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Donkeys generally strike us as stubborn and not particularly fast animals.
Yet a new study in the Sept. 9 issue of the journal Science now reveals that donkeys spread like wildfire out of Africa about 4,500 years ago and reached Europe and Asia within a few centuries.
This shows the key role that donkeys played in early human societies, especially as beasts of burden that mobilized people, culture and goods across arid deserts and steep mountains.
Surprisingly, where and when donkeys were first domesticated has remained a long-standing scientific mystery. Archaeological finds, as well as textual and iconographic evidence, have suggested the broad region of northeastern Africa as a possible source.
But they also pointed to regions outside of Africa, possibly the Arabian Peninsula or even further afield in Mesopotamia.
Previous work aimed at tracing the evolutionary history of donkeys using their genetics failed to find significant support for any candidate region.
“We decided to sequence the genome of donkeys that live in regions that were previously overlooked,” stated Ludovic Orlando, CNRS Research Director at the Center for Anthropobiology and Genomics of Toulouse (CNRS / University Toulouse III Paul Sabatier) and senior author of the study. in the press release. “This was expected to reveal important missing pieces of the puzzle.”
This gave the research team, which includes 49 researchers from 37 laboratories around the world, the most comprehensive genome panel of donkeys. But mapping the genetic diversity of donkeys today may not have been sufficient, as modern breeding and trade may have exchanged animals over very distant regions. Therefore, the researchers should also characterize the genomes of donkeys that lived in the past by utilizing state-of-the-art technologies in ancient DNA research.
Dr. Laura Patterson Rosa, assistant professor of animal and equine science at Sul Ross State University, was part of the team that analyzed the donkeys’ genome as part of the study.
Dr. Patterson Rosa’s efforts included sampling a unique South American donkey breed, the Pega donkey, which has been heavily selected for its smooth movement—as a “gaited” donkey breed—and endurance since the colonization of the Americas. The worldwide and historical comparative genomic analysis showed that the written historical accounts of the origin of this breed may not be completely accurate. Furthermore, the results revealed the high genomic inbreeding, which is a cause for concern as conservation of the Pega donkey may require planned outcrossing to maintain health and population diversity.
“Modern donkeys living in different regions of the world show quite strong genetic differences, especially between the African, European and Asian continents,” stated Dr. Evelyn Todd, first author of the study, in the press release.
In addition to striking geographic differences, the team found that the timing of the separation between the different populations followed a clear trend, starting first in Africa and spreading to Eurasia and Asia. The researchers report an African origin about 7,000 years ago, a time roughly close to when the once verdant Sahara region became one of the driest deserts on the planet. They estimate that donkeys spread out of Africa at least 4,500 years ago and rapidly expanded both eastward into Asia and westward into Europe within no more than 1,000 years.
The expansion did not just follow one direction, but also returned to Africa. For example, donkeys were already being exchanged between Europe and Africa across the Mediterranean by Roman times, the study shows. While these exchanges went both ways and continued after the collapse of the Roman Empire, they left the most important genetic footprint in modern donkeys from West Africa.
Ancient genomes provide securely placed timelines that helped scientists track the expansion of donkeys around the world. They also revealed the presence of previously unknown genetic lineages. One such lineage was found in the Levant about 2,000 years ago, but likely inhabited a much larger geographic area, as researchers could identify traces of their genetic heritage in modern donkeys throughout Eastern Europe, Central Asia, and East Asia.
Furthermore, the researchers reveal that relatives of wild donkeys also contributed a fraction of their genes to different regions of the world.
For Evelyn Todd: “This likely reflects free-range management of local donkey populations in some African regions and the Arabian Peninsula.”
Clearly, the study reveals an important difference between the donkey and their close relative, the horse. Unlike horses, inbreeding in donkeys did not particularly increase in modern times, suggesting similar reproductive strategies now and in the past.
But the researchers went beyond simply tracing global patterns of donkey handling techniques. They found a mule-breeding center at Boinville-en-Woëvre, a Roman site from northern France dating to the second to fifth centuries. There, breeders appear to have produced specially inbred families of giant donkeys at a time when mules provided the labor force to deliver military equipment and goods across an empire that stretched thousands of miles. Mating between these giant donkeys and female horses allowed breeders to produce highly prized sterile mules.
Here, the genetic evidence echoes the texts of Roman authors who described that selective breeding for animals of exceptional stature was already common practice and a lucrative business at the time.
“This is the beauty of ancient DNA to provide data that allows us to test hypotheses from other classical historical sources,” Orlando concluded.
The genetic study went a step further: the limited presence of horses in Boinville-en-Woëvre suggests that either female horses are brought for mating or donkey breeders visiting surrounding farms with their giant males. A different kind of journey, admittedly over a more limited region than that which brought their ancestors out of Africa thousands of years earlier, but which nevertheless helped build the mighty Roman Empire.
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