If a single find like Ardi can rewrite the way we think of evolution, why should we believe science at all?

Over the years I've noticed how one archaeological find can entirely change the way scientists think of certain ancient history. If a single find can change everything, why believe anything at all? It means every hypothesis has a 50/50 chance of falling at any given moment. Shouldn't it take more than one find to rewrite history? What if future historians found only one set of bones of our present time and they were Yao Ming's... wouldn't that lead our future geniuses spouting new theories overnight about an exaggerated height of present day man? My question is what guidelines does archeology use to ensure their finds are not deviations of what was the norms?