On the day of the North American disaster, which occurred about 12 million years ago, horses of several different species were grazing a grass-covered plain in what’s now Nebraska. Perhaps a few of the horses sheltered themselves from the sun under the walnut and hackberry trees that then dotted the landscape. Perhaps a few nibbled at leaves on shrubs. But most were probably busy eating the grasses that had replaced the Eocene wetlands. Accompanying the horses were hump-free camels, saber-toothed deer, strange rhinoceroses, several species of dogs, elegant cranes, and long-tailed secretary birds.
While the horses grazed, a thousand miles to the northwest a supervolcano exploded. Unlike the Laetoli volcano, the Bruneau-Jarbidge eruption was deadly. Its ash spread across hundreds of thousands of square miles, including the plain where the horses grazed. Tiny bubbles of silica—like soap bubbles but much, much smaller—emerged from the volcano and then shattered, creating a multitude of glassy, curved microscopic shards that wafted, like parachutes, a thousand miles distant on the winds that blew to the east. When they finally landed on the grass, the horses and other grazers breathed these shards into their lungs while they ate. Imagine taking several glass Christmas tree ornaments and pulverizing them with a hammer, then spreading those sharp-edged infinitesimals out over a field of grass. That’s what the animals inadvertently ingested.
Paleontologists have worked out the order in which the animals died. First, tiny birds fell out of the sky. Their lungs were the smallest and the most easily damaged. Then the smaller land animals succumbed. Then slowly, the larger animals, including the horses, died, their lungs destroyed by the glass-like micron-size silica that entered with every inhalation. The last to die were the largest animals with the largest lungs, the rhinos.
It was a slow, agonizing death. It must have hurt the horses to breathe, but of course they had no choice. In their misery, many of the animals sought out a local water hole, no more than a slight indentation in the plain that filled when the rains came. There wouldn’t have been a lot of water, but apparently even this tiny oasis offered some kind of solace. Perhaps the horses wanted to drink, or perhaps they were just seeking wet mud to cool their terrible fevers. Their slow suffocation caused bone decay, lung damage, swelling of body organs—damage that’s still visible even today on the many skeletons that have been left in situ for visitors to see.
After they died, the prairie wind continued to blow. The light ash covering the plain drifted over the cadavers, entombing them.
Twelve million years later, in 1971, the paleontologist Mike Voorhies and the geologist Jane Voorhies walked through the area and found an American Lagerstätte, one of those sites like Messel where, no matter how hard you work, there will always be more science to be done. Like Messel, Nebraska’s Ashfall Fossil Beds were unique. Messel fossils were flattened between paper-thin layers of clay and algae. At Ashfall, where the falling ash was light, the animals were preserved in three dimensions, similar to Vesuvius victims. The same tiny glass shards that killed the animals also preserved them. Voorhies likens the preservation material to the ultralight packing peanuts used for mailing delicate objects.
“The reason those peanuts work so well is that they’re curved,” he told me. “If the peanuts were flat, they’d settle out. The thing about the curved shape is that it traps air. When the ash came out of the volcano, it came out as bubbles, and the bubbles crashed against each other and made little curved pieces of volcanic glass.”
You can’t see any of these curved pieces with the naked eye, of course. When I visited the site, I rubbed the silica bits between my thumb and finger and it felt powdery, like flour. It seemed innocent enough. But then I felt a few mild, almost imperceptible pinpricks in my skin. I realized that if I breathed in this material, in only a short period of time, a few days perhaps, it could destroy my lungs.
Ashfall, a National Natural Landmark open to visitors, marks a key turning point in the history of the horse. That its preservation is so spectacular is like a gift from the gods. Even the most casual visitor, like me, can walk into the building where the skeletons have been left in the ground and see a flash point in time when horses (and other animals) lived on the cusp of global change. Paleontologists have found five different intact species of horses at Ashfall, ranging from three-toed horses who were only a bit bigger than the dawn horses to one-toed horses who were almost as large as our own modern horses.
