The camel cousin evolved fluff instead of fat because it was able to linger in an evolutionary slow lane, suggest newly sequenced genomes.
Camels have been helping people out for more than 5,000 years, ever since they were domesticated in Somalia and Arabia. Centuries of conquest, trade, exploration and expansion were carried on the humps of those animals, thanks to their incredible adaptations to extreme desert life. Camels can tolerate temperatures greater than 100°F for days on end, and they can safely lose up to 25 percent of their total body weight between visits to a water hole. By comparison, losing just 15 percent of body weight is fatal for most mammals.
The root of these adaptations, however, has been a mystery. To try to close the knowledge gap, a large team of researchers from institutions in China, Saudi Arabia and Denmark decided to perform high-quality whole genome sequences of the world’s two camel species, Bactrian camels—the two-humped, shaggy variety that lives in Asia—and dromedary camels, the single-humped ones from Africa and the Middle East. For comparison, the team also turned to the alpaca, a close camel relative that is less tolerant of harsh, hot environments. Alpaca have no humps and they lead more relaxed lives in South America’s cool highlands.
Reporting today in Nature Communications, the team reveals that camels and alpaca share about 83 percent of their genomes with both humans and cattle. The genes suggest that the common ancestor of camels and alpaca most likely broke away from cattle around 42.7 million years ago, a time that corresponds with the earliest camel-like fossils found in North America. That creature then split into camels and alpacas about 16.3 million years ago. Bactrian and dromedary camels began to diversify through small, accumulated mutations after their common relative migrated from North America to Eurasia about 4.4 million years ago.
Being so closely related, both camel species and the alpaca have just a few key differences in their genetic architecture. But after examining more than 20,000 genes in each of the three species, the team found that camels in particular showed exceptionally high rates of evolutionary change over a relatively short period of time. That “raises the possibility of camel-specific evolution to adapt to a desert environment,” the authors write. In other words, camels had to hit fast forward on their evolutionary trajectory to survive in the desert.
Adaptations include enhanced stress responses to things like heat, intense UV radiation and dust storms; tweaked fat and water metabolism; better eye protection in the form of long lashes and eyes that can withstand bright sunlight; and greater resistance to respiratory diseases to battle the desert dust. Bactrian camels in particular seem to have evolved special kidney functions that allow them to use high blood glucose, which acts to regulate osmotic pressure between cells and their environment and ultimately to retain water. This explains why the camels seem to constantly exist in a state of hyperglycemia, the authors think.
The camels’ humps—their most conspicuous feature—appeared alongside these genetic edits. Those giant fat mounds can weigh up to 80 pounds and act as an energy reserve for when food gets scarce. The authors think that the number of humps the camels have correspond to the different ways their fat metabolism developed along the evolutionary timeline, although more research is needed to prove that link.
While camels were going through the natural selection meat grinder, alpaca had to contend with just some chilly weather that arrived around the last glacial maximum, about 44,000 years ago. Otherwise, those long-legged, fuzzy creatures enjoyed growing their numbers and expanding down from Central America into South America, never encountering the need for something as cumbersome and outlandish as a hump. So while camels were prized for their incredible resistance to otherwise certain death in the desert, alpaca became prized for their thick, colorful fluff.
In addition to increasing our understanding of these historically and economically important domesticated animals, the authors point out that their findings could help us predict how other species might respond to a hotter, drier world—and it might aid breeding programs that seek to engineer a “supercamel” that can thrive in ever harsher conditions.
cc: smithsonianmag