{Quote} What drove horses to extinction in the Americas? Did humans have a hand in their demise, or did climatic changes and altering vegetation trigger it?
An artist's impression of the Yukon Horse, dating back 26,000 years. © Yukon Beringia Interpretive Centre
A definitive answer has eluded scientists, but improving techniques and the growing value of DNA analysis has painted a clearer picture of events surrounding their demise.
While climate change dominates headlines in the modern era, it loomed large in the lives of the many species that inhabited the Americas thousands of years before mankind began belching carbon dioxide into the atmosphere.
The end of the Pleistocene epoch - the geological period roughly spanning 12,000 to 2.5 million years ago, coincided with a global cooling event and the extinction of many large mammals. Evidence suggests North America was hardest hit by extinctions.
This extinction event saw the demise of the horse in North America. It survived only because the Bering land bridge that once connected Alaska and Siberia had enabled animals to cross into Asia and spread west.
The end of the Pleistocene also saw the end of the woolly mammoth, American camels, dire wolves, short-faced bears, saber-toothed cats, stag-moose, woolly rhinos and giant ground sloths.
The story of the North American extinction of the horse would have been cut and dried had it not been for one major and complicating factor: the arrival of humans.
Humans, too, made use of the land bridge, but went the other way - crossing from Asia into North America some 13,000 to 13,500 years ago.
Why could the continent that gave rise to the horse no longer provide a suitable home?
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The Bering Strait is a storm-prone stretch of water that separates two continents.
When we talk of a land bridge we tend to conjure up images of a narrow strip of terrain. The Bering land bridge was no such modest affair.
Its fortunes - and very existence - ebbed and flowed with rising and falling ocean levels. During cooler periods in the earth's past, glacial ice would build up, dropping sea levels to expose or expand the land bridge.
A colder period that ended some 10,000 years ago saw the land bridge reach about twice the size of Texas, and scientists have even given it a name - Beringia.
You could even consider the current state of affairs, with a body of water separating Siberia and Alaska, as unusual. The land bridge has actually been in place more often than not during the past two million years or more.
It has come and gone for far longer than that. It first developed at least 70 million years ago and was a dry land route for the movement of plants and animals, including dinosaurs.
A quagga, pictured at London Zoo about 1870. DNA analysis has shown that the Quagga was a subspecies of the Plains Zebra (Equus Quagga). The quagga was hunted to extinction in the late 1800s.
When submerged, sea-dwelling life was able to move between the Pacific and Arctic Oceans.
The distribution and nature of much life on earth has been greatly influenced by this crucial land bridge. Its appearance and disappearance would also have had an influence on climate, with the closing of the land bridge affecting ocean currents.
The bridge enabled near-global distribution for some species. Mammals from as far away as Africa were able to spread north and east through Eurasia and into the Americas. Camels and horses instead went westward from the Americas, where their respective species had developed.
Horses originated in North America 35-56 million years ago. These terrier-sized mammals were adapted to forest life. Over millions of years they increased in size and diversified.
Horses got larger in size and underwent other changes to their feet and teeth to adapt to changing environments. From five million to 24 million years ago, a number of horses occupied niches to which they had adapted, including grazing the spreading grasslands.
It was about four million years ago that the genus of all modern horses arose. The modern horse, known as Equus, evolved from the horse Pliohippus, which arose around 5 million years ago and was extinct by two million years ago.
The genus comprised three species, but quickly diversified into at least 12 species in four different groups.
They co-existed with other horse species which had evolved different features, but it was members of Equus which made a move that not only saved the genus from extinction, but profoundly changed the path of humankind.
Equus occupied North America for the entire Pleistocene epoch, from about 2.5 million years ago until their extinction. Scientists believe Equus crossed the Bering land bridge around the beginning of the epoch.
Some made it as far as Africa to evolve into the zebras we know today. Others moved across Asia, the Middle East and northern Africa, evolving into the onagers and wild asses of today, both well suited to desert environments.
A Somali wild ass filly foal with her dam. © Saint Louis Zoo
Still others spread across Asia, the Middle East and Europe, evolving into the true horse, Equus caballus.
North America remained home to Equus species for most of the next 2.5 million years until they died out. On latest evidence, that was just 7600 years ago.
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While the genus adapted to life outside North America, the "home bodies" did not fare so well. Their extinction came quickly, as it did for many other large mammals on the continent.
They faced a changing climate, altering vegetation - and the arrival of man.
