North American mammoth origins rewritten

Scientists previously believed that the primitive southern mammoth migrated from Asia to North America around 1.5 million years ago and evolved into the Columbian mammoth.

New research shows that the southern mammoth never made it to North America, and that the Columbian mammoth's ancestor was actually the more advanced steppe mammoth, a species very similar to the Columbian mammoth that migrated from Asia into North America over the Bering Strait land bridge around 1.5 million years ago.

The researchers also found evidence that Columbian mammoths interbred with woolly mammoths, after the woolly mammoth arrived in North America around 100,000 years ago.

Lead author and Museum palaeontologist Prof Adrian Lister says: 'Until now, we thought North American mammoth evolution and adaptation ran separately from other continents. This research shows that mammoth evolution is a lot more complex and surprising.'

Teething problems

Prof Lister and Dr Andrei Sher from the Institute of Ecology and Evolution in Moscow re-examined over 600 fossil mammoth teeth held in the Museum and other collections around the world. They found that many had been misidentified.

As Columbian mammoths fed on sedge, grass and other plants on the plains of North America, their teeth were ground down, changing the shape and size of the chewing surface. This makes them difficult to distinguish from teeth of the southern mammoth, which also have low crowns and fewer enamel ridges.

The researchers mapped the changing appearance of mammoth teeth over a lifetime using micro-CT scanning at the Museum. This proved that molars identified as southern mammoth teeth were actually worn-down Columbian mammoth teeth.

As a result of the study, many southern mammoth fossils held in museums around the world will need relabelling as Columbian or even steppe species.

The findings, published today in Science, suggest that there is no evidence southern mammoths ever reached North America. So who is the Columbian mammoth's ancestor?

Mapping out mammoths

Columbian mammoth teeth are extremely similar to those of the steppe mammoth, a species that lived in large numbers in northeast Siberia at least 1.2 million years ago. The steppe mammoth was in the right place at the right time to migrate over the Bering Strait into North America around 1.5 million years ago, where its remains are now identified as Columbian mammoth.

In fact, teeth of the steppe mammoth are virtually identical to those of the Columbian mammoth. Prof Lister's future research will look at whether the two are actually the same species:  'The challenge now is to study other parts of their body, especially their skulls, to see whether the Columbian mammoth had become a separate species by the time it settled in North America.'

Woolly relations

The woolly mammoth evolved later than the Columbian mammoth in the same region linking North America and Asia.

We know woolly mammoths crossed the Bering Strait around 100,000 years ago, living mainly in what is now Alaska, Canada and the northern US mainland. The Columbian mammoth was already present in large numbers at this time, living in the warmer regions of modern-day USA and Mexico.

Columbian and woolly mammoths probably bumped into each other along an overlap zone between their northern and southern ranges. 

Fossils found along this overlap zone were previously identified as several distinct species. Prof Lister now believes they represent hybrids that were the result of interbreeding between Columbian and woolly mammoths, a theory backed up by recent DNA analysis.

Prof Lister adds: 'Mammoths are a great example of "evolution in action" in the fossil record. This study shows the value of fossil collections in helping us understand the diversity of life, hand-in-hand with genetic evidence.'