In science-nerd circles, people lately have been really into talking about how many times crabs evolve — that is, how many unrelated invertebrate families wind up mutating into genetically distinct, unrelated species that happen to look exactly like a crab. Crabs, but alien to one another. Well, says Quanta Magazine, the same thing happened with vertebrates — but we don’t form crab-bodies. Creatures as distant from each other as birds and mammals have in separate ways evolved intelligence:
“A bird with a 10-gram brain is doing pretty much the same as a chimp with a 400-gram brain,” said Onur Güntürkün(opens a new tab), who studies brain structures at Ruhr University Bochum in Germany. “How is it possible?”
Researchers have long debated about the relationship between avian and mammalian intelligences. One possibility is that intelligence in vertebrates — animals with backbones, including mammals and birds — evolved once. In that case, both groups would have inherited the complex neural pathways that support cognition from a common ancestor: a lizardlike creature that lived 320 million years ago, when Earth’s continents were squished into one landmass. The other possibility is that the kinds of neural circuits that support vertebrate intelligence evolved independently in birds and mammals.
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A series of studies published in Science in February 2025 provides the best evidence yet that birds and mammals did not inherit the neural pathways that generate intelligence from a common ancestor, but rather evolved them independently. This suggests that vertebrate intelligence arose not once, but multiple times. Still, their neural complexity didn’t evolve in wildly different directions: Avian and mammalian brains display surprisingly similar circuits, the studies found.
“It’s a milestone in the quest to understand and to integrate the different ideas about the evolution” of vertebrate intelligence, said Güntürkün, who was not involved in the new research.
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In 1969, [MIT neuroanatomist Harvey] Karten wrote a “very influential paper that completely changed the discussion in the field,” said Maria Tosches(opens a new tab), 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(opens a new tab). 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.
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At the same time, [Achucarro Basque Center for Neuroscience’s Fernando] García-Moreno was collaborating with [brain researcher Bastienne] Zaremba and her colleagues at Heidelberg University. Using RNA sequencing, they created “the most comprehensive atlas of the bird pallium that we have,” said Tosches, who wrote a related perspective piece(opens a new tab) published in Science. By comparing the bird pallium to lizard and mouse palliums, they also found that the neocortex and DVR were built with similar circuitry(opens a new tab) — however, the neurons that composed those neural circuits were distinct.
“How we end up with similar circuitry was more flexible than I would have expected,” Zaremba said. “You can build the same circuits from different cell types.”