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Evolution does not consistently favour bigger brains. Instead, it's more common for mammals to have evolved smaller body sizes which makes it look like the brain is getting bigger.
A study of 1,400 living and extinct mammal brains, published in Science Advances, has given researchers an in-depth timeline of how mammal brains and bodies have evolved over the last 150 million years.
They have found a simple story, but one that completely changes our previously held perception that cognition is the primary driver of brain size evolution in mammals.
Over nearly 200 million years of mammal evolution, the researchers have shown that brain size is actually secondary to body size. Their research also shows that there are many different evolutionary paths to having a big brain.
Prof. Anjali Goswami, a Research Leader at the Natural History Museum and an author of the study, explains 'A lot of the time where it looks like brain size in increasing, it's actually not that brains are getting bigger, but evolution is acting to decrease body size.'
Habitat, diet, reproduction and metabolism all impact body size and may be more consequential for survival compared to cognition and intelligence.
The study included data from not just living mammals but also 107 mammal fossils including ancient whales and the oldest Old World monkey skull ever found. Looking at fossils gave the researchers a more accurate picture of how brain and body sizes have changed through time.
'When researchers talk about brain size what we usually mean is relative brain size, which takes into account the relationship between your body and the brain,' says Anjali.
'For example, a blue whale has a much bigger brain than humans, but it’s much smaller than ours would be if you blew a human up to the size of a blue whale without changing our proportions.'
'In this study, we developed a method that discriminated brain size from body size and looked at how both are evolving separately. This gives us an entirely new, and surprising, picture of how mammals evolve big brains.'
Over evolutionary time, 'big-brained' mammals including humans, dolphins and elephants have attained their proportions in different ways.
Elephants have increased their body size but, perhaps surprisingly, continued to increase their brain size even more.
Great apes, such as chimpanzees and gorillas, show a wide variety of body sizes, with a general trend towards increases in both brain and body size.
In comparison, ancestral hominins (which represent our own evolutionary history), showed a relative decrease in body size and increase in brain size compared to other great apes. This suggests that cognition was likely a factor in driving brain size evolution of our species.
With our big brains relative to body size, it's hard not to assume that humans are the best evolution can do. But bigger brains are energetically expensive, and this cost doesn't necessarily pay off in evolutionary terms.
The study found that primates hung onto the ancestorial form of brain and body size until quite late in their evolutionary history. It was not until around 23 million years ago when there was a bunch of rapid shifts across some primate groups.
'This is probably a little later than you would expect for the primates, which is surprising and may relate to changes in the global environment around that time,' says Anjali.
We think of baleen whales, such as blue and humpback whales, as extreme animals due to their massive body and brain. But this research shows that they are actually a typical mammal. They have not deviated from the ancestral mammal relationship of body and brain size: as the body gets bigger, so does the brain.
On the other hand, toothed whales such as killer whales and dolphins, have large brains because of a huge reduction in body size but not in brain size, making their brains look relatively bigger in comparison.
The study showed that most changes in brain size occurred after two cataclysmic events in Earth's history: the KPG mass extinction 66 million years ago and a climatic transition 23-33 million years ago.
After the mass extinction event at the end of the Cretaceous period, the researchers noticed a dramatic shift in brain and body size in lineages such as rodents, bats and carnivorans as animals filled the empty niches left by the now extinct dinosaurs.
Roughly 30 million years later, a cooling climate in the Late Paleogene led to more profound changes, with seals, bears, whales and primates all undergoing evolutionary shifts in their brain and body size.
This is what has led to the pattern in brain sizes that we see in modern mammals today.
All of this adds up to a much more complex picture of what drives brain size than we previously thought, with everything from climate change and mass extinctions to body size and, of course, a little bit of intelligence (at least for humans and dolphins).
In another study, Anjali and her team have found that the only consistent predictor of marsupial brain size is the size of a mother's litter.
The paper, published in the Proceedings of the Royal Society B, has found that dividing the mother's energy between many babies has evolutionarily impacted the size of their brains.
'Maternal energy is a big driver of brain evolution because brains are energetically expensive and you need a high calorie diet to maintain a big brain,' says Anjali.
The study looked at a range of factors including gestation and lactation times, both of which have previously been suggested to drive the evolution of big brains, but litter size emerged as the only significant predictor of brain size.
Marsupials display a much lower diversity of reproductive traits than placentals.
'This emphasizes the high importance of reproductive investment for the evolution of relatively larger mammalian brains,' says Anjali.