Some evolutionary gene modifications might prove fatal when exposed, but are nevertheless are useful while they remain dormant and hence are continued to be carried.
For instance, Sickle Cell anemia is a fatal disease where RBCs sickle up causing life threatening complications.
The human immune system will attack any foreign substance entering the body by generating antibodies which will attack and destroy them.
Hence, these substances which trigger antibody creation are called antigens.
Some recent studies have indicated that Rh negative people are resistant to some parasites like Toxoplasma, or to their effects on the body.
So it might as well have served an advantage in this geography NOT to have the Rh D antigen, rather than have it. And one can find many such instances across human evolution.
Tests are done during early stages of pregnancy, and if the mother is Rh -ve and the child is Rh ve, then an injection which prevents the mother’s immune system from generating antibodies against Rh D antigen is given during pregnancy.
This will ensure that the fetus remains unaffected by the fact that the mother’s blood is Rh negative and ensures a safe delivery of the child.
However, the main Rh classification is based on one single antigen of special importance, called the D antigen.
So, if your blood has the Rh D antigen, then your blood group is Rh positive, else it is Rh negative.
Recently came across this theory which suggests that those with Rh negative blood group might be the descendants of ancient alien astronauts. Around 85% of humans are Rh positive, which means they have Rh D antigen in their blood, only around 15% are Rh negative which means they do not have Rh D antigen in their blood.
This is also the reason why blood groups like O-ve are very rare.
O-ve means a blood group which has neither A nor B antigens, nor Rh D antigen.