallium, a forebrain region conserved across all vertebrates, based on comparisons of embryonic development. This led to the concept that birds and mammals had evolved their cognitive skills separately, and that both branches had reinvented the wheel of brain design for elevated intelligence.
In recent studies, the debate has taken another step in the direction of resolution using cutting-edge tools, including single-cell RNA sequencing, to track which developmental pathways give rise to neurons in birds, mammals and reptiles. García-Moreno’s team tracked embryonic development of pallial neurons in chickens, mice, and geckos, and found similar layout for mature circuits but different developmental pathways to get to that mature design in these species. These results highlight the idea that birds and mammals arrived at similar cognitive abilities via independent evolutionary trajectories.
Birds’ DVRs, no more fan-shaped and angular than mammalian neocortices, are remarkably efficient. The DVR contains roughly 0.5–2 billion neurons, comparable in density with the neocortices of higher primates (chimpanzees). Such a small collection of nerve cells enables birds to behave very complexly, a feat most mammals could pull off. New Caledonian crows create tools to obtain food, and magpies demonstrate object constancy comparable to human cognition, and African grey parrots use human words to convey numerical and relational facts. In fact, such a feat highlights the premise that intelligence is not about the neocortex, but that it can emerge in different kinds of neural networks.
Clifton Ragsdale, a neurobiology professor who studied the brains of birds and reptiles and is affiliated with the University of Chicago, expressed a similar sentiment. “Birds are more intelligent than you think, and they do clever things. So, the question is: What kind of brain circuitry are they using?” His team had identified IT neurons in the DVR cells that enable one part of the brain to communicate with another, like those in the mammalian neocortex. Such large differences in appearance imply that birds and mammals have completely different types of brain cells, but the new results suggest that the different groups actually share the same suite of cell types in their brains.
What these discoveries suggest about avian intelligence goes further than that. They draw on longstanding challenges to the anthropocentric idea that human brains are the pinnacle of cognitive evolution. As Kempynck put it: “We are not this optimal solution to intelligence.” Instead, intelligence resembles more a collection of evolutionarily adapted adaptations that fit the ecology and social needs of multiple species.
That radical rethinking of human uniqueness has inspired a rethinking of how we study brain evolution. Now, researchers are testing the continuity hypothesis which emanates from the dorsal and ventral pallium as part of an integrated neural architecture in birds. Across a range of RNA sequencing studies in zebra finches, transcriptionally similar domains are enriched for homologous sequence elements between the two structures, suggesting the potential for common functional properties that have gone undetected in the past. Such results highlight the need for a more nuanced form of comparative neurobiology that accounts for the phylogenetic diversity of cognitive solutions.
The new perspective that emerged had its genesis in the pioneering work of Harvey Karten, whose influence is evident in the way researchers are now taught to challenge assumptions. It’s a reminder that science is born of wonder and disbelief, that it urges us through the crooked corridors of evolution that have given birds, mammals and others their architecture.
Studying avian brains not only broadens our definition of intelligence, but also emphasizes the flexibility of neuronal systems. Birds have their own version of DVRs, suggesting that there isn’t a single way to do cognition. Instead evolution has led to a panoply of brain architectures, capable of wondrous things. In these mysteries, as we analyze them more deeply, we may discover that the strangeness of intelligence is that it is not a unified phenomenon, but that it comes in many varieties.
