How people moved pigs across the Pacific

Prehistoric cave painting of two Sulawesi warty pigs from Leang Tedongnge Cave, Sulawesi, Indonesia. Dated to at least 45,000 years old, making it one of the oldest known cave art in the world and demonstrates the long-standing relationship between pigs and people in the region. Credit: Adam Brumm (Griffith University) and Adhi Agus Oktaviana (BRIN, Indonesia)

A new study, published today in the journal Science, reveals how millennia of human migration across Pacific islands led to the introduction of invasive pig species all over the Asia-Pacific region.

The study was led by Laurent Frantz, Professor of Palaeogenomics at Queen Mary University of London (QMUL), and the Ludwig Maximilians University of Munich (LMU), David Stanton, from Cardiff University, and Greger Larson, from the University of Oxford.

Why were pigs one of the only species to cross the 'Wallace Line' in Indonesia?

Plants and animals have not always spread naturally across the islands of Indonesia. The evolutionary biologist Alfred Russell Wallace identified a major biogeographic boundary, the “Wallace Line”, noting that wildlife on either side rarely crossed. Leopards and monkeys, for example, are found on the Asian side, while marsupials and cassowaries are largely limited to the Australasian side.

One notable exception is pigs. Pig populations occur on both sides of the Wallace Line and extend across Southeast Asia to New Caledonia, Vanuatu and remote Polynesia. Pigs are highly effective ecological invaders, and they are also culturally important across the region, raising a key question: what role did people play in their spread?

Pigs appear in c. 50,000-year-old cave paintings 

The new paper looked at the genome of over 700 pigs, including from living and archaeological specimens. This allowed the reconstruction of their movement across southeast Asia and identify when they arrived on certain islands and how they might have interbred with various native pig species.

The researchers found that people of different cultures have moved pig species in the region for millennia. The earliest evidence points to people living in Sulawesi perhaps as early as 50,000 years ago, known to be the earliest cave painters, who both depicted and transported warty pig species as far away as Timor, possibly to establish future hunting stock.

Domestic pigs transported for agriculture "escaped and became feral"

The introduction of pigs in Island Southeast Asia dramatically accelerated, around 4,000 years ago, when early agricultural communities transported domestic pigs in the region. Their journey began from Taiwan, extending across the Philippines, northern Indonesia (Maluku), into Papua New Guinea, and on to the outlying islands as far as Vanuatu, and remote Polynesia. The authors also found evidence for the introduction of pigs from Europe during the colonial period.

Many of these domestic pigs escaped, and became feral, in some cases, like on the Komodo islands, hybridising with the warty pigs brought by people from Sulawesi thousands of years earlier. These hybrid pigs are now a major source of food for the endangered Komodo dragons.

Study highlights human impact on local ecosystems

The findings of this study highlight the dramatic and enduring impact of human activity on local ecosystems in the Pacific, raising conservation conundrums. Pigs in the region today have dramatically different statuses and impacts across islands: some are considered spiritual beings, others pests, while some are now so ingrained in local ecosystems that they could almost be considered native. Efficient conservation policy will need to navigate these complexities, going beyond the traditional paradigm of conserving only native fauna.

The study included collaborators from around the world, with more than 50 authors being involved, including scientists from Cardiff University, the University of Oxford, the National Research and Innovation Agency of Indonesia, National Museum of the Philippines, and the Vanuatu Cultural Centre.

What counts now as a "native species"?

Prof. Laurent Frantz, senior author of the study: “It is very exciting that we can use ancient DNA from pigs to peel back layers of human activity across this megabiodiverse region. The big question now is, at what point do we consider something native? What if people introduced species tens of thousands of years age, are these worth conservation efforts?”

Dr. David Stanton of the University of Cardiff and Queen Mary University of London, the lead author of the study said “This research reveals what happens when people transport animals enormous distances, across one of the world’s most fundamental natural boundaries. These movements led to pigs with a melting pot of ancestries. These patterns were technically very difficult to disentangle, but have ultimately helped us understand how and why animals came to be distributed across the Pacific islands.”

Prof. Greger Larson, of the University of Oxford said: “Wild boar dispersed across all of Eurasia and North Africa and certainly don’t need people to help them disperse into new areas. When people have landed a hand, pigs were all too willing to spread out on newly colonised islands in South East Asia and into the Pacific. By sequencing the genomes of ancient and more recent populations we’ve been able to link those human-assisted dispersals to specific human populations in both space and time.”