The brains of primates and cetaceans arrived at the same cognitive space while evolving along quite different paths.
Researchers compared CT scans of current and past dolphin species' brains.
The ratio of dolphins' brain to body mass changed significantly twice over the past 35 million years.
Ecolocation may be one reason dolphins developed large brains.
Dolphins and humans share a similar brain size, according to the first map of cetacean brain evolution over the past 47 million years.
Experts have long known that toothed whales boast exceptionally large brains. Some species, including the famously bright dolphins, have capabilities previously only ascribed to humans and, to some extent, other great apes.
For instance, dolphins can recognize themselves in mirrors and understand symbol-based communication systems and abstract concepts.
Such intelligence is probably due their big brains, but the evolution of such brains has remained a mystery.
To investigate this question, Lori Marino, from Emory University in Atlanta, and colleagues carried out the largest fossil study ever done on animals, searching museum collections for four years.
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The team, whose research will be appear in the December issue of The Anatomical Record, tracked down 66 fossilized cetacean skulls.
After determining the specimens' brain sizes with CT scans, the researchers estimated body mass by examining the size of bones around the base of the skulls.
They also studied brain and body weight data from 144 modern cetacean specimens, for a total sample of 210 specimens representing 37 families and 62 species.
The aim was to find each creature's encephalization quotient (EQ)-- the ratio of brain to body mass. An average-sized brain for the body weight produces an EQ of one. An animal with an EQ less than one indicates a smaller-than-average brain; an EQ greater than one, a relatively large brain.
Humans are the brainiest of all creatures, with an EQ of seven.
The skull examination found two points in cetacean evolution when the EQ jumped.
"The time when the first whale suborder went extinct and the new suborders emerged, about 35 million years ago, is a critical phase in cetacean evolution because the changes in encephalization level were tremendous and there were also other changes in body size and dentition," Marino said.
Indeed, about 35 million years ago, dolphins' earliest ancestors, from the Archaeoceti group, were about nine meters (29.5 feet) long, had sharp teeth and an EQ of about 0.5.
The EQ jumped to 2.5 in Odontocete group, smaller creatures with smaller teeth that followed the Archaeoceti after they mysteriously died out.
That growth in brain size came as the first cetaceans developed echolocation -- emitting high-frequency sounds that bounce off objects, creating an echo that the animal interprets, Marino said.
The study showed that a second brain size boost occurred in the origin of the superfamily Delphinoidea -- oceanic dolphins, porpoises, belugas and narwhals -- about 15 million years ago. Their EQs jumped to four and five.
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"Essentially, the brains of primates and cetaceans arrived at the same cognitive space while evolving along quite different paths. What the data say to me is that we, as humans, are not that special. Although we are highly encephalized, it's not by much or for that long compared with odontocetes," Marino said.
According to Patrick Hof, a neurobiologist at Mount Sinai School of Medicine in New York City, Marino's study is one of the very few modern examples of paleoneurologic research on specimens other than the human fossil record.
"It is indeed most interesting and important research. It shows that several evolutionary points in cetacean evolution took place corresponding to the emergence of distinct suborders and later on of delphinoidea," Hof told Discovery News.
"It is rare to see such a good correlate of evolution in the fossil record, and, more to the point, to be able to make functional inferences from fossil taxa by comparison with extant species," he said.