But first, a quick recap. To understand these new results, you have to understand last year's big study on dog origins. That paper, written by an entirely separate group, proposed an idea that was new at the time: that dog domestication actually happened twice. It argued that there was a dog population in the east and another in the west. The researchers estimated that around 14,000-6,000 years ago, that divide between dog groups already existed—but since the oldest dog fossils in both the east and west are older than that, it follows that the populations were already separate. That is, they was never one big group of dogs that split into two over time. Rather, domestication happened twice, such that by 14,000-6,000 years ago there were already two distinct populations. Then, as humans migrated from east to west, eastern dogs mixed with the western dogs, largely supplanting the western population and creating the illusion of a single lineage.