A detailed analysis of mammoth teeth from around the globe suggests that the first mammoth species to arrive in North America was much more evolved than previously thought. While the evolutionary origins of Eurasia's mammoths are established, much less is known of Columbian mammoths, the species that later resided in North America. It's currently believed the earliest mammoths arrived in North America between 1.5 to 1.3 million years ago (MYA), and that these early settlers were of "primitive" morphology, with a closer relationship to the mammoths found in Europe, M. meridionali, than to their more advanced counterparts from eastern Asia, M. trogontherii. This would suggest that primitive mammoths migrated across the Bering Strait into North America, as well as into Europe, before further evolving into different species within their respective continents. Studies of mammoth evolution often rely on the nature of molar teeth to determine divergence between species. However, Adrian Lister and Andrei Sher note that previous studies do not account for the wear and tear that occurs over the mammoth's lifetime, and weakening of the teeth as the molars shift from the back of the mouth to the front. After adjusting for these factors, the authors believe that the more evolved M. trogontherii first crossed the Bering Strait 2 to 1.5 MYA and later evolved into the species of Columbian mammoths. The Eurasian woolly mammoth followed suit later, residing in an area north of the Columbian mammoths for some time before the two animals' ranges overlapped, perhaps leading to interbreeding, the authors suggest. This research appears in the 13 November 2015 issue of Science.