The international study – the largest of its kind ever carried out – involved a team of 22 scientists, who investigated 1,400 living and extinct mammal species. In the UK, this involved Dr Jacob Dunn and Max Kerney of Anglia Ruskin University (ARU) as well as academics based at UCL, the University of Salford, and the Natural History Museum.
Published in the journal Science Advances and led by Dr Jeroen Smaers from Stony Brook University in the US and Dr Kamran Safi of the Max Planck Institute of Animal Behavior in Germany, this groundbreaking study has pieced together a unique timeline of how brain and body size has evolved over the last 150 million years, showing how “big-brained” species have attained their extreme proportions in different ways.
Great apes have a wide range of body sizes but have demonstrated a general trend towards increases in both brain and body size. In comparison, ancestral hominins, which represent the human line, have shown a relative decrease in body size and increase in brain size compared to great apes.
The study found that most changes in brain size occurred after two cataclysmic events in Earth’s history. After the mass extinction event at the end of the Cretaceous period 66 million years ago, the researchers noticed a dramatic shift in relative brain-body size in lineages such as rodents, bats and carnivorans, as these animals filled the empty niches left by extinct dinosaurs.
Roughly 30 million years later, a cooling climate in the Late Paleogene period led to more profound changes, with seals, bears, whales, and primates all undergoing evolutionary shifts in their brain and body size. The likes of elephants and great apes evolved their extreme proportions after this climate change event.
As well as demonstrating that brain size relative to body size has not followed a stable evolutionary trajectory, the study also shows that relatively big brains, long considered an indicator of animal intelligence, can be the result of a gradual decrease in body size to suit a new habitat, climate, or way of moving. In other words, relative brain size may have nothing to do with intelligence at all.
The study reveals that the California sea lion attained a low brain-to-body size because of the strong selective pressures on body size, most likely because these aquatic carnivorans diversified into a semi-aquatic niche and their body increased in size at a faster rate than their brain as they adapted to also living on land.
Co-author Dr Jacob Dunn, Associate Professor in Evolutionary Biology at Anglia Ruskin University (ARU), said: “These results make us completely change the way we think about brain evolution in mammals.
“For decades, biologists have assumed that relative brain size is a reliable indicator of intelligence among species. This has been especially true for research on primates, including our extinct hominin ancestors. Our study shows that we need to be more cautious when making assumptions about a species’ cognitive ability from its relative brain size, as the relationship between brain size and body size is complex, and either or both can evolve to be bigger or smaller in response to different environmental challenges.
“Our work highlights how variation in body size is a key driver of shifts in relative brain size, and opens up new opportunities for thinking about the complex ways that brains evolve.”
Dr Jeroen Smaers, an Evolutionary Biologist at Stony Brook University and first author on the study, said: “At first sight, the importance of taking the evolutionary trajectory of body size into account may seem unimportant. 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.
“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.”
The full open access study will be published here https://advances.sciencemag.org/lookup/doi/10.1126/sciadv.abe2101