In the world of big brains, humans have very few competitors. Dolphins come closest, with a brain to body weight ratio just below ours and just above chimpanzees. Now, a new analysis of these sharp swimmers reveals for the first time some of the genetic changes that led dolphins to evolve such large noggins.
"Dolphins evolved from relatively small-brained animals like cows and hippos into this large-brained, highly specialized aquatic organism," says Caro-Beth Stewart, an evolutionary biologist at the State University of New York, Albany, who was not involved in the research. "This is one of the first comprehensive studies to look at rates of molecular evolution in dolphins."
Nearly 50 million years ago, the ancestor of all cetaceans—a group that includes dolphins and whales—began its transition from land lubber to aquatic all-star. To do so, it had to evolve several adaptations: it lost limbs, it developed fins, and it gained the ability to hold its breath for long periods of time. Its brain also grew about three times bigger.
To get a sense of how these large brains evolved, Michael McGowen, an evolutionary biologist at Wayne State University in Detroit, Michigan, and his colleagues compared the dolphin's genome with two of its closest land-loving, small-brained relatives, the cow and the horse, as well as the dog.
Out of the roughly 10,000 protein-coding genes the researchers examined in the bottlenose dolphin genome, they identified 228 mutations that had swept through the population. Since the pressures of natural selection had encouraged the spread of these mutations through the species, the researchers surmised that these mutations were advantageous. Twenty-seven of these mutations were in proteins specifically associated with the nervous system, including transthyretin, which helps transport glucose across the blood-brain barrier, and microcephalin, which partly governs brain and head size. The researchers also found changes to several other genes that allowed neurons to form and break connections more easily, which is crucial to learning and higher cognitive functioning.
In addition, McGowen and colleagues discovered evidence for positive selection in genes affecting the cardiovascular system, which the authors believe were adaptations for carrying blood and oxygen to tissues during prolonged dives. They found additional changes to the small number of genes carried within the mitochondria, the cell's powerhouses. These changes indicated a rise in metabolism, key to fueling the energy-hungry brains, the team reports online today in the Proceedings of the Royal Society B.
The physical changes required to adapt to life in the water appeared rather quickly, in roughly 5 million to 10 million years. McGowen and colleagues thought this would mean the dolphin's genome would have rapid mutation rates, but they found the opposite. The dolphin genome had mutation rates much slower than expected. Interestingly, human genomes also show a similar slowdown in mutation rate. Although he is not yet sure why, McGowen hypothesizes that this slow mutation rate may somehow be a side effect of fast metabolisms and big brains.
"Because dolphins have also evolved large brains, it gives us an example of the independent evolution of big, complex brains to compare to the evolution of the human brain," says Stewart. "By doing this, you can find out what is necessary for a big brain."
For example, both humans and dolphins had mutations in the microcephalin gene, McGowen says, which seems to indicate that this gene may play an important role in determining brain size. He is currently examining the genomes of other animals to see if all large-brained mammals carry mutations in this gene. Other mutations, such as those boosting metabolism to fuel the brain, seem to show more variation, indicating that the outcome is more important than the specific genes involved.