Few animals have evolved to survive the unforgiving Antarctica like penguins. Species like the emperor penguin have overlapping layers of insulating plumage, tightly packed veins to recycle body heat and just enough belly to endure chills approaching minus 80 degrees Fahrenheit.
With all these cold-weather adaptations, it’s hard to imagine penguins living anywhere else. But fossils of ancient penguins have turned up along the equator, and many of these prehistoric seabirds predate the formation of Antarctica’s ice sheets. “They’ve been 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, Connecticut. “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 reviewed the genomes of all living penguins, including peeps like the foot-long blue penguin, rarities like the endangered yellow-eyed penguin, and showstoppers like the yellow-crested rockhopper penguin. However, the genetics of modern penguins could only tell the researchers so much. Most modern genera date only a few million years old, obscuring most of the 60-million-year odyssey of penguin evolution.
dr. Ksepka said more than three-quarters of all penguin species “are now extinct.” He added, “You have to look at the fossil record or you’ll only get a snippet of the story.”
To supplement modern data, the researchers examined fossils from a motley crew of ancient sailors. Some prehistoric penguins navigated tropical waters off the coast of Peru and used spear-like beaks to harpoon fish. Others wore long legs, and the largest may have been pushed as long as seven feet. Some even had spots of rusty red feathers.
By comparing the genomes of modern penguins with fossil penguins, the team was able to reconstruct the evolution of penguins. In their findings, published Tuesday in Nature Communications, the researchers identified genes that helped penguins transition from wading through warm water to perfecting the Arctic dive. Some of these genes helped penguins pack on blubber, while others shaped their shriveled wings into streamlined fins. Some even boosted penguins’ immune systems or helped them tolerate low oxygen levels during deep dives.
The researchers also identified genes that helped tune penguins to peer through icy depths. While most birds have four-colored cones in their eyes, one of them is inactive in penguins, hindering their ability to see green and red. Instead, their eyes have adapted to the ambient blue of the ocean.
Some of the missing genes were confusing to the researchers. While modern penguins gobble up krill, the team found evidence that their ancestors lacked genes that would have helped break down crustaceans. This could be evidence that ancient penguins spear larger prey, such as fish and squid. Penguins maintain a restricted palate. Their taste receptors can only pick up on salty and sour flavors, which is “pretty good if you eat fish,” said Dr. ksepka. “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 rate of evolution of any group of birds. Because they look so bizarre, this icy speed of change seems surprising. But it shows how successful the penguin’s plump yet streamlined body plan is — it’s only changed slowly over millions of years. But emperor penguins, which breed during the bitter Antarctic winter, have the fastest evolutionary rate of all penguins, leading the researchers to conclude that colder temperatures somehow accelerate penguin evolution.
Juliana Vianna, an ecologist at the Pontifical Catholic University of Chile, says this idea is consistent with the southern march of penguins that takes place during periods of global cooling. “Their evolutionary history is pretty much associated with historical climate change and ice age,” said Dr. Vianna, who recently led similar research but was not involved in the new study.
Understanding how penguins have changed in the past can provide clues as to how these cold-weather specialists might fare in a warmer future. “Warming temperatures will affect the biogeographic range of penguins, the species on which they depend for food and the species that in turn hunt them,” said Daniel Thomas, a paleontologist from Massey University in New Zealand and a researcher. author of the new study.
While the research is a comprehensive look at the penguin family, said Dr. Ksepka, there is still one seabird missing – the last flying penguin. The small, parrot-like bird likely lived in ancient New Zealand, but its fossils have proved elusive. “That would be the number 1 I would ask for if I had a ghost,” he said.