After the asteroid wiped out the dinosaurs, the surviving mammals grew large before becoming intelligent. Their body mass and muscle strength helped them, but their brains shrank and took 10 million years to grow. Size was more important than intelligence to survive.
The asteroid that wiped out the dinosaurs some 66 million years ago left mostly smaller creatures in its wake, from rodent-like insect-eaters to beaver-sized herbivores. Over time, these animals grew larger, and some even became giants.
Scientists have long thought that their brains developed in this expansion, gradually becoming larger than the rest of their bodies.
However, a new study suggests that brains were the last to grow, possibly because their bodies needed larger physical dimensions, rather than bulky brains, to survive.
What the new research has found is that mammals increased their body size immediately after the extinction of the dinosaurs, but that their brains only began to grow about 10 million years later, notes the journal Science.
evolutionary strategy
Ornella Bertrand, a palaeoneurologist at the University of Edinburgh, one of the authors of this research, believes that large brains only evolved when mammals needed them.
At first, he says, they were just trying to get as big as possible because there were so many more resources available, and besides, no one was trying to eat them. But once these larger mammals once again dominated the Earth, they began to get smarter, because the competition for food became more difficult and it was difficult to survive, adds Bertrand.
However, it is also thought that it was not just competition that led to larger brains in mammals, but rather a combination of abundant food and the absence of predatory dinosaurs.
With brains being incredibly expensive in terms of energy, once some mammals grew up, they could eat more and would have more energy to make bigger brains. If competition alone triggered brain enlargement, smaller mammals would have disappeared, Science also posits.
Lead author Professor Steve Brusatte, quoted by Reuter, believes that “the mammals that replaced the dinosaurs were pretty dumb, and it was only millions of years later that many kinds of mammals developed larger brains as they competed with each other to form new brains. ecosystems.”
Exhaustive analysis
To reach these conclusions, the scientists analyzed mammalian fossils from more than 120 extinct species, including dozens of newly discovered fossil skulls from the Paleocene (66 to 56 million years ago) and the Eocene (56 to 34 million years ago).
The team X-rayed the skulls and performed computed tomography (CT) scans, which allowed them to estimate the size of the animals’ brains and the dimensions of the areas of the brain that process sensory input, such as smell or vision. .
The investigations revealed that, in mammals of the time, the brain regions responsible for vision represented a relatively small proportion of the brain mass.
This only changed in the later Eocene era, highlights the German journal Spektrum, when mammalian species appeared in which the relative size of the brain increased again.
In these species, brain regions that process visual sensory impressions are important for movement control and perform integrative functions. However, the areas of the brain involved in the sense of smell lost importance.
Study Review
What is most surprising about this research is that it challenges previous studies, according to which mammals with larger brains are better at solving challenging cognitive problems and are more flexible in their behavior.
The new research has revealed that those skills weren’t needed for millions of years after the dinosaurs were almost completely gone.
Consequently, the idea prevailed that it was more important to be big and strong rather than smart. “Large brains would probably have been a disadvantage for early Cenozoic mammals in general,” Bertrand concludes, quoted by Spektrum.
What happened next is that mammals with better sensory performance and expanded motor skills were better positioned to evolve in other directions, creating selection pressure toward brain growth.
Predators and omnivores in particular increased in relative brain size and, in this regard, surpassed herbivores of the time. According to a study published in 2019, among animals, the carnivore represents 63% of the species, while 32% are herbivores. And only 3% of animals are omnivores.
Reference
Brawn before brains in placental mammals after the end-Cretaceous extinction. Ornella C. Bertrand et al. SCIENCE, 31 Mar 2022, • Vol 376, Issue 6588, pgs. 80-85. DOI:10.1126/science.abl5584