Pigs were domesticated from wild boars about 9,000 years ago. Researchers have long assumed that this process involved permanently isolating a few animals from the wild. But according to new findings published in Nature Genetics this week, pig domestication doesn’t follow this traditional model: Pigs continued to mate with wild boars even after they became domesticated.

During the Neolithic Revolution, the domestication of both plants and animals led to a major shift from a hunter-gatherer lifestyle to that of a sedentary agricultural one. The traditional view of animal domestication typically involves strong bottlenecks, or population reductions, since only a few individuals are selected as founders; soon thereafter, reproductive isolation between wild and domestic forms sets in. However, recent work has begun to take a less straightforward view – one that includes gene flow between wild and domestic populations, both during and after domestication. 

Pigs were domesticated in both Anatolia (roughly modern-day Turkey) and the Mekong valley in East Asia about a thousand years after the rise of agriculture. Now, an international team led by Laurent Frantz of Wageningen University examined 103 whole genomes from European wild boars and domestic breeds as well as genetic data from more than 600 pigs and boars in Europe and Asia. 

Domestic pigs in Europe, they found, originated from Anatolian domestic pigs; that was no surprise. However, they also found evidence of gene flow between these domestic pigs and wild boars – the two share a large portion of their DNA. The team tested several evolutionary models, and the best way to explain their data is this: Domestic pigs interbred with wild boars throughout their history, long after domestication. European domestic pigs are a mix of many wild populations (including extinct species) and therefore don’t constitute a homogeneous genetic group as we’d expected. (Asian pigs may have a similar mode of domestication but the data weren’t conclusive.)

Furthermore, the team found no evidence of a domestication bottleneck. Continuous selection by humans for agriculturally favored traits may have counteracted the effect of interbreeding and gene flow between wild and tamer pigs – allowing domestication to continue.