To put it mildly, not many recent issues in American politics have united people across party lines. But there is one notable exception: a big beefy mammal that has inspired unanimous agreement in the US House and Senate.

The American bison was voted America's national mammal in May 2016. The decision, while largely symbolic, offered support to conservation scientists, land managers and indigenous people, all of whom are working hard to bring wild American bison back to their home on the range.

But American bison are just one branch of an intriguing family tree, and their history is as tortuous as the horns on their heads. How did these formidable cow cousins arise and expand to graze widely across the wilds of North America? Why did so many of their relatives dwindle to extinction, while others still cling to existence today? And can they ever be restored to their former glory?

Modern-day American bison (Bison bison), also known as buffalo, are the largest land mammals on the North American continent.

One railway engineer claimed that it was possible to walk 100 miles alongside the Santa Fe railroad by stepping from one bison carcass to another

Continuously moving to find food, the bison, which weigh up to two tonnes, nibble native grasses down to nubs. They have thrived in a land of climatic extremes, their fur coats resistant to both hot and cold. During the winter, they use their gigantic heads as snowploughs, uncovering grass by sweeping them from side to side.

There are two subspecies: the wood bison (B. bison athabascae) and plains bison (B. bison bison). Their names reflect their choices of habitat: wood bison prefer boreal and aspen forests, while plains bison prefer open habitats like grasslands and meadows.

In the 1500s, an estimated 30-60 million of these shaggy brown beasts roamed widely across the interior of Canada, the United States, and far Northern Mexico. Vast herds of these humpbacked lawnmowers roamed grasslands, mountain meadows, scrublands and forest.

But today, American bison are only found in a few small pockets of their formerly massive range.

People began heavily hunting American bison in the 1700s. But as well as guns, bison were also harmed by diseases caught from their distant relatives: the cattle that Euro-American settlers introduced as they cultivated the land.

So many steppe bison bones are unearthed each year, they are called "the cockroaches of the Pleistocene"

By 1802, bison had disappeared entirely from Ohio, and the wave of destruction was sweeping westward. By the 1840s, bison had disappeared west of the Rocky Mountains.

Extensive trade fuelled this mass carnage. There was growing demand for their hides, and also for their bones. The latter were used in refining sugar, and for making fertiliser and fine bone china.

The bison slaughter is thought to have reached its peak in the southern plains around 1873. At that time, one railway engineer claimed that it was possible to walk 100 miles alongside the Santa Fe railroad by stepping from one bison carcass to another.

However, many centuries before American bison numbers plummeted at the hands of humans, bison populations had surged. We know this from fossils.

The oldest bison-like fossils have been discovered in southern Asia. They suggest that bison ancestors have been nibbling their way across our planet for at least 2 million years.

This near-complete mummified bison has skin tinged blue

At some point, those Asian proto-bison made their way to North America, and there they blossomed. The fossilised bones of bison litter the permafrost of Northern Canada and Alaska.

But these are not modern bison. They are an extinct species known as steppe bison (B. priscus). Steppe bison looked very similar to modern bison, but they had a double hump on their backs and much larger horns.

Grant Zazula is the palaeontologist for the government of Canada's Yukon Territory. So many steppe bison bones are unearthed there each year that he calls them "the cockroaches of the Pleistocene". They are also superbly preserved in the permafrost, so "we know a lot about them".

One of the most famous steppe bison finds is an individual nicknamed "Blue Babe". Discovered at a gold mine north of Fairbanks in 1979, this near-complete mummified bison has skin tinged blue by a phosphate-based mineral called vivianite. Its name is a reference to a mythical giant ox that accompanied the American folk hero Paul Bunyan.

The fossils were so different that dozens of different bison species were named

Blue Babe has scratch marks on its hindquarters, indicating it was killed by an American lion. However, very little of the flesh was missing from the dried and fossilised carcass. As a result, palaeontologists have surmised that Blue Babe froze shortly after death, making it difficult for the lions to eat.

The discovery of this unusual specimen inspired retired palaeobiologist Dale Guthrie, of the University of Alaska Fairbanks, to indulge in a peculiar commemoration. Guthrie and his team cooked and ate some of the bison's 36,000-year-old freeze-dried meat in a blue-hued stew. After tasting the stew, Guthrie wrote in his 1989 book Frozen Fauna of the Mammoth Steppe that, "The meat was well aged but still a little tough, and it gave the stew a strong Pleistocene aroma."

