Scientists at Stony Brook University and the Max Planck Institute of Animal Behavior have reconstructed a timeline of the evolution of brain and body size in mammals over the past 150 million years. The international team of 22 scientists, including biologists, evolutionary statisticians and anthropologists, compared the brain masses of 1,400 living and extinct mammals. For the 107 fossils examined – including ancient whales and the oldest Old World monkey skull ever found – they used endocranial volume data from skulls instead of brain mass data. Brain measurements were then analyzed along with body size to compare the scale of brain size to body size over a deep evolutionary period.
The results, published in Scientific progress, has shown that brain size relative to body size – long considered an indicator of animal intelligence – has not followed a stable scale over evolutionary time. Humans, dolphins, and famous “big-brained” elephants, for example, have reached their proportions in different ways. Elephants have increased in body size, but surprisingly, even more brain size. Dolphins, on the other hand, generally decreased their body size while increasing their brain size. Great apes have shown a wide variety of body sizes, with a general tendency for brain and body size to increase. In comparison, ancestral hominins, which represent the human lineage, showed a relative decrease in body size and an increase in brain size compared to great apes.
The authors say these complex models prompt a reassessment of the deeply rooted paradigm that comparing brain size to body size for any species provides a measure of the intelligence of the species. “At first glance, the importance of taking into account the evolutionary trajectory of body size may seem unimportant,” says Jeroen Smaers, evolutionary biologist at Stony Brook University and the study’s first author. “After all, many large-brained mammals such as elephants, dolphins and great apes are also large brain-to-body sizes. But it’s not always the case. The California sea lion, for example, has a low relative brain size, which contrasts with their remarkable intelligence. “
Taking into account the evolutionary history, the present study reveals that the California sea lion attained low brain-to-body size due to strong selective pressures on body size, most likely because aquatic carnivores have diversified. in a semi-aquatic niche. In other words, they have a low relative brain size because of the selection on increasing body size, and not because of the selection on decreasing brain size.
“We have overturned a long-held dogma that the relative size of the brain can be equivocal with intelligence,” says Kamran Safi, researcher at the Max Planck Institute of Animal Behavior and lead author of the study. “Sometimes relatively large brains can be the end result of a gradual decrease in body size to accommodate a new habitat or a new way of moving – in other words, nothing to do at all. with intelligence Using the relative size of the brain as an indicator of cognition the ability must be weighed against the evolutionary history of an animal and the nuances in how the brain and body have changed over the years. the tree of Life.
The study further showed that most of the changes in brain size occurred after two cataclysmic events in Earth’s history: mass extinction 66 million years ago and a climate transition there. 23 to 33 million years ago.
After the mass extinction event at the end of the Cretaceous, researchers noticed a dramatic change in brain and body scaling in lineages such as rodents, bats and carnivores, as animals radiated into empty niches left by extinct dinosaurs. About 30 million years later, a cooling climate in the Upper Paleogene led to more profound changes, with seals, bears, whales, and primates all undergoing evolutionary changes in their brains and body sizes.
“A big surprise was that much of the variation in the relative brain size of mammals that live today can be explained by the changes their ancestral lineages underwent as a result of these cataclysmic events,” says Smaers. . This includes the evolution of the largest brains of mammals, such as dolphins, elephants and great apes, all of which evolved to their extreme proportions after climate change 23 to 33 million years ago.
The authors conclude that efforts to truly capture the evolution of intelligence will require an increased effort to examine neuroanatomical features, such as regions of the brain known for their higher cognitive processes. “The size of the brain to body is of course not independent of the evolution of intelligence,” says Smaers. “But it may actually be more indicative of more general adaptations to large-scale environmental pressures that go beyond intelligence.”