My Theory...

Due to all of the fertility treatments these days, there will be a surplus of multiple births (ie. twins, triplets, octuplets, etc.). The gene pool will slowly diminish. The twins will have children with other twins and phase out the "normal" people. Pretty soon the planet will be filled with people that look exactly like each other and have the same genetic make-up. The earth will be occupied by a race of mutant clones, who will, due to genetic inbreeding, become evil and being to consume one another. Utter chaos will ensue and before too long the "human" race will have destroyed itself. Thanks, Jon and Kate...