Artifacts from the first Americans, known as the Clovis, cast some light on the relationship of these people with the horse.
A remarkable find of a cache comprising 83 stone implements within the city limits of Boulder City, Colorado, in 2008 provided scientists with invaluable insights.
Biochemical analysis showed that some of the 13,000-year-old implements were used to butcher ice-age camels and horses.
The University of Colorado study was the first to identify protein residue from extinct camels on North American stone tools and only the second to identify horse protein residue on a Clovis-age tool. A third tool tested positive for sheep and a fourth for bear.
All 83 artifacts were shipped to anthropology professor Robert Yohe, of the Laboratory of Archaeological Science at California State, Bakersfield, for the protein residue tests.
"I was somewhat surprised to find mammal protein residues on these tools, in part because we initially suspected that the cache might be ritualistic rather than utilitarian," Yohe said.
"There are so few Clovis-age tool caches that have been discovered that we really don't know very much about them."
Anthropology professor Douglas Bamforth, who led the study, said the discovery of horse and camel protein on the tools was the clincher for him that the tools were of Clovis origin.
"We haven't had camels or horses around here since the late Pleistocene."
The artifacts that showed animal protein residues were each tested three times to ensure accuracy.
Douglas Bamforth, Anthropology professor for the University of Colorado at Boulder, left, and Patrick Mahaffy, show a portion of more than 80 artifacts unearthed about two feet below Mahaffy's Boulder's front yard during a landscaping project in 2008. © Glenn J. Asakawa/University of Colorado)
Bamforth believes the type of people who buried the cache "lived in small groups and forged relationships over large areas".
"I'm sceptical that they wandered widely, and they may have been bound together by a larger human network."
Evidence of early Americans hunting horses had earlier been uncovered by University of Calgary scientists, who discovered the remains of a pony-sized horse while excavating the dry bed of the St Mary Reservoir in southern Alberta.
Several of the horse's vertebrae were smashed and it had what appeared to be butcher marks on several bones.
About 500 metres from the skeleton, they found several Clovis spearheads. Protein residue testing and examination confirmed they had been used to hunt horse.
So does evidence of horse hunting place humans in the frame as being responsible for horse extinction? The weight of evidence suggests not.
One compelling argument centres around the timeline: that the comparatively few humans were unlikely to have played a major part in the demise of a species that was already in decline from climate and vegetation change.
That said, recent discoveries point to a rather longer overlap during which both horses and humans lived in North America.
Some scientists had earlier believed the evidence pointed to horses dying out some 500 years before the arrival of the first humans - a view since disproved by the discovery of horse protein on Clovis tools.
However, statistical analysis by Andrew Solow, of the Woods Hole Oceanographic Institution in Woods Hole, Massachusetts, offered a different view on the possible role of humans. He explored the radiocarbon dating of the 24 most recent known ancient horse fossils.
His analysis indicated the ancient horses of Alaska could have persisted until perhaps 11,700 years ago, providing an overlap of several hundred years.
Solow noted that the fossil record was very incomplete.
"Just because the most recent remain is from 12,500 years ago, that doesn't mean that the horse became extinct at this time," he said in one interview after the 2006 publication of his findings - an observation that was later to be proved correct.
It was, he suggested, impossible to rule out human hunting as a cause or major contributing factor to North American horse extinction.
Fast-forward to 2009 and DNA analysis added another dramatic twist to the mystery.
Horses, the evidence now suggests, may have survived in North America until 7600 years ago - some 5000 years longer than previously thought. The new timeline suggests an overlap with human habitation approaching 6000 years.
Researchers who removed ancient DNA of horses and mammoths from permanently frozen soil in central Alaskan permafrost dated the material at between 7600 and 10,500 years old.
The findings suggested populations of these now-extinct mammals endured longer in the continental interior of North America, challenging the conventional view that these and other large species disappeared about 12,000 years ago.
It is possible the researchers unearthed the tiny genetic footprint of the last few hundred ancient horses to roam North America.
"We don't know how long it takes to pinch out a species," said Ross MacPhee, curator of mammalogy at the American Museum of Natural History.
"Extinctions often seem dramatic and sudden in fossil records, but our study provides an idea of what an extinction event might look like in real time, with imperiled species surviving in smaller and smaller numbers until eventually disappearing completely."
The researchers' remarkable findings were made possible thanks to the DNA-preserving properties of permafrost.
