Oldest-ever DNA extracted from a 1.2-million-year-old mammoth tooth

Up until now, the oldest DNA to have been extracted and studied came from a horse that had been frozen in the permafrost for 700,000 years.

A new study has now pushed this record back by 500,000 years, after researchers managed to extract and sequence DNA from three mammoth teeth that range from 700,000 to 1.2 million years old.

The DNA that has been retrieved from these teeth has not only revealed that there may have been a new, unknown type of mammoth roaming the grasslands of Siberia about a million years ago, but it is also helping to shed light on the origin of the Columbian mammoth that lived in North America.  

In addition to this, the DNA from the teeth are giving clues as to how some mammoth species adapted to the extreme cold conditions of the north.  

Prof Adrian Lister is an expert on mammoths at the Museum, and was involved with the study.

'Until now ancient DNA has rarely probed beyond a few tens of thousands of years, limiting what we can learn about the process of evolution,' says Adrian. 'With million-year-old DNA we extend beyond the date of origin of many iconic species like the mammoth, so we can track how species evolve and how their adaptations have arisen.'

The paper has been published in Nature.

Mammoth hybridisation

Mammoths belong to a group known as the proboscideans, which includes the living African and Asian elephants along with a whole raft of extinct elephant-like creatures.

Mammoths first appeared in Africa around five million years ago and went on to colonise much of the northern hemisphere across both Eurasia and North America. Some species adapted to the cold, grew long fur and became some of the most iconic ice age fauna.

Woolly mammoths are probably the most well know of the mammoths, but they are just one of about ten different species.

One of these was the steppe mammoth, which lived on the cold, dry grasslands of Eurasia between 1.7 million and 200,000 years ago. It is from these animals that the woolly mammoth is thought to have evolved in Eurasia, while one population crossed over into North America and gave rise to the huge Columbian mammoth.

The DNA from the animals' teeth reveals more about how these three species interacted and came to be.

The results show that rather than there being one distinct population of steppe mammoths that lived right across Siberia, there were two distinct genetic lineages that diverged at least two million years ago. Whether or not these were still part of the same species, or represent two different species alive at the same time, is yet to be resolved.

What the researchers were able to show, however, is that it was this hitherto unknown genetic lineage of steppe mammoth crossed over the Bering land bridge into North America around 1.5 million years ago. 

Around a million years later woolly mammoths also made this journey, and some individuals mated with the mammoths already in North America. From that point on, the Columbian mammoths were hybrid animals.

'Earlier studies by myself and colleagues had suggested an origin of North American mammoths from Siberian steppe mammoths, based purely on their tooth shape,' explains Adrian. 'But we could never have guessed that it was a genetic lineage that had separated off in Siberia a million years previously, nor that it hybridised with woolly mammoths well before the last ice age.'

The importance of hybridisation to the evolution of new species is a topic that has been gathering interest. It is now thought, for example, that the European bison arose as a result of a hybridisation event between the extinct steppe bison and aurochs.

Delving into the DNA

The youngest of the teeth studied, dating to around 700,000 years ago, is thought to have belonged to one of the earliest known woolly mammoths.

By comparing the DNA from the million-year-old steppe mammoth, the slightly younger woolly mammoth and woolly mammoths that were alive just a few thousand years ago, researchers have an unprecedented look at how genetics of the species changed over a period of hundreds of millions of years as they adapted to the cold, harsh environments in the northern latitudes.

It reveals that the genes already associated with life within the Arctic Circle, such as long hair, cold tolerance, fat deposits and changes to their sleeping cycles were already present in the ancestors of the woolly mammoth. This suggests that the adaptations of mammoths to their environments occurred slowly over a long time. 

The shapes of mammoth teeth suggest a similar story.

'The tooth shape of all these northern species is adapted to grass eating in a cold, open environment, unlike their more distant ancestor, the so-called 'southern mammoth' which comes from more southern regions, including the UK,' explains Adrian. 'The southern mammoth had teeth adapted to eating trees and shrubs in a warmer, forested environment, and so hence this fits the genetic evidence of cold adaptation in the steppe mammoths.'

As we get better at studying ancient DNA, similar stories of hybridisation could come to light.  

While there are reports of ancient DNA from a 1.7-million-year-old rhino tooth, this was actually looking at the proteins contained within the remains, rather than the actual DNA itself. But that doesn't mean it won't be possible to extract DNA of that age in the future.

'One of the big questions now is how far back in time we can go,' says Prof Anders Götherström, an expert in molecular archaeology and joint research leader at the Centre for Palaeogenetics. 'We haven't reached the limit yet.

'An educated guess would be that we could recover DNA that is two million years old, and possibly go even as far back as 2.6 million. Before that, there was no permafrost where ancient DNA could have been preserved.'