What’s interesting about this, Voorhies told me, is that only a few million years earlier, at least twenty species of horses roamed this region at the same time. Then a drying event occurred. Geologists know this because they’ve found a layer of caliche—hardpan—just below the layer containing the Ashfall horses. This hardpan is evidence, Voorhies said, of a “significant drying event” that may have helped whittle down the number of horse species in the area from twenty to five.
Most of the time when fossils are found, they’re found in bits and pieces. A piece of the skull in one place. A bit of tail bone ten feet away. Perhaps scavengers pulled them apart, or maybe the animals died in floods and the flowing water scattered their bones. It’s not always easy for researchers to figure out how the bones fit together, but at Ashfall there’s no uncertainty.
The skeletons are intact—thanks in large part to yet another bit of serendipity. Millions of years after the horses died, an ice sheet grew over much of the northern half of the continent, shredding everything in its path. Because of the destructive nature of the ice, we lack good records of the life forms it covered.
But the ice sheet stopped just seven miles away from where the horses lay fully articulated and covered in ash. Had it continued, there would have been little, if anything, for Voorhies to find.
I asked why the ice sheet hadn’t come farther south, but was quickly corrected on my wording.
It wasn’t that the ice hadn’t come farther south, Voorhies explained, but that the ice couldn’t flow that far uphill. The most recent ice sheet to cover much of North America was, in some places, miles thick. The weight of all that ice pushed lobes of ice forward in pulses, but the tips of those pulses could only climb so far above sea level before they ran out of power. In eastern Nebraska, the lobe pushed its way all the way up to 1,650 feet above sea level.
Ashfall, Voorhies said, is 1,700 feet above sea level.
When I thought about that, I thought about all the paleontological evidence of horses that must have been destroyed by the relentless ice. How much more we would know, I said to Voorhies, had it not been for the ice.
“There’s so much that goes on in the biological record,” he answered, “that doesn’t leave any trace. Fossils are just a pitiful remnant.” His voice was full of regret.
Because the Ashfall skeletons are intact—and because, remarkably, some of the flesh was desiccated rather than destroyed by bacteria—there’s a wealth of information available. Researchers can see the structure of the cartilage in the horses’ legs, the connections of the horses’ bones with tendons and ligaments, and even what plants the horses ate. Marvelously, the neck flesh on one animal was preserved. We know that the volcano probably exploded in late winter or early spring, because foals have been found in some of the mares’ uteruses, and foals, Jason Ransom and other colleagues have found, tend to be born in the wild in late winter or early spring.
The Ashfall bones will remain where they have rested over the past 12 million years, available during the summer months for the public to see. Excavators are carefully brushing the ash from the fossils, but the fossils themselves will stay in place rather than be removed. Voorhies prefers this approach.
I asked him why.
“I felt like I was vandalizing the site,” he told me. “There’s so much more information here, with the fossils in situ.” For example, by examining the fossils in situ we can recognize that the multitude of animals found here are not touching each other. Had the animals died in a sudden panic, they ought to have been grouped together in terror. But instead, they are a
ll separate.
It was as though, Voorhies said, touching each other would have been too painful.
Ashfall is a snapshot of a critical moment in horse evolution. The five species of horses preserved in this ash include three Hipparion species. In all three, the side toes are much more pronounced than the side toes of the much younger Laetoli Hipparion. These side toes were definitely functional. In the mud surrounding the water hole the Hipparion left tracks, as at Laetoli.
“You can see what you’d expect to see,” Voorhies told me. “They look like tracks left today by the unshod hoof of a modern horse. But then, behind the main hoof are the impressions of the side toes. Looking at the anatomy of the foot, you can see that the side toes had their own sets of ligaments.”
But, phenomenally, at Ashfall there are also one-toed horses. In other words, at Ashfall, we can see the precise point in time when horses evolved the modern hoof: horses with three toes and horses with one toe lived simultaneously. So much for the Victorian ideal of evolution. Here, at Ashfall, is a clear overlap showing that evolution is not linear—not one thing, then the next thing, but many different things, sometimes all at the same time.
