Stranded on a desert island, a unique DNA subgroup
There’s an island off the coast of Yemen that you’d be forgiven for never having heard of.
Socotra sits in the Arabian Sea and is home to just 60,000 people; for the most part, they fish, raise livestock, and grow dates. Scheduled air transportation services were severed several years ago, and the island is currently only reachable by charter flights and cargo ships. It’s warm year round, it almost never rains, and it’s reputed that Thomas the Apostle was once shipwrecked there and built an early Christian church out of his boat’s remains.
The island was designated a UNESCO World Heritage Site in 2008, and for good reason. A survey found that just a handful of other places on Earth have more endemic species than Yemen. Only here can you find cucumber trees, dragon blood trees, Socotrine aloe, and the Socotran pomegranate, as well as unique species of sparrows, grosbeaks, sunbirds, and starlings.
Another oddity you can only find on Socotra is the mitochondrial DNA (mtDNA) haplogroup N in the female population.
Haplogroups are groups of alleles in an organism and can be thought of an a sort of ancestral clan—haplogroups can tell you where your ancestors came from a long, long time ago. A haplogroup map can show how humans evolved in Africa and spread across the globe.
On Socotra, the male population is predominantly of the unique Paragroup J-P209, which is only found elsewhere in Oman, while most of the female population falls into the even rarer N group.
The N group descends from L3, which likely developed in east-central Africa, near modern-day Kenya and Uganda, and spread both northward and south, giving birth to the so-called “out of Africa” hypothesis of early human migration. The movement of N is ascribed to the subsequent “out of Arabia” hypothesis of human movement.
That migration, described in the American Journal of Physical Anthropology, posits Socotra along the southern migration route of anatomically correct modern humans about 60,000 years ago and suggests the island “may harbor traces of that first dispersal.”
In that study, researchers, led by Viktor Cerny, collected and analyzed mtDNA and Y-chromosomal variations from island residents and found little African influence but some Arabian and southwestern Asian influence.
“The Soqotri population shows evidence of long‐term isolation and autochthonous evolution of several mitochondrial haplogroups,” Cerny, et al., wrote. “Specifically, we identified two high‐frequency founder lineages that have not been detected in any other populations and classified them as a new R0a1a1 subclade.”
A more recent study in Nature suggests that a subclade of R0a1a1, R0a1a1a, is evidence of “increased maritime activity and trade” and predates periods of heightened rainfall on and around the southern Arabian Peninsula and could have involved then-coastal regions which are now under water.
“Our results do indicate population growth within Arabia at ~3 ka, which may be implicated in a late Holocene range expansion across the Arabian Sea involving perhaps HV1, and perhaps also of R0a1a1a lineages into the island of Socotra, where the age of the R0a1a1 lineages date to the same timeframe,” the Nature study notes. “Populations survived along the southeast Arabian coast during the extreme aridity of the so-called ‘Dark Millennium’ after 5.9 ka and may have prospered as climatic conditions improved again in the Arabian Bronze Age. Although there is less evidence from Yemen, this phase saw marked re-settlement of southeast Arabia during the Hafit phase of oasis agriculture after 5.1 ka, and a similar trajectory seems likely to have taken place to the west.”
If so, according to a 2010 study published in American Journal of Human Genetics, “Arabia was indeed the first staging post in the spread of modern humans around the world.”