Penguins are perhaps best known for being flightless birds whose wings help them “fly” through the frigid Antarctic waters. But penguins lost their ability to fly and instead became streamlined swimmers around 60 million years ago, long before the Antarctic ice sheet formed, and researchers have now revealed how that happened.
A new study of penguin fossils and the genomes of current and recently extinct penguins has identified a variety of genetic adaptations the birds made to live an aquatic lifestyle; from a vision sensitive to blue tones underwater to genes related to blood oxygenation, and even changes in bone density. Together, the findings suggest that penguins as a group adapted to survive some serious environmental changes that played out over millions of years.
From flight to no flight
The oldest penguin fossils date back 62 million years, said study co-author Daniel Ksepka, a paleontologist at the Bruce Museum in Greenwich, Connecticut. At that time, penguins could no longer fly, although they looked very different from modern penguins. They had longer legs and beaks, and their wings still looked more like wings than flippers, Ksepka told LiveScience.
“These early ones are probably evolving from a puffin-like animal that could still fly through the air,” Ksepka said. (This flying ancestor has yet to be discovered in the fossil record, so it is not known precisely when penguins lost their aerial abilities.)
Over time, evolution created a “motley crew of interesting penguins,” Ksepka said, from penguins with long lance-shaped beaks to penguins with red feathers to birds that were a foot or two taller than the largest penguin species on Earth. today, the emperor, who is about 3 feet 7 inches (1.1 meters) tall.
In the study, the researchers evaluated the fossil evidence along with the genomes of all penguins still living and the partial genomes of those that became extinct in the last few hundred years. The findings suggest that penguins originated near what is now New Zealand sometime before 60 million years ago, dispersed to South America and Antarctica, and then returned to New Zealand. Most species alive today diverged from each other in the last 2 million years, Ksepka said. During that period, the Earth has gone through cycles of glacial and interglacial periods in which the polar ice expanded and retreated. The advancing ice pushed the penguins north, probably separating some populations from each other and allowing them to follow their own evolutionary paths for about 100,000 years. When the ice retreated, the separate penguins had become different species.
“It doesn’t affect all species equally, but it’s almost as if someone is turning a crank to create more penguin species,” Ksepka said.
Despite all the changes they’ve been through, penguins have the slowest rate of evolutionary change of any bird, the researchers report July 19 in the journal. nature communications (opens in a new tab). This was surprising and remains unexplained, Ksepka said. Larger animals and those that reproduce relatively slowly, such as penguins, tend to have slower evolutionary rates, she said. However, some birds that are larger than penguins evolve faster than penguins. Other types of birds that reproduce at similar rates to penguins also evolve more quickly, so more work is needed to understand why penguins evolve so slowly, Ksepka said.
Although the evolution of penguins may be comparatively slow, it has provided them with many adaptations for life in and near the sea. They share a set of genes with other flightless birds that likely shortened their wings, and they also have unique genes that may have turned many of the penguin ancestors’ wing muscles into tendons, which stiffened the wings. of the penguins and made them more like flippers. The researchers also found mutations in genes associated with calcium storage, which may contribute to the dense bones that help penguins dive.
Evolution has also brought about many other changes, such as genes associated with fat storage and temperature regulation. An interesting finding was that penguins lost several genes early in their evolution that were related to the digestion of crustacean exoskeletons. This suggests that early penguins had a diet focused on prey like fish and squid, Ksepka said. But the expansion of the ice sheets created an Antarctic ecosystem rich in krill, which are small crustaceans. Fortunately, the researchers found that the penguins had one gene left, the CHIA gene, that allowed them to digest crustaceans.
“If that last one had gone out, they might have had a hard time digesting [krill]Ksepka said.
About 75% of all penguin species that have ever lived are now extinct, and climate change may drive even more extinct, Ksepka warned. This is especially true for species with a niche lifestyle, such as emperor penguins (Aptenodytes forsteri) that breed entirely on sea ice. If sea ice melts, Ksepka said, emperor penguins could struggle to find breeding ground. At the other end of the spectrum, the tiny penguins that inhabit the rocky Galapagos islands live so far from other lands that they have nowhere to flee if their equatorial habitat gets too hot.
“We definitely think these animals are sensitive to environmental change and in many cases are already considered endangered,” Ksepka said. “In other cases, they could become much more vulnerable in the coming decades.”
Originally published on Live Science.