It's because of sexual reproduction,
this causes everyone to get unique DNA patterns, (except for twins,
which from by one fertilised egg splitting into 2). 50% from mum 50%
from dad
When ever the sex cells go through reproduction, our 46 cells come together to make about .01% of us, in that is everything about us, eye color, skin color, hair color, emotions. So really we all are barely different.
