Or that's what everyone thinks. The REAL truth is, that when the nukes were sent back in time, it was a period where earth was barren, no life existed yet.
When the nukes detonated, a lot of mars debris was ejected into space due to the explosion. The debris also harboured mars based bacteria/life, some of which found their way to earth, the mars bacteria thrived on earth, and became the origin of life for earth. We humans are descends of martians, meanwhile the earthlings in the alternate timeline are something entirely different.