Â鶹ÒùÔº

North America's mammoth species were breeding together within the past 40,000 years. Fossil teeth found in Canada show that Columbian and wooly mammoths regularly had calves together where the different species mixed.

North America's mammoth species weren't as separate as they first appear.

Two different mammoths lived on the continent during the last Ice Age—the wooly mammoth in what is now Canada and the northern U.S., and the Columbian mammoth further south. Adapted to different climates and food sources, it was assumed that these species lived largely independent lives.

Two mammoth molars found in western Canada, however, tell a different story. Genetic analysis of the fossilized teeth shows that they belonged to mammoths that were hybrids between the species.

In fact, because the younger fossil has more Columbian mammoth DNA than the other, Columbian and wooly mammoths must have bred many times over thousands of years. Professor Adrian Lister, one of our scientists who co-authored the new research, says that it suggests that hybrids play a much bigger role in evolution than was previously thought.

"Traditionally, we're taught that different species can't breed together," Lister says. "As our ability to investigate genetics has developed, however, we're finding that this has actually happened many times."

"The approach we've taken to investigate these mammoths could potentially be applied to other extinct animals as well. By reconstructing their history, we'll be better able to see the role that hybridization has played in the evolution of the species we see today."

The findings of the study are in the journal Biology Letters.

How did hybrid mammoths evolve?

Part of the research team first uncovered evidence of mammoth hybrids back in 2021, when they extracted 1.2-million-year-old DNA from a steppe mammoth tooth found in Krestovka, Siberia.

Historically, steppe mammoths were thought to have evolved into the wooly mammoth in Eurasia around 700,000 years ago, and the Columbian mammoth in central and southern North America around 300,000 years later. However, the Krestovka tooth showed that the story isn't quite that simple.

The Krestovka lineage turned out to be a distinct group of steppe mammoths whose members bred with wooly mammoths. It's now thought that this hybridization event is what created the Columbian mammoth. This interbreeding likely happened in North America after the wooly and steppe mammoths crossed a now-submerged land bridge between Siberia and Alaska.

As a result, as much as half of the DNA of Columbian mammoths was inherited from the wooly mammoths. But this new study shows that some wooly mammoths were also inheriting Columbian mammoth genetics.

One of the newly unearthed wooly mammoth teeth, dating from about 36,000 years ago, shows that the animal it belonged to inherited over 21% of its genome from Columbian mammoths. In the younger tooth, from about 11,000 years later, Columbian mammoths are responsible for just under 35% of the mammoth's ancestry.

The increased level of Columbian mammoth DNA suggests that interbreeding between wooly and Columbian mammoths continued for thousands of years. Analysis of the sex chromosomes suggests that these encounters were mostly male Columbian mammoths breeding with female wooly mammoths.

The mixing of both species also made the North American wooly mammoths more genetically diverse than any other population known so far. Species with greater variety in their DNA are normally better able to adapt to changes, so it's possible that this could have helped these mammoths to survive.

Mammoths under pressure

While these hybrid mammoths are genetically distinct from other members of their species, their teeth are surprisingly similar to other wooly mammoths. Lister explains that this is likely the result of natural selection.

"We tend to think that hybrids average out, so if a long animal bred with a short animal they'd have a medium-length offspring," Lister says. "So, in these hybrids, we might expect their teeth to have elements from wooly and Columbian mammoths."

"Instead, we found that their teeth are still very like those of wooly mammoths, which are well-adapted to eating grasses on cold, open plains. As the hybrids live in a similar environment, there's a pressure for them to keep their wooly mammoth-like teeth."

It would have been similar for hybrid Columbian mammoths living further south. As they lived in a warmer environment with a wider range of foods, there was a pressure for these animals to keep their more generalist teeth despite having significant amounts of wooly mammoth DNA.

Studying the impacts of mammoth hybridization further could also help protect living animals. Species such as the Scottish wildcat, for example, are in because of breeding with closely related domestic cats. Having past examples for conservationists to draw on can help them to understand potential outcomes for at-risk species.

The changing conditions of the Ice Age also help scientists to understand how modern elephants, and other animals, might adapt to modern climate change.

"Understanding how species can mitigate environmental change is very important at the moment, and we can look to past climate change to help with that," Lister adds. "We know that mammoths ultimately didn't survive the end of the Ice Age, and delving into their adaptability could help us better understand why."