Few animals have evolved to survive the unforgiving Antarctic like penguins. Species like the emperor penguin have overlapping layers of insulating plumage, tightly packed veins to recycle body heat and just enough paunch to weather wind chills that approach minus 80 degrees Fahrenheit.
With all these cold-weather adaptations, it’s difficult to envision penguins living anywhere else. But fossils of ancient penguins have popped up along the Equator, and many of these prehistoric seabirds predate the formation of Antarctica’s ice sheets. “They lived through some of the hottest times in Earth’s history, when it was five degrees warmer at the Equator,” said Daniel Ksepka, a paleontologist at the Bruce Museum in Greenwich, Conn. “They basically evolved in an ice-free context.”
To determine how penguins transitioned from balmy, tropical waters to polar seas, Dr. Ksepka and his colleagues recently analyzed the genomes of all living penguins, including pipsqueaks like the foot-tall blue penguinrarities like the endangered yellow-eyed penguin and showstoppers like the yellow-tufted rockhopper penguin. However, the genetics of modern penguins could tell the researchers only so much. Most modern lineages date back only a couple million years, obscuring most of the 60-million-year odyssey of penguin evolution.
Dr. Ksepka said that more than three-quarters of all penguin species “are extinct now.” He added, “You have to look at the fossil record, or you’re only getting a fragment of the story.”
To complement the modern data, the researchers examined fossils from a motley crew of ancient seafarers. Some prehistoric penguins folded tropical waters off Peru, using spearlike bills to harpoon fish. others sported long legs, and the largest may have pushed seven feet tall. Some even had patches of rusty red feathers.
Comparing the genomes of modern penguins with fossil penguins allowed the team to reconstruct penguin evolution. In their findings, published Tuesday in NatureCommunications, the researchers pinpointed genes that helped penguins transition from wading through warm waters to perfecting the polar plunge. Some of these genes helped penguins’ ability to pack on blubber, while others molded their shriveled wings into streamlined flippers. Some even bolstered penguins’ immune systems or helped them tolerate low oxygen during deep dives.
The researchers also identified genes that helped fine-tune penguin eyes to peer through here depths. While most birds have four color cones in their eyes, one of these is inactive in penguins, hamering their ability to see green and red. Instead, their eyes have adapted to adjust to the ambient blue of the ocean.
Some missing genes were perplexing to the researchers. While modern penguins gobble krill, the team found evidence that their ancestors lacked genes that would have helped break down crustacean shells. This may be evidence that ancient penguins were spearing larger prey, like fish and squid. Penguins retain a restricted palate. Their taste receptors can pick up only salty and sour tastes, which is “pretty good if you’re eating fish,” Dr. Ksepka said. “That’s probably why they’re pretty happy with sardines.”
When these changes occurred in ancient penguins, they stuck. The genetic analyzes revealed that penguins generally have the lowest evolutionary rate of any group of birds. Because they look so weird, this glacial rate of change seems surprising. But it reveals how successful the penguin’s plump yet streamlined body plan is — over millions of years, it has changed only in slow increments. But emperor penguins, which breed during the bitter Antarctic winter, have the highest evolutionary rate of any penguin, leading the researchers to deduce that colder temperatures somehow speed penguin evolution.
Juliana Vianna, an ecologist at the Pontifical Catholic University of Chile, says this idea is consistent with the southward march of penguins occurring during bouts of global cooling. “Their evolutionary history is pretty much associated with historical climate change and glaciation,” said Dr. Vianna, who recently led similar research but was not involved in the new study.
Understanding how penguins changed in the past may offer clues to how these cold-weather specialists could fare in a hotter future. “Warming temperatures will impact the biogeographic ranges of penguins, the species they rely on as food and the species that, in turn, hunt them,” said Daniel Thomas, a paleontologist from Massey University in New Zealand and an author of the new study.
While the research is a comprehensive look at the penguin family, Dr. Ksepka said, there’s still one seabird missing — the last flying penguin. The small, shearwaterlike bird probably lived in ancient New Zealand, but its fossils have proved elusive. “That would be the No. 1 thing I’d ask for if I had a genie,” he said.