In an attempt to understand the mysteries of the human brain, scientists have been trying to find general principles that underlie brain evolution across species. In their quest, they have been confronting controversial questions that resemble the chicken-egg dilemma. One of these questions revolves around the cause of brain evolution: which came first, bigger brains or larger brain regions that control specific behaviours? Neuroscientists have keenly debated this for decades, but a new Cornell University study provides fresh clues by focusing on the evolution of the brain of songbirds.
The study, conducted by Timothy DeVoogd, Professor of psychology at Cornell University, and published in the Proceedings of the Royal Society, analysed the evolution of the brain of 58 species of songbirds, and was able to show that there is some truth in both the theories involved in the debate, and therefore neither of them is an accurate model of brain evolution on its own.
The debate began in 1995, with a study published in Science by psychologists Barbara Finlay and Richard Darlington. According to them, size comes first. The argument follows this logic: if you have more brain mass in general, you also have more brain mass in specific areas. So with a bigger area dedicated to, say language, you can develop more complex linguistic skills. Bigger brains would therefore mean greater capabilities. But how did bigger brains evolve in the first place? The researchers argued that the growth happens in reaction to external alterations that extended the neurogenesis, that is, the period over which neurones are born.
“This idea just does not work”, comments Professor Robert Barton from Durham University. Barton himself replied to Finlay and Darlington in 2000 with a study in Nature, showing that, in mammalian brains, the overall size of the brain is only a side effect. Bigger brains would not come first, but they would be the result of some “evolutionary pressures that, at some points, require the development of some specific capabilities. To develop them, some different brain components grow in a functionally and anatomically correlated way.” For example, “if you are going to be a good song learner, you also need to have good hearing. And you will develop both the systems, growing different parts of your brain altogether.” The idea is that if many components grow together, an overall bigger brain is the result.
After sixteen years, DeVoogd has finally shown that these theories are not mutually exclusive. There is an advantage in studying songbirds instead of mammals: “While in mammals it’s difficult to measure component areas of the brain accurately, specific areas can be easily seen in the brain of songbirds,” explains DeVoogd. Another reason is that “there is substantial overlap with human brains.” The researchers focused on those areas associated with beak and facial muscles, concluding that there is a strong correlation between brain size and brain evolution. “More brain means more everything,” points out DeVoogd. However, there are also some brain components that are independent from brain size. These findings suggest that brains grew because some pressures required the development of new capabilities, but also that bigger brains became the boost for new control, that would have never been allowed otherwise.
The bottom line here is that there is some truth in both of the theories. However, despite considering the findings aligned with those of his own study, Barton claims that he disagrees with the authors of this paper, “where they think that Finlay and Darlington would be to some extent right. They don’t need to argue that. Instead they should say that the way brain components grow together is very complex.”
The state of the debate suggests that no straight answers are forthcoming. What we can understand from it, however, is that the chicken-egg dilemmas do not account for all the intriguing complexities of the brain.
Silvia Lazzaris is studying for an MSc in Science Communication at Imperial College London
Banner image: neurons, Rost9