Tales of giants have been with us since before writing was invented.  The earliest writings include tales of giants.  We have all heard of David & Goliath, Cyclops, Jack and the Bean stock and many other giant tales.  With most myths and legends, there is some truth behind the stories, so were there ever real giants?  I think the answer has to be "yes".  The question is, what were they?
    One possibility is that they were mutations with regular size parents.  We know that organic compounds of mercury can cause mutations in animals including gigantism, so maybe that is what happened.
    It seems though, that in many tales of giants, people simply don't seem to be very surprised about running into a giant, as if it was something unusual, but not extremely rare.  That would point more to the idea that giants were a separate race or sub-species of man that died out for some reason.  There is some evidence of this.  Over the past two centuries, there have been a number or reports of skeletons found that were nine, ten, even twelve feet tall.  There are also a number of ancient tools such as axes and swords that are much too big to be handled by regular size men.   Perhaps they were just cerimonial, but they might have been the tools of giants.
    So if there were giants, what happened to them?  One possibility is that they had big bodies but small brains and there their lack of intellegence allowed the smaller but smarter humans to defeat them and eventually wipe them out.  Another possibility is the ice age.  If the giants existed before the ice age, and some survived, the shortage of food during the ice age would cause them to have smaller babies and their growth would be stunted.  That stunted growth would be a positive trait during the ice age so it would be passed on to future generations.  If that is what happened, than some of us may be descended from giants!
    I'm not saying that is what happened.  It's just speculation on my part.