A Hint About How Life Made It Onto Land

Mangrove rivulus fish hate enforced water aerobics. Despite her best efforts, Giulia Rossi, a biologist who recently received her doctorate from the University of Guelph, cannot coax the fish to swim against a current in a laboratory tank. “They refuse to exercise in water,” she told me. “They just let themselves hit the back mesh.”

When plucked out of water, however, the stubborn swimmers quickly whip themselves into shape. Rossi boops their snoots with a clicky ballpoint pen, and the fish—which are amphibious, and can survive on land for weeks at a time—backflip out of reach.

In goading the rivulus into these workouts, Rossi becomes a trainer of both body and brain. Terrestrial CrossFit, she and her colleagues have found, is an excellent way to juice up the piscine mind. Just a few in , are a ringing endorsement of the benefits of cardio. They also provide a potential glimpse, the researchers argue, into the colonizing tactics of our long-gone fish ancestors, who might have eased their by first hopping back and forth—and reaping the cognitive rewards.