first of all,
http://www.csicop.org/si/9903/ten-percent-myth.html
and second, when we create new life, we don't even use our brains. it's not like we sit and design the DNA sequence of every child. we have no control over the mutations passed along to our offspring, nor did any of the species from whom we evolved. the changes were/are essentially random, and those chosen by natural selection to survive will be passed on and will therefore improve the chances of survival of the organisms holding that gene
finally, you take it on faith that a car won't hit you when you cross a street? damn, you're gonna be naturally selected soon if you don't start looking both ways...