Kumimanu wasn’t just exceptionally big; it also ranks among the oldest penguin fossils yet found. Both its age and its size make Kumimanu important to understanding the astonishing transformation that turned a lineage of flying birds into flightless swimmers.

The 18 modern species of penguin, ranging from the coast of Antarctica to the Galápagos Islands at the Equator, are impressively adapted to aquatic life. Rigid, blade-shaped wings enable them to shoot through the water at up to 22 miles an hour. Record-setting human swimmers don’t even reach six m.p.h.

But their adaptations to water have also left them unable to fly. When penguins haul out to rest or rear their young, they can only waddle about on stumpy legs. “They’re so unbirdlike that many people would not know they are birds,” Dr. Mayr said.

While penguins may look profoundly different from other birds, their DNA points to a close kinship to such species as albatrosses and petrels. These birds all fly over water to hunt for prey, hinting that the ancestors of penguins may have, too.

Auks, which can dive over a hundred feet underwater, may be a living model of the first penguins.

Birds accumulate mutations in their DNA at a roughly clocklike rate, allowing scientists to estimate when their lineages branched apart. Studies suggest that penguins diverged around the time of the mass extinction that struck the planet about 66 million years ago.