New insights into the peopling of Siberia and human migration into the Americas have been found in what might seem like an unlikely place: gut bugs.

Helicobacter pylori is a type of bacterium that lives in people's digestive tracts and can cause stomach ulcers. It has evolved alongside (and inside) humans for at least the past 100,000 years, accompanying people out of Africa, on cross-continental migrations and beyond.

Now an international team has added more detail to the patchy fossil record of how and when people migrated from Siberia into the Americas, by reconstructing H. pylori's own evolutionary journey as it moved about in the stomachs of early humans.

But more than that, the study opens a window into the complex history of people living in Siberia, some of whom it seems weathered the worst of the last ice age.

"The peopling of Siberia and the Americas is intriguing for archaeologists, linguists, and human geneticists, but despite significant recent developments, many details remain controversial," the research team, led by zoologist Yoshan Moodley, writes.

Everything from the timing of human migration from Eurasia into the Americas, to the routes the first migrants used, has been called into question in recent years, challenged by newfound evidence.

Did people move across land bridges or traverse coastal kelp highways? Did they venture into the Americas as trans-Siberian glaciers melted away, or did they arrive much earlier than that?

"Recently, genomic studies of ancient human DNA have offered further insights, confirming that Siberia was the gateway for human migrations into northern America as well as into western Eurasia," Moodley and his colleagues explain.

But questions linger, mostly because ancient human remains are rarely preserved and hard to find, and genetic studies which follow slow steps in human evolution can only tell us so much.

As it turns out, there are more historical clues to be found in the DNA sequences of bacteria that have been living inside us this whole time.

The team behind this new study sampled over 550 distinct strains of H. pylori from 16 ethnic and multiple traditional language groups currently living in modern-day Siberia and Mongolia, looking to understand past human movements.

More than half the world's population is today infected with H. pylori, but little is known about the gut bug's presence or diversity in these remote regions.

"The diversity of language families spoken in the region," the team writes, "hints at a complex history of migration and isolation."

"The patterns of human diversity between these ethnic groups are also largely understudied," they add.

Being a bacterium, H. pylori replicates real fast in the human gut, evolving ever so slightly as it goes. This makes it a useful marker of human migrations, as comparisons of divergent strains can reveal how diverse groups of people across the world are actually related.

Moodley and the team reconstructed the evolutionary histories of H. pylori sampled across Siberia and in the Americas, then modelled how people and H. pylori strains might have migrated across the continental divide.

Because people across "the entire extent" of Siberia shared H. pylori strains with people in North America, it suggests that there was likely a single migration event as recent as 12,000 years ago, the researchers concluded.

But, as the team acknowledges in their paper, mounting archaeological evidence, along with ancient remains and genetic studies, suggest that human migration into the Americas occurred much earlier than that, anywhere between 13,000 and 23,000 years ago.

Another contentious point is whether or not people who first appeared in Siberia around 45,000 years ago stuck it out in Siberia through the coldest part of the last ice age (roughly 20,000 years ago) or retreated further south.

During the last glacial maximum, ice sheets covered a quarter of Earth's land area and a third of Alaska. Sea levels also retreated, exposing land bridges such as the one that once spanned the Bering Sea between Russia and Alaska.

The same ancient H. pylori strains were found across Siberia, leading the researchers to conclude that some people must have persisted in northern Siberia through the height of the last ice age.

However, the team also identified a few more recently admixed H. pylori variants across the region. This suggests that isolated populations living in central or southern Siberia likely re-joined the hardy groups in the north once the weather warmed during the Holocene, which began roughly 12,000 years ago.

The study was published in PNAS.