Based on studies of fossilized bones and horns like Blue Babe's, paleontologists were able to piece together an outline of the story of bison.

Early paleontologists noted that the bison fossils they uncovered looked very different from place to place: in technical terms, they had different morphologies. The fossils were so different that dozens of different bison species were named, says Beth Shapiro at the University of California Santa Cruz.

Bison first arrived in the Americas somewhere between 300,000 and 130,000 years ago

During the 19th and early 20th Centuries, each new type of horn – big or small, wide or narrow, curly or straight, forward- or backward-facing – meant the classification of a new species. These extinct bison were given names like B. antiquus (ancient bison), B. crassicornis (thick-horned bison) and B. latifrons (giant ice age bison). The latter had horns over 6.6ft (2m) long, more than three times the length of their modern relatives.

"Bison are a classic problem because there's a lot of morphological variability in the fossil record," Zazula says. It was tricky to figure out which species was descended from which, and in particular which was the ancestor of modern bison.

However, in the last 15 years geneticists have also started studying the history of bison. Zazula says there has been a "struggle between the old guard – the potbellied, bearded guy sitting in the back of the museum – versus these new 'gel jocks' who can't even identify a bone, but they can work with sequence data."

The geneticists have rewritten the history of bison.

A major study that Shapiro and colleagues published in 2004 sampled 442 bison fossils from Alaska, Canada, Siberia, China and the lower 48 US states. Their data suggests that North American bison of the past were all closely related. Instead of many different species, there was probably just one: steppe bison.

Much of Canada was covered by glaciers several kilometres thick

Furthermore, Shapiro's findings suggest that bison first arrived in the Americas somewhere between 300,000 and 130,000 years ago.

They did so by crossing the Beringian land bridge, a wide strip of land that periodically connected Siberia to North America during the Ice Age.

The exact date when bison arrived in North America "is still a matter of some debate," says Shapiro. But the fact that they did so may explain why steppe bison took so many forms.

North American steppe bison had arrived in a new home with few competitors. This would have created a field day for evolution, in which different groups evolved different characteristics and diversified to occupy the underexploited ecological niches. This phenomenon is called "ecological release".

The Ice Age itself may also have helped drive bison evolution.

When bison arrived across the land bridge from Asia, much of Canada was covered by glaciers several kilometres thick. But the land bridge and the regions on either side – known as Beringia – was different. Its unique local climate and geology created a refuge from the glaciers.

The study "set the bison world on its ass end"

More recent studies of fossil DNA by Shapiro, Zazula and others suggest that traffic flowed both ways across the land bridge.

It was not just bison moving back and forth, but mammoths and musk ox too.

On the North American side, whenever the ice sheets shrank, bison also spread to the south. So as the ice sheets grew and shrank over the millennia, North America's bison populations were alternately isolated and reconnected, says Shapiro. This meant that they sometimes evolved in isolation.

Around 20,000 years ago, the ice sheets reached their greatest extent: a time called the Last Glacial Maximum. At this time, the bison living south of the ice became distinct from those living to the north. The northern population later went extinct, and Shapiro's research suggests that the southern population eventually gave rise to the wood and plains bison that exist today.

Shapiro's 2004 study shook up many previous assumptions about bison evolutionary history. It "set the bison world on its ass end," Zazula says.

European bison are difficult to trace, because they appear abruptly in the fossil record about 11,000 years ago

The study also hinted at an explanation for the decline of North American bison. Contrary to what had been previously thought, the population began falling thousands of years before human overhunting.

The key may have been that the climate was changing radically as Earth warmed up and the ice sheets retreated. This meant that different plants were growing and some of the bison's foods were no longer available.

On the other side of the world, European bison were confronted with similar threats. Their own story of a near-miss with extinction has proved to be just as epic as that of their American cousins.

The European bison, also known as the wisent, is physically similar to its American cousin. They tend to weigh slightly less, but have longer horns.

The origins of European bison are difficult to trace, because they appear abruptly in the fossil record about 11,000 years ago – just after the end of the most recent Ice Age. That sudden appearance may be because their ancestors lived and died in regions where fossils are not well preserved.

