Tracing America’s First Migrants

Early migrations of humans to the Americas from Siberia around 12,000 years ago have been traced using the bacteria they carried by an international team including scientists at the University of Warwick.

Originally, all modern humans came from Africa. About 60,000 years ago small groups of hunter-gatherers left Africa on foot and made their way into Eurasia where they settled. These were the world’s first human immigrants. Astonishingly, by the end of the ice age some 50,000 years later, modern humans had already reached the American continent which, if travelling over land, is almost as far away from Africa as it is possible to get.

These ancient human migrations took place during the last glacial period, or ice age, which lasted from 115,000 to 11,700 years ago. At that time, most of northern Eurasia, also known as Siberia, would have been a frozen wasteland, and presumably inhospitable to long-term human settlement.

How then, did humans manage to migrate across this vast region and find their way to North America? This is one of the most important, and as yet unanswered, questions in human prehistory, because it would explain how humans were able to colonise the whole world from an African origin, in such a short space of time.

Arctic Ocean today credit Scotland’s Arctic policy framework

Crossing The Bering Land Bridge

During the ice age, much more water was frozen at the Earth’s poles, making the sea level at that time over 100 metres lower than the present-day sea level, thus exposing a land bridge between Eurasia and North America and allowing human migration.

Using samples of a stomach bacteria called Helicobacter pylori, which has shared a tight co-evolutionary relationship with humans for at least the past 100,000 years, analyses using new statistical techniques provide evidence that humans colonised the Americas through a pre-Holocene migration of evolutionarily ancient northern Eurasians across the Bering land bridge.

Professor Mark Achtman of Warwick Medical School at the University of Warwick, explained:

“This project began in the early 2000s, when nothing was known about the genetic diversity of Helicobacter pylori in central Asia. By 2007, hundreds of Siberian H. pylori strains had been cultivated and selected genes had been sequenced. But repeated attempts by multiple talented population geneticists failed to shed light on their evolutionary history.

“This study now uses the powerful approach of ABC statistics to reconstruct and date the migrations of Siberian H. pylori (and their human hosts) across Siberia and to the Americas.”

The research used genetic information on H. pylori catalogued in EnteroBase at the University of Warwick to trace the evolutionary history of the bacteria. H. pylori is a stomach bacteria that infects approximately half of individuals worldwide, but scientists have found that its genetic sequence also varies with the region that it is identified in.

A Single Migration Event

To reconstruct the most likely evolutionary history for H. pylori in Siberia, researchers compared the most likely evolutionary models and timings using a technique called approximate Bayesian computation (ABC).

The results showed that a tiny population of H. pylori colonised the Americas in a single migration event approximately 12,000 years ago.

This population subsequently expanded to give rise to the indigenous Americans we see today.

The bacterial DNA sequence database they generated also suggested that some groups of humans, known as ancient northern Eurasians, did manage to reside in Siberia throughout the bitter ice age.

Other human groups who originally inhabited warmer latitudes in Asia, colonised Siberia after the end of the ice age, leading to the complex mix of human populations we see in that region today.

The study entitled “Helicobacter pylori’s historical journey through Siberia and the Americas” was published 14 June in the prestigious international journal Proceedings of the National Academy of Sciences of the USA (PNAS) by a team of researchers led by Professor Yoshan Moodley at the University of Venda, South Africa.

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