Viking rodents reveal paths of historic human exploration

When Portuguese explorer João Gonçalves Zarco’s ship struggled into the harbor at Madeira during a crashing storm in 1418, the only witnesses were the mice watching from the hills. Zarco believed this island off the coast of northwest Africa had never been discovered, but to today’s archeologists his rodent audience tells a different story — someone else got there first.

But just who remained a mystery until 2001. With no archeological records of settlements before the Portuguese, and inconclusive historical documents, researchers were at a loss. It turned out the key was hidden in the genes of the mice, Madeira’s earliest immigrants. The mice had colonized Madeira for the Vikings.

To scientists, mice are the unsung companions of human exploration. The small stowaways explored new worlds in step with the Vikings, the travelers in the ‘age of exploration,’ and beyond. They harbor a genetic fingerprint that can be used to track mouse movement, and indirectly, human movement, across the globe. Researchers have begun using this fingerprint to look at the effect ancient humans had on the environment and how disease-resistant populations arose.

“The mice are a kind of living artifact of those first colonizing humans,” says Jeremy Searle, evolutionary ecologist at Cornell University. House mice, which are native to northern India and Iran, started coexisting with people when ancient humans began to settle and store grain, he says. From there, it was a small step for the mice to sneak into the food stores of large ships and caravans when people started to travel on a large scale in the Iron Age.  Although Searle is particularly recognized for tracking Viking mice, over the past few years he and other researchers have tracked mice across Europe, Australia, China, Russia and Japan.

Anywhere people went, their intrepid rodent counterparts explored too. And the mice created quite the colonial empire. Mice would jump ship as the vessels landed, and get left behind unwittingly — with nothing left to do but explore and try to survive. And survive they did.

The genes carried by the descendants of these valiant vermin allow researchers to “follow lineages,” says Diethard Tautz, director of the department of evolutionary genetics at the Max Planck Institute of Evolutionary Biology in Munich, “and lineages are always easier for tracing history.”

To trace a lineage, Tautz says, researchers collect DNA from mice in a specific area — like Madeira — and compare it to the DNA of mice in other regions. If a mouse had ancestors on the Portuguese mainland, sections of its DNA would match the DNA in Portuguese mice. And unfortunately for Zarco’s alleged discovery, mice on Madeira did not have Portuguese ancestry.

Instead, Searle found the Madeira mice were descendants of 9th century Viking mice. “It’s just an utterly extraordinary result,” he says. And it’s unexpected because there are no records of the Vikings on Madeira except for the mice. This lack of records makes it difficult for some archeologists to accept the result.

“You’d expect there to be more evidence of Vikings than just mice,” says Catherine Batt, Viking archeologist at the University of Bradford in the UK. She says that researchers typically look for signs of Vikings in the physical changes they leave behind — like their distinctive longhouses, tall structures roughly 20 feet wide and up to 250 feet long. This is supplemented by extensive work on ancient texts, says Neil Price, chair of European Archeology at the University of Uppsala in Sweden. There has been a rise in interest in ancient DNA to identify remains, says Batt, but this has mostly been done with human bones.

But ancient mouse bones have offered yet more confirmation of Searle’s theory that the Vikings were the first to Madeira. In 2014, a team of biologists in Spain radiocarbon dated a fossilized mouse skeleton they discovered on Madeira. The skeleton dates to 1031 — corroborating Searle’s prediction that mice arrived on Madeira in the era of Viking exploration. Josep Antoni Alcover, a member of the team working with the skeleton, and a conservation biologist at the Mediterranean Institute of Advanced Studies in Esporles, Spain, was particularly interested in the timing of this invasion with an extinction event of native birds on Madeira that occurred after the Portuguese arrived.

Had Darwin visited a thousand years earlier, he would have found it just as incredible as the Galapagos, says Alcover. But the arrival of the Viking mice altered the ecology of the island, initiating the collapse of the native species. By the time the Portuguese arrived 400–600 years later, the breeding colonies of marine birds had all but collapsed. “It’s two different worlds” before and after the mice, he says.

Prior to the radiocarbon dating, the extinction of Madeira’s native birds was a mystery. But knowing that mice arrived 400–600 years before the Portuguese makes a lot of sense to evolutionary biologists. “We know that when mice have the opportunity to move into a new environment they inhabit all the niches,” Tautz says. To him, the extinction is a clear result of the mice taking over an island where they lacked native predators. The mouse invasion on the island of Gough in the South Atlantic Ocean, where the mice have become carnivorous and eat birds, is perhaps a modern analog to what happened on Madeira. Once a mouse population is established, it adapts to a new environment relatively quickly, he says. Latecomer mice aren’t able to compete as well, and so don’t reproduce — a fact that allows researchers to identify the genetics of the first mouse migrants.

These small initial populations can lead to major effects today — particularly in the realm of disease-resistant pests, says Carine Brouat, a biologist at the Institute of Research for Development at the French National Center for Biology and Management of Populations. Brouat is using the genetic marker techniques developed by Searle to study the effect of invasion on disease resistance. Although she is working with black rats — not mice — in Madagascar, she saw a similarly devastating effect of invasion of stowaway animals.

On Madagascar, rats are resistant to plague, which allows them to carry it and pass it to humans. This is a constant problem for the country — there were ten plague related deaths this September, part of a larger outbreak that started in 2014. Brouat’s genetic work indicated that all the rats came from the Arabian Peninsula before the 6th century, bringing plague resistance with them. “We have a species that invaded an island a millennium ago and has become a big problem for the last century,” she says.

The impact of these animals, and of ancient people, makes following mice across the globe more than just a mission of curiosity. In many cases, researchers don’t have the archeological settlements or ancient texts to tell them when and where ancient people went. And this is where the work of Searle and other researchers has really had an impact.

“There isn’t this conversation yet about how ancient people influenced dispersal,” says Courtney Hofman, molecular anthropologist post-doc at the University of Oklahoma. Researchers are just starting to understand the impact ancient humans had on the ancient world, she says. And it has been a significant impact, particularly on island ecosystems like Madeira and Madagascar. Ancient DNA and genome tracking, she adds, is “literally a time machine, it’s telling you what genetic diversity looked like at a certain point in time. And that’s really valuable information about understanding human history.”