Soubrier's team has nicknamed the original hybrid animal "the Higgs bison"

"Morphologically and genetically, nobody has ever been able to characterise a direct ancestor for this species," says Julien Soubrier of the Australian Centre for Ancient DNA at the University of Adelaide.

The working assumption for many years was that, like North American bison, European bison evolved from steppe bison.

To figure out where European bison came from, Soubrier and his colleagues analysed DNA from bison bones obtained across the Urals, Caucasus, and Western Europe, as well as examining cave art depicting bison. Their work was published in October 2016.

Their results suggest that the European bison is the result of inter-breeding, over 120,000 years ago, between steppe bison and aurochs; the wild ancestors of modern domestic cattle. Soubrier's team has nicknamed the original hybrid animal "the Higgs bison". European bison today share up to 10% of their genome with the ancient cow.

In the millennia since their origins, European bison came to dominate the landscape.

In a study published in October 2016, Eva-Maria Geigl at the French National Research Center in Paris and her team constructed a bison family tree. They used ancient DNA from bison remains spanning a 50,000-year time period, with samples obtained from France, Switzerland, the Caucasus, Poland and Siberia close to Beringia.

At the end of World War I, there was not a single [European bison] left in the wild

This revealed how the populations of European and steppe bison rose and fell over the millennia. The team found that, as the climate alternately warmed and cooled, the abundances of the two bison species flip-flopped.

"When the climate became really cold before the last glacial period, the steppe bison came from Siberia into France, and stayed there over the Last Glacial Maximum," says Geigl. Then, when the climate warmed up again about 14,000 years ago, "we saw that the bison from the southern Caucasus – the European bison – appeared in Europe."

For some reason, European bison then became more numerous while the steppe bison disappeared. We do not know why the steppe bison disappeared, says Geigl, but it may have been due to competition over resources with the European bison, which was better adapted to a warming world.

However, in more recent times the European bison have been plunged into a crisis of their own.

"At the end of World War I, there was not a single animal [European bison] left in the wild," says Geigl. The main reason was that the bison's habitat had been degraded and fragmented, thanks to farming, logging and unlimited hunting.

It is possible to figure out what sort of food an extinct animal ate by examining the chemical makeup of preserved bones

However, a handful survived in captivity. After the war, a small herd was released into the forest of Bialowieza, Poland, reconstituted from just 12 zoo-housed individuals.

Today, there are about 6,000 European bison and they remain a vulnerable species. All of them trace their roots to the progeny of those dozen survivors.

The key question now is how to give them the best chance of surviving in the long term.

In particular, what is their natural habitat? The Polish bison live in forests, but that may not be the ideal place for them. At the moment, the bison in the forests "have to be fed in the winter, otherwise they will survive in very low numbers," says Hervé Bocherens at Tübingen University.

In February 2017 a small herd of plains bison was reintroduced to Banff National Park in Alberta, Canada

It is possible to figure out what sort of food an extinct animal ate by examining the chemical makeup of preserved bones. In this way, Bocherens has compared the diets of ancient European bison with those of grass-grazing aurochs and shrub-browsing moose that lived around the same time. He published his results in 2015.

Bocherens discovered that, 10,000 years ago, European bison fed on both grass and leaves in an open landscape. The implication is that European bison were only recently forced into forests, when their numbers plummeted.

That is no surprise to European bison specialist Yvonne Kemp of Rewilding Europe, who manages a free-ranging herd in the Netherlands. These Dutch bison have lived in an open dune habitat for nine years, and Kemp says they seem to be doing just fine.

North American bison are also enjoying a resurgence, after decades of occurring in such low numbers that biologists considered them "ecologically extinct". Now, reintroduction programs are attempting to restore the population.

Bison were a "force of nature"

For example, in February 2017 a small herd of plains bison was reintroduced to Banff National Park in Alberta, Canada. Early reports suggest they are settling in nicely.

American bison are, on the whole, slightly better off than their European cousins. They are classed as "near threatened" by the International Union for Conservation of Nature, which is a step up from the European bison's "vulnerable".

Still, they are not what they once were. Bison were a "force of nature", says Keith Aune of the Wildlife Conservation Society in New York. They once helped shape the ecosystem of an entire continent.

It remains to be seen whether they can reclaim their pivotal ecological role, and once again become guardians of America's grasslands.

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