Whales, seals and manatees are so at home in the water that it’s easy to believe their recent ancestors were also aquatic. That conclusion is harder to sell for lumbering elephants, burrowing moles or bumbling spiny echidnas. And yet, new evidence from Scott Mirceta at the University of Liverpool suggests that all of these groups recently descended from ancestors that spent a lot of time in the water.
Mirceta’s conclusion is based on a single protein called myoglobin, and it shows just how much one molecule can tell us about the evolution of an entire group of creatures.
Myoglobin is an oxygen-storing and iron-carrying protein found in the muscles of all mammals. It gives meat its red colour. Animals that live in the water tend to have more myoglobin than those that live on land, and those that dive regularly, like sperm whales or elephant seals, have the highest concentrations of all. With every inhalation, the myoglobin bonanza in their muscles traps a huge amount of oxygen, allowing them to carry out a lot of activity before needing another influx. So, if you know how much myoglobin an animal has, you can work out how long it can hold its breath.
Sperm whales are among the best mammals at breath-holding, and can stay underwater for 90 minutes at a time. Sure enough, their muscles are rife with myoglobin, and their version of this molecule is one of the best studied of all proteins. In 1959, English biochemist John Kendrew worked out its structure, down to the position of every atom—the first time anyone had done that. He earned a Nobel prize for this breakthrough three years later.
Fifty years on, and myoglobin is still yielding new secrets. For example, Mirceta’s team found that diving mammals don’t just have more of the stuff—their myoglobins also have more positively charged surfaces. This means that the individual molecules repel each other more strongly. Thanks to this charge, deep-divers can pack their myoglobin into ever higher concentrations without it clumping together uselessly.
The team found highly charged version of myoglobin in the muscles of many aquatic mammals, including seals and walruses, whales and dolphins, beavers and muskrats. Other partially aquatic species, like American water shrews and star-nosed moles, have myoglobins with less charge than those full-time swimmers but still more than land-living species.
Of course, mountaineers and burrowers also need a lot of oxygen, but the team found that their myoglobins lack the positive charges of diving mammals. It seems that a positively charged myoglobin is a clear sign of a diving lifestyle.
That’s huge news.
Muscles don’t fossilise, so you can’t measure how much myoglobin an extinct animal had. But you can work out how charged its myoglobin was. A protein’s charge depends on which amino acids it contains. By working back from the myoglobins of modern mammals, the team could reconstruct the amino acids of their ancestors’ proteins, and deduce how positively charged they were. And from that, they could work out how much myoglobin these long-extinct species had, whether they were divers, and even how long they could hold their breaths.
“This is really cool,” says Jennifer Burns from the University of Alaska, who studies marine mammals. “The evolutionary history of marine mammals has been worked out from hard parts but while body shape and size hinted at diving behaviour, it couldn’t elucidate diving capacity. This combination of approaches is a unique and important advancement in the field.”
The team team applied their methods to a family tree that included 130 living mammal species. Some of the conclusions were unsurprising.
The whales gradually developed highly charged myoglobins between 54 and 36 million years ago. Early members like wolf-sized Pakicetus weren’t particularly specialised for life in the water, and couldn’t even hold their breath for 2 minutes. Later species like the huge Basilosaurus had strongly charged myoglobin and were fully adapted to life in the water, but even this giant could only hold its breath for 17 minutes. This suggests that more recent species like sperm whales and beaked whales evolved to exploit extremely deep sources of food that their ancestors could never reach.
Other results were more surprising. The team found that echidnas and moles both descend from ancestors with charged myoglobins. These species might spend their time burrowing today, but they hail from a line of swimmers. The same was true for dedicated landlubbers like elephants and hyraxes—small mammals that look like guinea pigs but are actually close relatives of elephants.
“At first, we thought: There goes that theory!” says Michael Berenbrink from the University of Liverpool, who led the study. But on closer inspection, he found that the conclusions weren’t as far-fetched as they seemed. “We looked up echidnas and found a recent paper that proposed, based on fossils, that echidnas had an amphibious past,” he says. Other palaeontologists had also suggested that elephants and moles had aquatic ancestors. Myoglobin was just reiterating the story that the bones were starting to tell.
Still, Berenbrink says, “We always wanted to look at loopholes.” Among living aquatic mammals, the manatees and dugongs have myoglobins with little positive charge. But that’s probably because they have little need for oxygen-hogging muscles. They’re sluggish animals with no need to dive, since the plants they eat grow in shallow waters.
The team concluded that 65 million years ago, when most dinosaurs were going extinct, the common ancestor of elephants, hyraxes and manatees was swimming around, using muscles powered by highly charged myoglobin. If they’re right, it’s a wonderful conclusion. We don’t have any bones for this ancestor and we can only speculate what it looked like, but we can say with confidence that it was partially aquatic.
Berenbrink is quick to point out that his team’s impressive study depended on a huge amount of work by other scientists. They relied on an accurate family tree that showed how mammals are related, which could only be constructed thanks to a lot of sequencing and fossil-hunting. They used measurements of the diving times, myoglobin levels, and oxygen capacities of different mammals, which physiologists have amassed over decades. “We just added a new dimension to the mix,” says Berenbrink. “On its own, we couldn’t do much with myoglobin.”
Reference: Mirceta, Signore, Burns, Cossins, Campbell & Berenbrink. 2013. Evolution of Mammalian Diving Capacity Traced by Myoglobin Net Surface Charge. Science http://dx.doi.org/10.1126/science.1234192