MacPhee and his colleagues decided that the permafrost around wind-blown Stevens Village, on the banks of the Yukon River, fitted the bill perfectly.
In this location, sediments were sealed in permafrost soon after deposition.
Cores collected provided a clear picture of the local Alaskan fauna at the end of the last ice age. The oldest sediments, dated to about 11,000 years ago, contained remnant DNA of Arctic hare, bison, and moose; all three animals were also found in higher, more recent layers, as would be expected.
But one core, deposited between 7600 and 10,500 years ago, confirmed the presence of both mammoth and horse DNA. To make certain there was no contamination, the team did extensive surface sampling around Stevens Village.
No DNA evidence of mammoth, horse, or other extinct species was found in modern samples, a result that supports previous studies which have shown that DNA degrades rapidly when exposed to sunlight and various chemical reactions.
"The fact that we scored with only one layer is not surprising," says MacPhee. "When you start going extinct, there will be fewer and fewer feet on the ground, and thus less and less source material for ancient DNA such as faeces, shed dermal tissues, and decaying bodies."
His team also developed a statistical model to show that mammoth and horse populations would have dwindled to a few hundred individuals by 8000 years ago.
"At this point, mammoths and horses were barely holding on. We may actually be working with the DNA of some of the last members of these species in North America," says permafrost expert Duane Froese, associate professor in the Department of Earth and Atmospheric Sciences at the University of Alberta.
Why then, with such a substantial overlap in human and horse habitation, does the weight of evidence rest elsewhere?
The fossil record indicates that major changes in climate and vegetation at the end of the Pleistocene may have been the last nail in the coffin for the horse.
Extinction is not a rare event among life on Earth. In fact, the vast majority of species that have inhabited the planet are now extinct.
While the extinctions around the late Pleistocene saw the end to mammoths, giant sloths, horses and the like in the Americas, the extinction rate of North American mammals actually reached its highest level some six million years ago, resulting in the demise of about 60 genera. Several species of horses were driven to extinction at that time.
That period delivered the highest rate of extinctions in the Americas in 30 million years.
Evidence of climate change and the resulting change of vegetation is considered the most likely cause for horse extinction, but investigations by Johns Hopkins paleobiologist Steven Stanley may have pinned down the cause even more specifically.
Stanley, a professor in the Johns Hopkins Department of Earth and Planetary Sciences, looked at the findings of other scientists and found evidence that it was the grittier nature of grass that may have caused the demise of equine species.
For tens of millions of years, as the Earth's climate became cooler and dryer, the trend toward expanding grasslands and receding forests continued in North America.
About 13 million years ago, the 15 or so species of horses in North America were split between those with long teeth and those with shorter teeth. Also at that time, a few new species emerged that had very long teeth.
Grasses have a gritty compound called silica, which is contained in sand and is used to make glass. As animals chew grass, the silica wears down their teeth. Therefore, animals with longer teeth live longer because their teeth don't wear as fast, and they can continue to feed.
As grasslands expanded, the horses with long teeth lived longer because they were best adapted to eating grasses instead of leaves. Living longer enabled them to produce enough offspring to guarantee survival of their species and the evolution of new species.
By 11 million years ago, only the horses especially adapted to eating grasses - those with longer teeth - were surviving in North America.
"Then, there is this sudden event, six million years ago, more or less, and what you see is a big extinction pulse, a big drop in total diversity, and the survivors are all the ones with very long teeth," Stanley said.
The conventional wisdom has suggested that the long-toothed horses disappeared because of expanding grasses. But that just didn't make sense, Stanley said, because the horses with long teeth were especially adapted to eating grasses.
"So, why would more grass be a problem for them?" Stanley asked.
Somehow, something about the grasses must have changed, he reasoned.
Meanwhile, other scientists had discovered that, as the climate became dryer and cooler, a different type of grass began to dominate North America. Those grasses, known as C4 grasses, which thrive in dryer climates, replaced many of the previously dominant grasses, known as C3 grasses.
"I thought, well, this seems like a long shot, but I wonder if there are on average more silica bodies in the C4 grasses than C3 grasses," Stanley said.
His hunch proved correct. Stanley found that, on average, C4 grasses contained about three times as many of the silica particles as do C3 grasses.
"Think about a species that was doing all right eating C3 grasses. Maybe it lived 10 years on average and produced enough colts to reproduce the species. Well, what happens if that horse is suddenly only living seven years, or six years? It may not produce enough colts to perpetuate its species.
