The findings emerge in a world enraptured by artificial forms of intelligence, and they could teach us something about how complex circuits in our own brains evolved. Perhaps most importantly, they could help us step “away from the idea that we are the best creatures in the world,” said Niklas Kempynck, a graduate student at KU Leuven who led one of the studies. “We are not this optimal solution to intelligence.”

Birds got there too, on their own.

Pecking Disorder

For the first half of the 20th century, neuroanatomists assumed that birds were simply not that smart. The creatures lack anything resembling a neocortex — the highly ordered outermost structure in the brains of humans and other mammals where language, communication and reasoning reside. The neocortex is organized into six layers of neurons, which receive sensory information from other parts of the brain, process it and send it out to regions that determine our behavior and reactions.

“For the longest time, it was thought that this is the center of cognition, and you need this kind of anatomy to develop advanced cognitive abilities,” said Bastienne Zaremba, a postdoctoral researcher studying the evolution of the brain at Heidelberg University.

Rather than neat layers, birds have “unspecified balls of neurons without landmarks or distinctions,” said Fernando García-Moreno, a neurobiologist at the Achucarro Basque Center for Neuroscience in Spain. These structures compelled neuroanatomists a century ago to suggest that much of bird behavior is reflexive, and not driven by learning and decision-making. This “implies that what a mammal can learn easily, a bird will never learn,” Güntürkün said.

The conventional thinking started to change in the 1960s when Harvey Karten, a young neuroanatomist at the Massachusetts Institute of Technology, mapped and compared brain circuits in mammals and pigeons, and later in owls, chickens and other birds. What he found was a surprise: The brain regions thought to be involved only in reflexive movements were built from neural circuits — networks of interconnected neurons — that resembled those found in the mammalian neocortex. This region in the bird brain, the dorsal ventricular ridge (DVR), seemed to be comparable to a neocortex; it just didn’t look like it.

In 1969, Karten wrote a “very influential paper that completely changed the discussion in the field,” said Maria Tosches, who studies vertebrate brain development at Columbia University. “His work was really revolutionary.” He concluded that because avian and mammalian circuits are similar, they were inherited from a common ancestor. That thinking dominated the field for decades, said Güntürkün, a former postdoc in Karten’s lab. It “sparked quite a lot of interest in the bird brain.”

A few decades later, Luis Puelles, an anatomist at the University of Murcia in Spain, drew the opposite conclusion to Karten. By comparing embryos at various stages of development, he found that the mammalian neocortex and the avian DVR developed from distinct areas of the embryo’s pallium — a brain region shared by all vertebrates. He concluded that the structures must have evolved independently

Karten and Puelles were “giving completely different answers to this big question,” Tosches said. The debate continued for decades. During this time, biologists also began to appreciate bird intelligence, starting with their studies of Alex, an African gray parrot who could count and identify objects. They realized just how smart birds could be.

However, neither group seemed to want to resolve the discrepancy between their two theories of how vertebrate palliums evolved, according to García-Moreno. “No, they kept working on their own method,” he said. One camp continued to compare the circuitry in adult vertebrate brains; the other focused on embryonic development.

In the new studies, he said, “we tried to put everything together.”

Same but Not the Same

Two new studies, which were conducted by independent teams of researchers, relied on the same powerful tool for identifying cell types, known as single-cell RNA sequencing. This technique lets researchers compare neuronal circuits, as Karten did, not only in adult brains but all the way through embryonic development, following Puelles. In this way, they could see where the cells started growing in the embryo and where they ended up in the mature animal — a developmental journey that can reveal evolutionary pathways.

For their study, García-Moreno and his team wanted to watch how brain circuitry develops. Using RNA sequencing and other techniques, they tracked cells in the palliums of chickens, mice and geckos at various embryonic stages to time-stamp when different types of neurons were generated and where they matured.

They found that the mature circuits looked remarkably alike across animals, just as Karten and others had noted, but they were built differently, as Puelles had found. The circuits that composed the mammalian neocortex and the avian DVR developed at different times, in different orders and in different regions of the brain.