Recent research by Marino and her colleagues has traced the changing encephalization, or relative brain size, of cetaceans during the past 47 million years by using magnetic resonance imaging and histological studies of the fossil record. While modern humans have brains that are seven times bigger than would be expected for our body size, giving us an encephalization level of seven, some modern dolphins and whales have an encephalization level close to five—not a huge difference, says Marino. For example, Homo sapiens’ closest relatives, the great apes, have encephalization levels of only two to two-and-a-half.
“While humans are the most encephalized—the brainiest—creatures on earth, we are relative newcomers to that status,” says Marino. “The cetaceans enjoyed a tremendous increase in brain size and organization about 35 million years ago, whereas humans got their big brains much more recently during the past one to two million years.”