Brown HEREDITARY line on fingernail?

Since you've had it for a very long time, without any changes; and since the line is uniform in color and width; and since there is no leeching of the pigment into the nearby nail fold area, I think you can feel safe in knowing that your brown line is nothing more than the result of having a mole occur under the nail plate. However, should any of the above mentioned changes occur, then it should be evaluated by your dermatologist immediately, as melanomas can occur under the nail plate.