Illustration of two elephants and a shrew. Blue brains are on their heads

How mammals evolved big brains

Scientists have now pieced together a 150-million-year timeline to determine how mammals evolved big brains.

An international team, led by Jeroen Smaers of Stony Brook University, US, compared the brain mass of 1400 living mammals and 107 fossils and compared them to body size to determine how the scale of the two has changed through time.

The result? Brain size and body size didn’t evolve in a stable way.

Instead, the researchers found that big-brained animals like humans, elephants and dolphins all evolved their brain-to-body-size proportions in different ways.

Sometimes, brains got bigger as bodies got bigger, such as in the case of elephants, while other mammals like dolphins instead evolved smaller bodies while their brain size increased.

Humans also appeared to evolve both bigger brains and smaller bodies in comparison to their other ape cousins, the authors show in their paper, published in Science Advances.

Phylogenetic tree of animal evolution. The closer together animals are on the tree, the more closely they are evolutionarily related. Credit: Smaers et al. / Science Advances

These findings challenge the idea that comparing brain size to body size is an indicator of intelligence.

“At first sight, the importance of taking the evolutionary trajectory of body size into account may seem unimportant,” says lead author Smaers.

“After all, many of the big-brained mammals such as elephants, dolphins, and great apes also have a high brain-to-body size. But this is not always the case. The California sea lion, for example, has a low relative brain size, which lies in contrast to their remarkable intelligence.”

This means that not only is relative big-brain-to-body size not a clear measure of intelligence, but it is also not explicitly selected for in evolution, because body size had a greater impact on survival.

“We’ve overturned a long-standing dogma that relative brain size can be equivocated with intelligence,” says Kamran Safi, a research scientist at the Max Planck Institute of Animal Behaviour and senior author on the study.

“Sometimes, relatively big brains can be the end result of a gradual decrease in body size to suit a new habitat or way of moving – in other words, nothing to do with intelligence at all. Using relative brain size as a proxy for cognitive capacity must be set against an animal’s evolutionary history and the nuances in the way brain and body have changed over the tree of life.”

Credit: Smaers et al. / Science Advances

If brain size is not necessarily an indicator of intelligence, that could be good news for our beloved Aussie icons.

“Scientists are often prejudiced against marsupials,” says co-author Vera Weisbecker of Flinders University. “They are considered primitive and small-brained because they are born at tiny sizes and their brain mostly develops after birth.

“However, this study shows that marsupial brains have a similar relationship with body size as other mammals, such as bats, some rodents, and shrews”

The research found that the biggest changes in brain size occurred after two major cataclysmic events in Earth’s history: the mass extinction 66 million years ago (when dinosaurs disappeared, at the end of the Cretaceous period) and a major climate shift 23-33 million years ago (in the Late Paleogene, after which many major evolutionary changes happened).

The climate shift appeared to have triggered the evolution of the biggest brain-to-body ratios, such as those of dolphins, elephants and apes.

“A big surprise was that much of the variation in relative brain size of mammals that live today can be explained by changes that their ancestral lineages underwent following these cataclysmic events,” says Smaers.

“Brain-to-body size is of course not independent of the evolution of intelligence. But it may actually be more indicative of more general adaptions to large scale environmental pressures that go beyond intelligence.”

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