Taking the Plunge

We like to think of boundaries as being clear-cut borders, but at least in the biological world they generally turn out to be fuzzy zones of change. The line between land and sea is my own favorite example. Last summer my wife and I would sometimes take our oldest daughter Charlotte to the beach. At the time she was a year old and refused to put her toe in the water. This summer she heads straight in, but only about up to her knees. She runs back out and goes back in, repeating the circuit a few dozen times. Next year, I still expect to see her chin above the water line. In her own tadpoling way, Charlotte is reenacting an evolutionary journey taken many times by her fellow mammals--the evolutionary transition back to the water.

These treks have something profound to say about biological change--how life can start out exquisitely adapted to one world and then eventually become adapted just as exquisitely to an utterly different one. Before creationists began marketing bacterial flagella and other examples of intelligent-design snake oil, they loved to harp on the transition from land to sea. Who could possibly believe the story those evolutionary biologists tell us, of a cow plunging into the sea and becoming a whale? And it was true, at least until the 1980s, that no one had found a fossil of a whale with legs. Then paleontologists working in Pakistan found the fossil of a 45-million year old whale named Ambulocetus that looked in life like a furry crocodile. Then they found a seal-like whale just a bit younger. Then they found tiny legs on a 50-foot long, 40-million year old whale named Basilosaurus. I wrote about these discoveries and others like them in my first book, At the Water's Edge, in 1998. I'm amazed at how the fossils have continued turning up since then. Paleontologists have found goat-like legs on a dog-sized whale that lived 50 million years ago, known as Pakicetus. They've found other whales that may have been even more terrestrial than Pakicetus, and many others that branch off somewhere between Pakicetus and Basilosaurus. In the latest review of fossil whales, the evolutionary tree of these transitional species sports thirty branches.

All these discoveries have apparently made whales unsuitable for creationist rhetoric. Yes, you can still find some pseudo-attacks on the fossils, but you have to look hard. The more visible creationists, the ones who testify at school board meetings and write op-eds for the Wall Street Journal, don't bring up whales these days. The animals apparently no longer serve the cause. It's hard to distract people from evidence when it can kick them in the face.

Whales, moreover, were not the only mammals that moved into the water. Seals, sea lions, manatees, and other lineages evolved into swimmers as well, and paleontologists are also filling in their fossil record. It's fascinating to compare their invasions, to see how they converged on some of the same strategies for living in the water, and how they wound up with unique adaptations. The June issue of The Journal of Vertebrate Paleontology has two papers that shed light on one of the weirdest of these transitions--a transition, moreover, we know only from fossils. The animals in question were sloths.

That's right--I'm talking about the sort of animals that hang from trees by their three toes. Sloths may seem an unlikely choice for a sea-going creature; if you threw one of these creatures in the water, I'd imagine it would sleepily sink away without a trace. I've never hurled a three-toed sloth myself, so I can't say for sure. But the sloths alive today are actually just a vestige of a once-grand menagerie that lived in North and South America. Many species prowled on the ground, growing as tall as ten feet. And one lineage of these giant sloths that lived on the coast of Peru moved into the ocean.

In 1995 Christian de Muizon of the National Museum of Natural History in Paris and his colleagues announced the discovery of sloth fossils in Peru dating back somewhere between three and seven million years. The rocks in which they found the bones had formed in the sea; the same rocks have yielded other ocean-going creatures including fish, sea lions, and weird dolphins with walrus-like tusks. The sloths, de Muizon concluded, were aquatic as well. Terrestrial sloths have much longer lower leg bones than upper ones, but the Peruvian sloths had reversed proportions. Manatees and otters also have reversed legs, which suggests that the sloths' limbs were adapted for powerful swimming strokes. The front of their skull was manatee-like as well: its jaws extended out well beyond its front teeth, with a rich supply of blood vessels. Like manatees, de Muizon argued, the sloths had powerful muscular snouts they used to root out sea grass.

In their initial report, the paleontologists dubbed the fossils Thalassocnus natans. But it was already clear that they might have more than one species on their hands. In the years since, they've dug into the Peruvian rocks and found hundreds of sloth fossils, which they have been carefully studying and comparing. The new papers are not the last word on Thalassocnus, but the sloths are already shaping up as a great illustration of a transition to the water.

Instead of a single species, de Muizon's team has now identified at least five. They lived, respectively, seven to eight million years ago, six million years ago, five million years ago, three to four million years ago, and, finally, 1.5 to three million years ago. The earliest species look more like ground sloths on land, while later species show more adaptations to the water. For example, the radius, one of the lower bones of the foreleg, became much broader. The change--which can also be seen on sea lions--allowed the forelegs deliver a better swimming stroke. The teeth become less like those of ground sloths, adapted for browsing on leaves and assorted vegetation. Instead, they became adapted for full-time grazing. The coast of Peru is a bone-dry desert with nothing to graze on, and so the only thing to graze on would be sea grass.

The sloth skull changes as well. Both the upper and lower jaws stretch out further and further. From the oldest species to the youngest, the distance from the front teeth to the tip of the jaw nearly doubles. At the same time, the entire skull became stronger, to withstand the forces involved in tearing sea grasses from the sea floor. And finally, bones in the palate evolved to support muscles that could keep the digestive tract separate from the sloth's airway--something important when you're feeding underwater.

The changes documented in these fossils suggest that the earliest Thallassocnus sloths eked out an existence on land along the Peruvian shore. In a bleak desert, the sea grass that washed up on the beach would have been like manna. De Muizon and his colleagues have found another clue in the early sloths that supports this beach-comber hypothesis: their teeth bear scrape marks that suggest they were getting a lot of sand in their mouths; later sloths show no such marks. Over five million years or so, the sloths evolved adaptations that allowed them to move further and further out into the water, to feed on sea grass beds. Natural selection would have put a strong premium on these adaptations, since they would let sloths graze in lush underwater forests rather than pick through sandy flotsam and jetsam on the beach.

De Muizon's group have yet to sort out all the differences throughout the entire skeletons of all five species. We'll have to wait for those papers. But there's enough in print now to raise some interesting questions. In whales, seals, and manatees alike, their arms and hands became flippers--stubby, webbed, fin-like limbs. Thalassocnus still had big, long-clawed fingers on its hands. De Muizon proposes that they would have enabled the sloths to hold onto rocks to stay submerged as they fed on sea grass. Manatees don't need to do this because their bones are especially dense; the sloths had not yet acquired this adaptation. It seems that Thalassocnus only traveled part of the way down the road to a marine life before they became extinct.

Why they became extinct (as opposed to manatees, for example), is also intriguing. Did something happen 1.5 million to 3 years ago that ruined their home? Perhaps the coastal waters off Peru became too cold. If the sloths had spread further along the coast, they might not have been so vulnerable. Other mammals moved into the water at very restricted sites as well. For their first few million years or so, whales could only be found off the coast of Pakistan. If some Indian volcano had blanketed the neighborhood in ash, we might never have known what a whale looks like.

UPDATE Monday June 21, 7 pm: PZ Meyers has put photos of one of the skulls on Panda's Thumb