Siberia has become the scene of an anthrax outbreak. Reportedly, a region of permafrost melted in a heatwave, exposing an infected reindeer carcass.

Anthrax has a reputation as a bioweapon, particularly after the infamous 2001 anthrax attacks in the US. But the disease has a surprisingly long history.

Ancient Chinese texts document anthrax-like outbreaks 5,000 years ago. The Roman poet Virgil described an anthrax-like plague in First Century BC Europe that "raged through an animal's veins and shrivelled its flesh" before "virtually dissolv[ing] the bones".

Some historians think the disease was present in pharaonic Egypt, and it may have been behind the fifth of the ten plagues. As the Bible puts it: "Behold, the hand of the Lord is upon thy cattle which is in the field, upon the horses, upon the asses, upon the camels, upon the oxen, and upon the sheep: there shall be a very grievous murrain [plague]."

But it is actually strange that so many regions of the world experienced anthrax problems in antiquity. Anthrax kills within days, so infected animals fall ill before they can travel far from the source, and this means anthrax should remain localised.

Somehow, a disease that is intrinsically unfit for international travel became a globetrotter thousands of years ago – and there is a good chance humanity is, inadvertently, to blame.

Anthrax is a potentially lethal infection caused by the bacterium Bacillus anthracis.

The anthrax letter attacks really helped spur the development of whole-genome sequencing

The microbes form spores that can survive in soil for several years, although exactly how long is not clear. "The figure I always hear is 10 years," says Talima Pearson at Northern Arizona University in Flagstaff. "But I don't think that's been rigorously tested."

An animal or human can become infected if they breathe in the spores, eat food tainted by them, or even pick up spores through an open wound. Without prompt treatment, death can come quickly.

B. anthracis bacteria are very similar at the genetic level, which initially thwarted researchers' efforts to study their evolutionary history. However, this changed about 20 years ago when geneticists developed state-of-the-art technology that allowed them to identify the small genetic differences between strains.

"Then the anthrax letter attacks really helped spur the development of whole-genome sequencing," says Pearson.

In a 2004 study, Pearson and his colleagues showed that B. anthracis bacteria fall into three main groups dubbed A, B and C. All three shared a common ancestor, but when and where that first bacterium lived is unclear.

A disease that is intrinsically unfit for international travel became a globetrotter

It probably originated in Africa, argues Martin Hugh-Jones of Louisiana State University in Baton Rouge. In parts of southern Africa the genetic diversity of B. anthracis in soil samples is particularly high, and that is often a good indicator of where a species first appeared.

However, Pearson thinks geneticists need to analyse many more soil samples to be sure. "In terms of diversity I could argue for Africa or Europe," he says.

What is not in doubt is that B. anthracis has come a long way since its point of origin. In particular, the A group strains are now found on almost all of the major continents. They account for about 85% of recent anthrax cases around the world.

It is unlikely that they made it so far on their own.

Pearson once took part in an anthrax study in South Africa. Historically, the region had lots of the large herbivores that are particularly vulnerable to infection. These animals migrate across southern Africa, so in theory anthrax strains could travel with them. But they do not.

As early humans migrated across Eurasia... they unwittingly carried anthrax spores with them in their clothes

"When we looked at the anthrax, the different groups tended to be highly localised," says Pearson.

This initially prompted some head-scratching, he says. But once the researchers considered how rapidly anthrax kills its host, it no longer seemed so unusual. Pathogens often rely on their hosts to spread the infection, but B. anthracis kills its host before it can travel far. Anthrax is a highly localised problem.

Or, at least, it is in southern Africa. However, in Eurasia some strains have a much wider distribution. Somehow they have managed to become long-distance travellers.

"Why is there this difference in Europe and Asia?" asks Pearson. "We don't really know the answer. But we can speculate."

He suspects human behaviour is the key difference.

The first Americans almost certainly arrived in the New World from north-east Asia

Prehistoric humans were always looking for ways to exploit their environment. When they came across the carcass of a dead animal, they might well have taken its skin and used it for clothing. If some of those animals had died from anthrax, their skins might have contained anthrax spores.

As early humans migrated across Eurasia, Pearson thinks they unwittingly carried anthrax spores with them in their clothes, scattering them in their wake.

Ancient humans were just as active in southern Africa, but the climate here was far more benign than in Eurasia, says Pearson. "They perhaps didn't need to carry animal skins with them for warmth at night," he says. As a result, B. anthracis in southern Africa retained its ancestral localised distribution.

If prehistoric humans did help spread B. anthracis outside Africa, it might help explain the unusual distribution of A group anthrax in North America.

Until a few years ago most researchers assumed anthrax arrived in the Americas a few hundred years ago, with the first European settlers. But when Pearson and his colleagues looked at the data for a 2009 study, they realised it did not fit that story.

The farming revolution might well have intensified the anthrax problem

The data showed that the earliest forms of B. anthracis are restricted to the north-west of the continent. Later forms are found further south, with some reaching into Texas. This suggests that anthrax arrived in the Americas through Alaska and Canada, then spread south and east.

That pattern of spread is quite unlike the movement of European settlers, who arrived on the eastern seaboard. But it does match the route that the earliest Americans took when they first settled on the continent over 13,000 years ago. These first Americans almost certainly arrived in the New World from north-east Asia, taking advantage of a land bridge that linked Asia and North America during the last Ice Age.

This implies that, by the time people began farming about 10,000 years ago, their hunter-gatherer forerunners had already carried anthrax across Eurasia and into the Americas.

However, the farming revolution might well have intensified the anthrax problem.

In 2007, researchers including Pearson produced the most comprehensive analysis to date of the global history of B. anthracis. It suggested that the A group went through a dramatic radiation between about 6,500 and 3,300 years ago.

There is no definitive evidence that humans are responsible for the global spread of B. anthracis

This coincides with the dawn of the Bronze Age and the beginning of long-distance trade of commodities – including farming goods – across Eurasia. That long-distance trade, coupled with the fact that livestock farming brought people into closer contact with large herbivores, may well have created the perfect conditions for A group B. anthracis to spread and diversify.

In keeping with this idea, a 2009 study of B. anthracis in China revealed that the microbe is at its most diverse in western China, near to the city of Kashi. This city was a crossroads on the famous Silk Road, which might first have been used as a trade route in the Bronze Age.

As yet there is no definitive evidence that humans are responsible for the global spread of B. anthracis, or of A group strains. But there is a lot of circumstantial evidence pointing that way.

Such a story might even explain why A group B. anthracis in particular became so prevalent.

A study published in 2000 suggested that A group strains are better-adapted to a wider range of environments than B or C group strains. The A group B. anthracis might also have an edge when it comes to surviving in the soil.

"Maybe it has a mutation rate that allows its fast adaptation to whatever soil it finds itself in," says Hugh-Jones.

The A group strains are now found on almost all of the major continents

However, Pearson thinks these biological differences alone cannot really explain why A group strains have so spectacularly outperformed B and C strains. The difference, he says, might simply be that A group strains were swept up by human activity to a far greater extent.

"They were just in the right place at the right time," he says.

Either way, it is looking increasingly likely that anthrax is as common as it is, not because it is good at spreading itself, but because our distant ancestors carried it all over the world – without realising it.

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