"I think that's what happened. I think there was a big grind down."
A blow it might have been, but the horse was far from finished in North America. But what led to their ultimate demise, along with a raft of other large mammals?
Several explanations have been offered by scientists, ranging from overhunting by humans to a meteor or comet impact, and novel infectious diseases.
However, most scientists find it hard to look beyond yet another period of substantial climate and vegetation change wrought by the end of the last glacial period.
The last glacial period began about 110,000 years ago and ended about 12,500 years ago, around the end of the Pleistocene epoch. Glaciation was at its peak about 18,000 years ago.
Some 70 per cent of North American large mammals became extinct between 20,000 and 10,000 years ago.
"The causes of this extinction - the role of humans versus that of climate - have been the focus of much controversy," Dale Guthrie, of the University of Alaska in Fairbanks, wrote in the journal Nature.
"Horses have figured centrally in that debate, because equid species dominated North American late Pleistocene faunas in terms of abundance, geographical distribution, and species variety, yet none survived into the Holocene epoch.
"The timing of these equid regional extinctions and accompanying evolutionary changes are poorly known," he said.
He believes climate change and a shift from grasslands to tundra is the likely cause, resulting in a reduction in the animals' food supply.
"Horses underwent a rapid decline in body size before extinction and I propose that the size decline and subsequent regional extinction ... are best attributed to a coincident climate/vegetation shift," he said.
Guthrie radiocarbon-dated bones from two species of extinct Alaskan horses. The bones date back about 12,500 years - 500 years before the first signs of human settlement in the area.
He found that the bones were about 12 per cent shorter than those from another horse that lived nearly 15,000 years earlier.
The evidence did not support human overkill and several other extinction causes," he said.
"Comparable size declines at the end of the Pleistocene are not unique to horses," he pointed out. "Bison declined more dramatically in an even shorter time span, but at a later date.
"The significance of this size decline among Alaskan horses just before their regional extinction is that environmental pressures provoking smaller body size may well have been the same ones that ultimately resulted in their extinction."
What of the environmental changes in Alaska at the time of these extinctions? The last glacial period was a time in which the cold/arid northern Mammoth Steppe was most extreme, he said, though still capable of supporting a rich diversity of large mammals.
Evidence pointed to arid and windy conditions with a treeless, short grass-sedge-sage sward.
"Although the region's large mammals were evidently adapted to handle cold/arid extremes, each species was evolutionarily fine-tuned to different optimal diets and habitats."
A dramatic pollen shift occurred around 12,500 to 13,000 years ago. Landscape changes included the creation of lakes, bogs, shrub tundra, forests, low-nutrient soils, and plants highly defended against herbivore grazing.
"Vegetation in the north now supports a relatively small biomass of large herbivores, and almost no grazers," he noted.
"The present data suggest that Alaskan horses prospered during the last glacial maximum, and appear to have been particularly well adapted to the more intense versions of the cold/arid Mammoth Steppe.
"Perhaps the declining body size of Alaskan horses and their extinction relate not only to the absolute decline of their access to optimal food resources, but also to increasing competition with other large mammals possessing the physiological capacities to thrive on the vegetation characteristic of this ... end-Pleistocene transition."
However, not all scientists ascribe to this view.
Recent findings from J. Tyler Faith, Ph.D candidate in the hominid paleobiology doctoral programme at the University of Wyoming, and Todd Surovell, associate professor of anthropology at the university, suggest the mass extinction occurred in a geological instant.
Faith's research revealed the extinctions as a sudden event that took place between 13.8 and 11.4 thousand years ago.
Faith's findings provide some support to the idea that this mass extinction was due to human overkill, an extra-terrestrial impact or other rapid events rather than a slow attrition.
"The massive extinction coincides precisely with human arrival on the continent, abrupt climate change, and a possible extraterrestrial impact event" Faith said.
"It remains possible that any one of these or all, contributed to the sudden extinctions. We now have a better understanding of when the extinctions took place and the next step is to figure out why."
So was it climate change and a resulting change in vegetation that drove horses to extinction? With evidence that changes in grass resulted in the extinction of roughly half of North America's equine species six million years ago, is it not reasonable to assume that a similar vegetative change some 10,000 to 12,000 years ago could not have done the same thing?
The weight of evidence still rests in this camp, but totally dismissing the role of over-hunting is no longer so easy.{End Quote}
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