Will humans grow a third arm?

Possibly...it is shown that after about 500,000 years of evolution a species often have a jump of rapid evolution that has physical and internal effects. This jump isn't as fast as you would think, it happens over about 100 years. In the past these "jumps" have included appearance of tails on a species once tail-less, an extra finger/toe on each hand/ foot (dinosaurs and other animals only had 4 fingers/toes on each hand/foot) Scientists don't know what humans next jump will be. However, it is quite unlikely to have it be a third arm it is quite possible, but it is more likely to be something less dramatic.