Domestic pigs have a lot more "wild" in them than originally thought — at least from a genetic standpoint.

By studying the genes of pigs and wild boars, researchers have discovered that domestic pigs in Europe interbred with European wild boards at multiple occasions after their initial domestication by humans. This contradicts a common assumption that pigs were domesticated when humans isolated a certain population away from other wild animals and selectively bred them to promote certain behavioral and physical traits. Instead this study, published today in Nature Genetics, shows that domestic pigs of European descent are actually a mosaic of multiple wild populations — some of which may be extinct.

Humans first domesticated pigs about 9,000 years ago. Since then, they've become pets, movie stars, and of course a source of food. The domestic pig wasn't always this docile, however. Scientists have known for a long time that pig domestication took place over a number of generations. But today's study reveals that this process wasn't nearly as simple, or as streamlined as researchers once thought.

"More exchange with wild populations than we thought."

"There were a lot of theories that when humans started to domesticate plants or animals, that it was a lot more restricted in time and region," says Martian Groenen, a geneticist at Wageningen University in the Netherlands and a co-author of the study. "But what our results really show is that over the years, there has been a lot of exchange with wild populations present at many different occasions."

In the study, researchers came up with various evolutionary models that they then tested against genetic data they had gathered from European wild boards and domestic pigs. They found that the models that best fit the genetic information were ones that included breeding events with wild boars well past the initial period of domestication. "The data shows that there was much more exchange with wild populations than we thought there would be," Groenen says.

The study raises a whole bunch of questions. For example, the data weren't able to tell researchers exactly why domestic pigs mated with wild populations. "It's very difficult to really go back in time," Groenen says. Humans may have seen some benefit to breeding their animals with wild boars, or this genetic flow may have been accidental; researchers really don't know.

It's also not clear whether humans intentionally tried to counteract the effects of genetic mixing between wild and domestic populations. But regardless of whether it was intentional or not, continuous artificial selection likely helped promote the neurological and anatomical traits that European pig breeds display today, Groenen says.

Finally, the boars that the researchers sampled weren't enough to explain all of the genetic variation they saw in domestic pig DNA. That means that some other wild population — one that might be extinct — likely also contributed genetic material to domestic pigs in Europe.

"Going back in time would be the best thing to do."

The researchers now hope to collect fossils that will be able to fill in some of the gaps. "Going back in time would be the best thing to do," Groenen says. "The fossil record might be able to answer some of these questions."

Because of its role in agriculture, animal domestication is widely thought of as one of the most important events in human history. So, for people who might not be interested in pigs, this study can also be regarded as a window into lives of humans of yore, Groenen says. "The pig is an important food source, and trying to understand how these pigs relate to their wild relatives is an interesting question — but it's also a question of how society evolved, how civilizations started, and what humans have done to get to the situation that we're in now," he says. "As a historical concept, that's fascinating."