by Janet A. Ginsburg

Introduction

Eohippus. Mesohippus. All the “hippi” and that led to horses. Growing up in Chicago, my favorite display at the Field Museum was, without a doubt, “The Evolution of the Horse.” Dinosaurs and mammoths may have been more popular, but the delicate forms of prehistoric horses, the toes that kept merging until they became hooves, the wide eyes, the grace, the family-tree of it all – nothing could compare with this skeletal herd, tucked away in a hushed and dim back hall. I was a horse nut girl. My copies of "Black Beauty" (both the picture book version and the “real” book by Anna Sewell), "Misty of Chincoteague," and basically anything by Marguerite Henry (illustrated by the incomparable Wesley Dennis) were well-thumbed and much loved. Model horses roamed the bookshelf range, while trusty stick-horse mounts munched imaginary hay in the stall over in the corner. The real stars of old “Roy Rogers” reruns? Trigger and Buttermilk, of course. Horses to me were simply a wonder.

I still look for horses whenever I go to natural history museums. So many fossils have been excavated that just about every museum of any size has a set. How could there have been so many, and then none at all?

The mystery deepened when I found myself on the Wyoming / Montana border, filming a television segment about a small band of horses, descendants of escaped Spanish conquistador mounts that had been living wild for the past 400 years. The horses were thriving in harsh terrain where natural predators abound: bears, mountain lions, lightning strikes, wildfires, drought, winter. It took a helicopter to round them up.

Over tens of millions of years, members of the horse family (Equidae) traveled from their North American home over the Bering Land Bridge.

“During the Pleistocene, equids were the most abundant, medium-sized grazing animals of the grasslands and steppes of Africa, Asia and the Americas,” according to a report by the World Conservation Union (IUCN). Seven wild species still survive in Asia and Africa. What could have happened 10,000 years ago to cause all the horses of North America to vanish?

Here is one possible explanation…

*******

THE MYSTERY OF THE ANCIENT HORSES

BACKGROUND

The horse is an American original. Between 55 and 60 million years ago,a dog-sized,

multi-toed Hyracotherium (nee Eohippus or “dawn horse”) was nibbling leaves in an Eocene forest. Dozens of species and genera followed, leading eventually to Equus caballus, the modern horse. Prehistoric equids crossed the Bering Land Bridge, settling and evolving everywhere from Siberia to Africa. Yet horses, clearly one of Nature’s most enduring and successful animal designs, died out completely in North America between 10,000 and 11,000 years ago, part of a mass extinction of large animals (megafauna) that saw the demise of dozens of species.

Bones, teeth and the occasional bits of fur are all that’s left of mammoths, mastodons,

saber-toothed tigers, and giant sloths. Horses were the only ones to return, hitching

a ride with Columbus on his second voyage to the New World in 1493. Their absence

for so many millennia may have changed the course of human history. In his book,

“Guns, Germs & Steel,” about why certain societies triumph while other fail, Jared Diamond notes that Spanish conquistadors had several tactical advantages that allowed them to quickly defeat much larger Indian forces. Perhaps the most important in the heat of battle: They were on horseback. Conquistadors appeared frighteningly powerful on these strange, never-before-seen animals. Would the battles have turned out differently if horses had survived in the Americas and been domesticated here as well?

THEORIES

What happened? Paleontologists have argued for years about whether the Pleistocene extinctions were due to climate change (loss of habitat), or overzealous human hunting. North America's climate had changed many times over millions of years without triggering anything so severe, but humans were new to the equation. And there is plenty of evidence for a pattern of wildlife extinctions in the wake of human settlement around the world. Hunting could easily have crashed populations of species such as mammoths that were slow to mature and reproduce, especially if they had few natural predators and simply didn’t recognize humans as a threat. Still, even the most talented hunters armed with the sharpest Clovis points would have had a more difficult time with the large of herds of horses, a species with a comparatively high birth rate (10%) and an ability to survive in harsh terrain. Horses also were literally born to run, shaped by evolution to flee first and ask questions later. How do you kill all of them?

No doubt climate change and hunting played important roles. But could there have been another contributing factor at work?

DISEASE

Imagine a deadly epidemic whose speedy spread across the continent was the result of a lethal combination of geography, vectors and virulence. It took West Nile virus -- which kills about about third of diagnosed horses -- just three years to spread from the Atlantic to the Pacific. Could a similar plague have led to the demise of their ancestors? Are there parallels between the Pleistocene and the present?

