Donkeys generally strike us as stubborn and not particularly fast animals.
However, a new study published in the September 9 issue of Science now reveals that donkeys spread like wildfire out of Africa about 4,500 years ago, reaching Europe and Asia in just a few centuries.
This illustrates the key role donkeys played in past human societies, particularly when beasts of burden mobilized people, culture, and goods across arid deserts and steep mountains.
Surprisingly, the place and date of the first domestication of donkeys has remained a long-standing scientific mystery. Archaeological finds, as well as texts and iconographic evidence, have suggested the vast area of northeastern Africa as a possible source.
But they also referred to areas outside of Africa, perhaps Arabia, or further afield in Mesopotamia.
Previous work aimed at tracing the evolutionary history of donkeys using their genetics had failed to find significant support for any candidate region.
“We decided to sequence the genomes of donkeys that live in previously overlooked regions,” said Ludovic Orlando, director of CNRS research at the Center for Biology and Genomics of Toulouse (CNRS/University Toulouse III Paul Sabatier). “This was expected to reveal important missing pieces of the puzzle.”
This provided the research team, which includes 49 scientists from 37 laboratories around the world, with the widest genome panel of donkeys. However, mapping the genetic diversity of donkeys today may not be sufficient because modern breeding and trade have exchanged the animals across very distant regions. Therefore, researchers must also characterize the genomes of donkeys that lived in the past, and take advantage of the latest technology in ancient DNA research.
Dr. Laura Patterson-Rosa, assistant professor of animal and equine sciences at Sol Ross State University, was part of the team that analyzed the donkey genome as part of the study.
Dr. Patterson Rosa’s efforts included sampling a unique breed of South American donkey breed, the Pega donkey, which has been highly selected for its smooth movement – as a “charged” donkey breed – and its stamina since the colonization of the Americas. Historical and global comparative genomic analysis has shown that written historical accounts of the origin of this breed may not be completely accurate. Furthermore, the results showed high rates of genomic inbreeding, which raises concern that the conservation of the bega donkey may require planned crossbreeding to maintain health and population diversity.
“Modern donkeys that live in different regions of the world show very strong genetic differences, particularly between the African, European and Asian continents,” Dr. Evelyn Todd, the study’s first author, said in a press release.
In addition to the striking geographic differences, the team found that the timing of the separation between different populations follows a clear trend, beginning first in Africa and spreading into Eurasia and Asia. Researchers have reported an African origin about 7,000 years ago, around the time when the green-tinged Sahara region became one of the most arid deserts on the planet. They estimate that donkeys spread out of Africa at least 4,500 years ago, and expanded rapidly from east to Asia and west to Europe within 1,000 years at most.
The expansion not only followed one direction, but also returned to Africa. For example, donkeys were actually exchanged between Europe and Africa via the Mediterranean by Roman times, the study found. While these exchanges went both ways and continued after the collapse of the Roman Empire, they left the most significant genetic impact on modern donkeys from West Africa.
Ancient genomes provide securely defined points in time, which have helped researchers track the expansion of donkeys around the world. They also revealed the existence of previously unknown genetic strains. One such lineage was found in the Levant about 2,000 years ago, but may have inhabited a much larger geographical area, as researchers have been able to identify traces of their genetic heritage in modern donkeys throughout Eastern Europe, Central Asia, and East Asia.
Moreover, the researchers revealed that relatives of wild donkeys also contributed a fraction of their genes in different regions of the world.
For Evelyn Todd, “This likely reflects free-range management of domestic donkey populations in some areas of Africa and the Arabian Peninsula.”
Obviously, the study reveals an important difference between the donkey and their kin, the horse. In contrast to horses, inbreeding in donkeys has not particularly increased in modern times, indicating similar reproductive strategies now and in the past.
But the researchers went beyond just tracking global patterns of donkey management techniques. They found a mule-breeding center at Boinville-en-Woëvre, a Roman outpost from northern France dating from the 2nd to the 5th centuries. There, breeders seem to have produced inbred families of giant donkeys while mules provided the workforce to deliver military equipment and goods across an empire that stretched over thousands of kilometers. Mating between these giant donkeys and female horses allowed breeders to produce highly prized sterilized mules.
Here, genetic evidence mirrors the texts of Roman writers who described that selective breeding of animals of exceptional stature was already a common practice and a profitable business at the time.
“This is the beauty of ancient DNA in providing data that allows us to test hypotheses from other classical historical sources,” Orlando concluded.
The genetic study went even further: the limited presence of horses in Boinville-en-Woëvre suggests that female horses were brought for mating, or donkey breeders visiting surrounding farms with their giant males. Another type of voyage, recognized across a more limited area than that which brought their ancestors out of Africa thousands of years ago, but nonetheless contributed to the building of the powerful Roman Empire.