If the great dinosaurs had not gone extinct, would one of their species have dominated Earth today? In a parallel universe, would there be intelligent feathered giants relaxing and reading Scientific American? Although the idea of sentient, technological reptiles immediately reminds us of Star Trek’s Gorns or Doctor Who’s Silurians, there has actually been a scientific debate about this possibility for more than four decades. Recently it has returned to the forefront, thanks to two analyses published in the Journal of Comparative Neurology. Together these studies put the surprising cognitive abilities of dinosaurs—and their potential limitations—in a new light.
In one study, Suzana Herculano-Houzel of the Vanderbilt Brain Institute at Vanderbilt University calculated the likely number of neurons in dinosaurs’ pallium, a brain structure that is responsible for advanced cognitive functions and corresponds to the cortex in mammals. Research suggests that it is the number of neurons in these areas, rather than the size of the brain, that gives an idea, albeit approximate, of an animal’s possible cognitive abilities. For example, despite having a very small head, birds have more densely packed brain cells than many mammals and so can possess roughly as many neurons as primates. The result is that some birds, such as parrots and corvids (the bird family that includes ravens and crows), show great cognitive abilities, comparable to the smartest non-human mammals.
And it is precisely birds, being the only surviving lineage of dinosaurs, that are Herculano-Houzel’s touchstone. By comparing the relationship between brain size, number of neurons and body size in numerous extant bird and reptile species—as well as considering the available fossils of extinct dinosaurs—Herculano-Houzel concludes that a large dinosaur such as Tyrannosaurus rex could have housed two billion to three billion neurons in its pallium, a number similar to that of a baboon. If so, it’s possible that large dinosaurs were highly intelligent animals. It therefore seems plausible that, under the right conditions, nothing would have prevented the evolution of a “dinosaur sapiens.”
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Herculano-Houzel says, “T. rexes had what it takes, in terms of number of neurons, to be as cognitively flexible as other animals today that have similar numbers of telencephalic neurons, which are macaws, baboons, macaques and also whales.” That brainpower, she adds, means dinosaurs could have had the capacity for tool use and planning for the future.
But the number of neurons may not be enough. For intelligence, brain architecture also matters. And this could have been the Achilles’ heel of dinosaurs, argues Anton Reiner of the department of anatomy and neurobiology at the University of Tennessee Health Science Center. According to Reiner, it is unlikely that dinosaurs could have ever evolved cognitive abilities comparable to ours.
A little bit of anatomy is needed here. Over 350 million years of separate evolution, mammals and dinosaurs found two rather different ways to organize cognitive functions. The mammalian brain developed the so-called neocortex, in which neurons are organized in a relatively thin layer formed by compact columns. In each column, different parts can communicate with one another over short distances (less than a millimeter). To use a simple metaphor, we can think of the mammalian brain as a modular supercomputer in which it is easy to increase computational power by adding new elements.
In contrast, in the dinosaurs that survive today, namely birds, the organization is less compact. Functions that are performed by a single column of the mammalian cortex require the communication of separate brain nuclei. This structure works perfectly well as long as there are relatively few brain areas to manage. But expanding brain capabilities beyond a certain point could make the structure far more complex and less efficient than it is in mammals, according to Reiner. For comparison with the earlier metaphor, it is as if our supercomputer were made up of many computing centers in separate buildings connected by a jumble of long cables.
If this were the case, an increase in brain size would correspond to a greater distance between the cores, slowing down their communication. Cognitive efficiency would then drop at some point as brain volume increased. The architecture of the bird brain would also, Reiner argues, render little use for another strategy that, in mammals, made it easy to enhance cognitive faculties: the folds of the cerebral cortex. In mammals, these folds not only allow the area devoted to cognition to increase without overinflating the brain’s volume but also allow areas that would otherwise be distant to communicate rapidly.
According to Reiner, therefore, animals such as parrots or corvids would be near the upper limit of cognitive capabilities among birds. Beyond this threshold, adding new brain areas would lead to an overall decline in brain efficiency. And if, as everything suggests, dinosaurs had a similar brain architecture, they could have been very smart but not as smart as we are.
Reiner stresses that he defines intelligence in his work in terms of human intelligence, which he sees as a trait that sets our species apart and that has helped us thrive despite limitations such as our relative physical weakness. Simply put, “we can do stuff that other animals can’t,” Reiner says, pointing to our species’ combination of complex language, planning and tool use. At the same time, “that doesn’t mean we’re better than anybody else,” he adds. “It is likely there would be no humans except for the dinosaur extinction.... Nothing about the mammalian brain makes humans inevitable.”
Is the debate closed? Certainly not. Reiner and Herculano-Houzel each published a commentary with counterpointsand rebuttals to the other’s argument. Meanwhile other scientists have entered the fray. Some, such as neurobiologist Giorgio Vallortigara of the University of Trento in Italy, critique the notion of defining intelligence based on the brain and behavior of our own species. “The idea that the end point of the evolution of intelligence is human intelligence and that it is embodied in the organization of the cortex makes no sense from a biological point of view,” he says. “In biology there is nothing that is ‘best’ absolutely. It is always a matter of cost and benefit. We already know that the brains of birds can perform better than those of mammals (including humans) in various cognitive tasks. With that organization, birds do certain things better than mammals.” According to Vallortigara, bird brains are not necessarily less efficient at communicating between neurons. On the contrary, speed in transmitting information between networks of neurons is precisely one of their strengths.
Whatever the truth, it is easy to dismiss these analyses as idle, albeit fascinating, academic pastimes. But they can tell us much about how evolutionary history shapes the development of cognitive abilities. Evolution can find many ingenious solutions but cannot invent something from scratch: it must work with what it has available. Understanding how and if brain architecture imposes limits on the development of higher intellectual faculties could reveal much about the evolution of abilities and behaviors of various types of animals.
Finally, if we look at the issue with detachment, this debate may tell us more about our own species than about dinosaurs. In 1982 paleontologist Dale Russell and taxidermist Ron Séguin published what they called a “dinosauroid,” the first purportedly scientific reconstruction of a hypothetical dinosaur that evolved to achieve intelligence similar to ours. For reasons more cultural than scientific, the proposed result was a reptilian yet very human-looking being. Biologists and artists later proposed alternatives, albeit fanciful, that were more plausibly dinosaurlike and less biased towards the form of our own species. The whole episode illustrates how, when asked to imagine dinosaur intelligence, people tried to put themselves at the center of the universe yet again. One can argue that the bias to imagine humans as the embodiment of intelligent species reflects a belief that our evolution was, somehow, inevitable.
As evolutionary biologist Stephen Jay Gould pointed out long ago, however, humanlike technological intelligence is unlike many other complex evolutionary adaptations such as flight or the eye. It arose only once after more than three billion years and only because of a series of fortuitous favorable circumstances. Conversely, there are already species on Earth, from crows to whales to octopuses, that are extremely intelligent, albeit differently from us. Maybe someday a bird or other kind of animal will evolve an intelligence similar to ours—or maybe not. As to the dinosaurs, their cognitive strengths and achievements could have been very different from our own—yet equally spectacular.
This article originally appeared in Le Scienze and was reproduced with permission with additional reporting by Daisy Yuhas.