Evolution.

I won’t spend a whole essay on this.  I think that there is a lot of evidence to support that biological evolution exists.  A quick study of paleontology, zygote development, cross-species DNA comparison, blah, blah, blah. should be enough to confirm this idea.  

Besides, even if you don't believe in the evolution of humankind from animals.  You have to admit that the evolution process occurs in several ways in our world.  In how businesses survive in capitalism, the way ideas survive and propagate (memology), how music styles change.  

The idea that things that have the best performance in a given time period have the ability to influence and change their following generation seems very basic overall.  It is also the most reasonable answer for explaining a multitude of questions about the world.