Over the past 10 years, a few paleontologists, notably Ross MacPhee at the American Museum of Natural History, have begun to promote a disease extinction theory. MacPhee believes that a “hyperdisease,” a super deadly pathogen capable of infecting dozens of species, caused the massive die-offs. Many wildlife biologists are skeptical that a single disease, or even a handful of diseases, could cause such high mortality in so many different types of animals. Even H5N1 – bird flu -- although extremely lethal to birds, a threat to felines and a potential pandemic in humans, would at its worst kill off only a small percentage of those species’ populations. Many if not most germs infect multiple species, but it is a rare pathogen that presents a significant danger to more than a few. (Foot & Mouth disease, for example, is extremely contagious to cows and pigs, though people rarely get it. Nor, for that matter, do horses living in the same barn.) Biologists, however, agree that disease can severely weaken a population, which could be enough to tip an already delicate balance toward extinction.

A SHORT COURSE ON GERMS: MEANS, METHODS and OPPORTUNITIES

For pathogens, life is all about getting from one host to the next.

In its wandering life, a parasite travels from species to species. The flatworm riberoria that causes limb malformations in frogs moves from bird to snail to tadpole. Germs can hitch rides on sneeze droplets (cold and flu viruses). Fly through the air (Q fever). Spread through water (cholera). Or simply wait, sometimes for years, for a suitable host to present itself (anthrax spores). Over the long haul, the most lethally effective strategy is to find a middleman, a vector such a mosquito, tick, louse or mite, that can provide a suite of convenient services: transport, targeted delivery, and a degree of physical protection en route. A virus in the sun quickly degrades. A virus in a mosquito is safe.

Evolutionary biologist Paul Ewald (“The Evolution of Infectious Diseases”) compared the mortality rates of insect-borne diseases against diseases using other transmission routes. He found that vectored germs tended to be more deadly because their top priority was an ability to coexist with their transport, not their ultimate hosts. If a germ kills its vector, nobody’s going anywhere. In contrast, there actually is an advantage to making hosts so sick, they don't have the energy to swat away vector bugs.

Pathogens that don’t use vectors are more likely to evolve to a non-lethal state of “benigness” with their hosts. This doesn’t mean they can’t make their hosts very, very sick. But they need their hosts to survive long enough to continue to the next infection cycle.

The arms race between germs and hosts is as old as life itself. Any time a pathogen finds a susceptible population that hasn’t had time — or reason — to develop defenses, the potential for a massacre exists. More people have died from infectious disease than from all the wars ever fought. Smallpox, along with the horse, helped seal the conquistadors’ victories.

Thirteen thousand years ago, humans weren’t the only species crossing the Bering Land Bridge into the New World. Everything, everybody, and all their pathogens and parasites were on the move -- some moving faster than others. With the climate warming, birds from South America began migrating further North during the summers to take advantage of vast stretches of newly opened habitat. There they met up with birds flying in other parts of the world. Meanwhile, insects were battling for territory and carving out new niches. The stage was set…

GEOGRAPHY AS DESTINY

No one knows for sure how West Nile virus got here. Perhaps in the body of a traveler, or a stowaway mosquito on a plane. But its quick trek West from its first sighting in New York City in 1999, to California three years later, was primarily by feather express.

Birds are “amplifying hosts," which means they generate enough virus in their blood to infect mosquitoes, which in turn infect more birds. When birds from New York flew South during the fall of 1999, they came in contact with birds from the other major North American flyways. The tapering shape of the continent forced them together. Given such close quarters, the disease spread next to birds from the Mississippi flyway, that then carried the pathogen into new territories the following Spring.

By 2002, the first cases were reported in California.

In some ways, West Nile fits MacPhee’s description of a hyperdisease. ("Was a 'Hyperdisease' Responsible for the Late Megafaunal Extinction?") It affects dozens of species, including birds, alligators, horses, humans, and even rhinos (zoo). But most infected animals and humans survive -- although many get sick enough to put them at

risk of predation.

A LIKELY SUSPECT & A SMOKING GUN

If West Nile is a model for what may have happened to the prehistoric horses, where is the evidence?

The fossil record on germs is thin. Viruses are fragile. The soft bodies of bacteria and parasites decompose. Even when scientists know where to look, the microbial evidence is elusive. It took years, and millions of dollars, to track down a sample of the 1918 pandemic flu virus. Pleistocene germs are more than a hundred times as old.

But maybe fossils are the wrong place to look. Western Equine Encephalitis (WEE) is another virus amplified by birds, vectored by mosquitoes and potentially fatal to both humans and horses. Curiously, its genetic make-up has evidence of both Old World and New World origins. Dating exactly when this mixing took place is tricky. Researchers think it happened about 1,700 years ago, long after the Pleistocene extinctions. But they could be off be off by as much as a factor of 10. Even if the dating is accurate and WEE had nothing to do with Pleistocene horses, the virus still provides a vital clue.