The condition affects the shape of the normally round red blood cells, turning them crescent or sickle-like. This can cause the blood to clot, resulting in infection and severe blood loss. Those with the most severe form of the disease often don't make it past their fifth birthday.
Now, scientists believe that this widespread, often deadly condition is the result of just one genetic mutation that occurred a little over 250 generations ago.
The study, published in the American Journal of Human Genetics last week, was carried out by a team of researchers at the Center for Research on Genomics and Global Health, an offshoot of the National Institute of Health (NIH).
Using historical records and analysis of the genomes of close to 3,000 people with some genetic history of the disease, the researchers were able to pinpoint the most likely time and place of its origin: The Green Sahara 7,300 years ago, which makes it roughly 259 generations old. However, they add that it may have originated somewhere in West-Central Africa.
At about this time, a child would have been born with a genetic mutation in one chromosome, affecting the shape of the blood’s hemoglobin. The child wouldn't have suffered because the other gene would have been perfectly normal, but he or she would have grown up and had children of his or her own, thus passing on the gene to the next generation.
The mutation would have endured, passing on for hundreds of years before one unfortunate child would have been born with two copies of the mutation and, therefore, have developed sickle cell anaemia. The first recorded cases in history occurred in Egypt during the predynastic period (3200 BCE).
During the Bantu migrations and, later, the slave trade, SCD would have travelled to other parts of Africa and to other parts of the world, including the Near East, India, and Southern Europe, developing into different sub-types along the way.
In most cases, a deadly mutation like this would not survive, but the sickle cell mutation has because it comes with an important genetic advantage. One affected chromosome can protect its carrier against malaria – it is not a coincidence that sickle cell is far more prevalent in countries at a high risk of malaria. It appears SCD effectively starves the parasite responsible for malaria because it cannot feed on the sickle-shaped cells.
The scientists involved in the study hope this research can help improve medical care for people with SCD, offering medics a way to better predict whether a patient will develop a severe or mild form of the disease.