A Brief History of Pigs

Ninety-seven million years ago pigs and humans were one of the same. Well, sort of. There wasn’t exactly a four-trottered, bristly-nosed Homo-something snuffling through the Cretaceous. But we did share a common ancestor, a small generic mammal prototype, whatever that might be.

Actually it probably looked like an extinct creature called Juramaia sinensis, the oldest known eutherian (non-marsupial) species. Pop a withered, earless Dachshund head onto an obese rat, and that might paint a very vague picture. Anyway, this is about pigs.

As eutherian mammals we reached a fork in the road: humans and rodents went one way; cows, horses, dogs, and pigs the other.

Recent analyses of the genome of domestic and wild pigs, under the auspices of the Swine Genome Sequencing Consortium, have shed new light on pig evolution.

The scientists compared the genomes of ten wild boar and six domestic pigs from different areas across Europe and Asia.

Their findings, published in Nature, confirm the fact that boars (and all pigs) emerged from South East Asia and subsequently spread across Eurasia.

About one million years ago colder climates during the Calambrian glacial intervals isolated the two populations. A deep chasm formed between European and Asian boars. Then – to leap forward 80,000 years – during the Last Glacial Maximum (LGM) about 20,000 years ago vast swathes of boar died. But a small proportion survived. They suffered a population bottleneck on both sides of the split. However, this drop in population was more pronounced in Europe than in Asia where the LGM had less impact. For this reason the researchers found that European boar had less genetic diversity than Asian boar. With fewer genotypes in circulation the difference between Asian and European pigs was driven further.

Enter humans and domestication. About 10,000 years ago pigs and humans embarked on an intimate and complex relationship. Hmm. This lead to even greater selective pressures that shaped the genome of domestic pigs. In Europe the result was Sus scrofa domesticus, the pork-chop-and-bacon variety. Incidentally, there are about one billion domestic pigs alive today, which makes S. s. domesticus one of the most numerous large mammals on the planet.

By 1868 the populations were so different that Darwin proposed they originated from two distinct boar species. But this study shows that both Asian and European pigs arose from the Eurasian wild boar (Sus scrofa). This analysis tells us that pigs were domesticated independently across Europe and Asia. It also reinforces the historical observation that Asian pigs were introduced into Europe during the 18th and 19th centuries to bolster European herds.

The researchers go on to describe the pig’s enormous repertoire of olfactory genes. Smelling is vital to their scavenging lifestyle. So have a bit of sympathy when you next pass a pig farm – their eyes are watering more than yours.

DOI: 10.1038/nature11622