Even more surprising is the fact that some of these one-toed horses, called Pliohippus, lay at the site beside Pliohippus horses with three toes. In Pliohippus, however, these side toes were useless.
I asked Voorhies how he knew they were useless.
“If you look at the bones in the side toes [of these Pliohippus], you can see that they did not have ligaments. These side toes would basically be functionless. In not too many generations,” Voorhies said, “they would have disappeared completely.”
“This is in one genus?” I asked.
“Just one species,” he answered.
I was amazed. One species with two clearly different styles of feet. Were the differences irrelevant, like black hair versus brown hair? Or did the difference between three toes and one toe dictate important differences in the lifestyles of the individual animals? Were both types living equally successfully? If so, why did the three-toed horses disappear entirely?
Perhaps Darwin would have been surprised to find one species with two different kinds of feet, but he might also have been pleased. What better proof could he have asked for that his theory of evolution was correct? Twelve million years ago, 8 million years before the Hipparion mare and foal crossed paths with A. afarensis at Laetoli, in the Northern Hemisphere all the way on the other side of the world, a one-toed horse was slowly evolving and also, apparently, slowly overtaking horses with three toes.
Standing there at that site, seeing right before my eyes clear evidence that horses changed in response to a changing planet, was exhilarating and a little bit eerie. I tried to imagine what Darwin would have felt if he had known about this site, or even been able to visit it. The visit would have confirmed his theory, but it might also have refined it a bit. Perhaps he would have understood that evolution was not about “improvement” or “direction,” but just about fitting in. In some situations, right there on the North American plain at that specific time, three-toed horses held an advantage. But in other situations, one-toed horses excelled.
So why did horses ultimately evolve to have only one toe? Traditionally, paleontologists have explained this phenomenon in terms of an improved ability to run over open areas in order to escape predators. The paleontologist and horse expert Christine Janis has suggested that the one hoof may also have allowed horses to roam farther in search of food. Three toes worked well in an Eocene world. In a drier world of grass-covered plains that provided mostly firm footing, running on a single toe had clear advantages.
I asked Voorhies if it would ever be possible for horses to re-evolve three useful toes—say, if the world returned to Eocene conditions.
Probably not, he said.
This is worth thinking about. When horses first appeared 56 million years ago, they had a wide array of options available, evolutionarily speaking. But once an animal begins to follow a certain path—running on one toe, for example—he may become so specialized that he can never go back. His fate is, to some extent, determined.
Voorhies agrees that many of the adaptations of horses during that time had to do with the proliferation of grass. By the time the Ashfall event occurred, grasses had become quite clever. They had learned to spread themselves by piggybacking onto animals. Some of the seeds of various grasses had tiny “hooks” on their seed coats. These hooks could attach themselves to the coat of whatever animal was passing by and get the seeds a free ride to some place where they could put down roots.
“If you walk through a field of high grass and get things stuck to your socks, you’ll know what I’m talking about,” he explained. “There’s a type of grass called needle-and-thread grass, where the seeds are protected by silica.” This grass has a silica-covered “needle” that can prick your finger as easily as a sewing needle and is excellent at embedding into an animal’s hide, or, in our case, into our clothing. It takes days to get all of them out of your socks and off your shoes and pant legs. In this way we, too, are unwitting actors in the plants-versus-animals knockdown.
At Ashfall, however, the triumph of one-toed horses was not yet in full swing. The paleontologist Darrin Pagnac and his colleague Nick Famoso studied a large number of horse fossils and found that 78 percent of the horses had three toes, while only 22 percent had one. Judging by these proportions, the future of three-toed horses seemed secure.
However, the world continued to change.
* * *
Ultimately, only one-toed horses survived. As evidence, paleontologists point to Idaho’s Hagerman Horse Quarry, where a number of broken-up horse bones have been found. Researchers assembled the fossils and determined that there were about 200 individual horses and that, a little more than 3 million years ago, they had all died, perhaps in a flash flood, then washed downstream until they came to rest along the riverbank.
All the horses at this site had only one toe. And they were all members of the same species—Equus simplicidens, the foundation animal for all modern horses, including our modern riding horses, zebras, asses, and the Przewalski’s horse. In a sense, Equus simplicidens is akin to Epihippus, in that the future of horses depended on just one species.
But unlike Epihippus, Equus simplicidens was not rare. He was prolific. His long legs, his single-toe hoof, his flexible digestive system, his large brain and his hardy teeth—all combined to create an animal well suited to North America at that time. His evolutionary history had made him capable of withstanding an extremely varied range of environmental stresses and of eating all kinds of foods that other grazing animals would pass up.
In short order, E. simplicidens spread, spawning many species.
Then something—some kind of perfect storm—occurred. After 56 million years of evolution, the horse became extinct in the Western Hemisphere.
Why? If horses had managed to survive sudden temperature increases and decreases, and if they had managed to learn how to eat silica-covered grass when grapes disappeared, and if they gave up four toes on their front feet in favor of one—all in order to survive—then why were there no horses at all when European colonizers arrived in 1492?
Darwin looked for an answer to this riddle, but couldn’t find one. Over the past 150 years, many of the other riddles of horse evolution that perplexed Darwin have been solved, but not this one. The problem remains the subject of heated, and sometimes overheated, debate. Indeed, these disagreements are sometimes representative of “science at its ugliest,” to borrow a phrase from the biologist Bill Streever.
I wanted to know two things: What happened to the horses of the Western Hemisphere? And—how did the science become so controversial?
5
EQUUS
A hoof is like a second heart in a horse.
—J. EDWARD CHAMBERLIN, Horse
The last meal of the golden-coated Yukon horse was buttercups.
It took 100 mill
ion years for the Cretaceous Terrestrial Revolution to get those buttercups up onto that Ice Age northern plain. And it took about half that time for the scampering, warmth-loving dawn horses of the Eocene to transform into Equus, an animal capable of surviving in the frigid Arctic in order to eat those flowers.
But finally, after tens of millions of years, in the Yukon horse, the mosaic of the modern horse—the high withers, the hocks well off the ground, the spine and legs specialized for speed and endurance, the single-toed hoof, the sensitive muzzle, and the deep-jawed mouth with huge, durable teeth—was present in one powerful package.
In 1993, the miners Lee Olynyk and Ron Toews were digging for gold near Canada’s Dawson City, just a bit south of the 60th parallel, close to the Arctic Circle. One September day, as operations were winding down for the year, their machines took a chunk out of the black muck oozing from the bank of Last Chance Creek.
“What’s that?” Olynyk’s young son asked.
Olynyk took a look. It was the carcass of a horse. Not just the bones, as at Ashfall, or an imprint, as at Messel, but a carcass of flesh and hair and tendons and mane and tail and gut and intestine. As well-preserved as if you’d just pulled it out of your freezer. But the animal seemed a bit odd. It wasn’t a horse as much as a pony, Olynyk decided. He figured maybe it was one of the pit ponies the old miners used to pull carts in the underground mines.
“It smelled strong, fresh, like a horse smells when you just come in from a ride. A nice horsey smell,” he told me. “The leg came out. Then we saw the hide.”
The more he looked, the less he bought into the pit pony idea. It just didn’t feel right. So he made some calls. Paleontologists arrived on the scene, pulled the horse out, and sent the flesh to a laboratory to find out its age.
The carcass turned out to be from the Ice Age and was almost thirty thousand years old. This horse lived in the Arctic Yukon at the time when, at the other end of the dry-adapted Eurasian steppes, Paleolithic artists were painting horses on the cave walls of France and Spain and craftsmen were decorating their weaponry with horse images. The Vogelherd horse had already been around for a few thousand years.
The Horse Page 13