The X-Program was a top secret military project that aimed to create sophisticated, combat ready androids that could replicate human thought processes.  Fifty-one of these androids were built, each possessing an advanced positronic central processor as well as a host of offensive and defensive capabilities.

Dr. Abel Stack was one of the lead scientists on the project and he posited that the only way in which an artificial intelligence could truly be taught to think independently was to raise it as though it were a human.  He took home the android model X-51 and cared for it as though it were his son, teaching it the differences between right and wrong and offering it love and nurturance.   

Stack’s hypothesis proved accurate, X-51 cultivated sentience and showed compassion and sympathy towards others.  The remaining fifty androids, those that were denied this special attention each became aggressive and began to attack the staff.  These androids were equipped with a self destruct feature and they were each destroyed before they could hurt anyone.  Dr. Stack had removed the self destruct mechanism  from X-51 yet it accidentally blew up, critically injuring the scientist.  With his dying words, Dr. Stack named X-51 ‘Aaron Stack’ and compelled him to live his life and find his own destiny.  

Hunted by the military, X-51 was able to make modifications to his form to remain hidden and pass as human.  He additionally updated his functioning, greatly increasing his capabilities.  He would go on to have numerous adventures as the superhero known as ‘‘Machine Man,’ all whilst trying find his place as an android in a human world.  He would go on to join the West Coast Avengers as well as the main Avengers.  

X-51’s artificial sentience intrigued the godly, intergalactic beings known as The Celestials.  These entities took in X-51 and offered him the opportunity to survey the cosmos and better understand the meaning of existence at their side.  He agreed and ventured off with the Celestials.  

When X-51 left with The Celestials, a replica was left in his place.  This android possessed all of the original’s memories and believed himself to be one and the same with the original.  Unlike his predecessor, however, this new X-51 is sardonic and irascible, possessing the attitude that androids are superior to humans.  He somehow ended up an operative of the Beyond Corporation, acting as a member of the hero team, NextWave.  He later assisted ARMOR in fending off a breach from an alternate reality populated by zombies and has additionally acted as a member of the Avengers Initiative and Deadpool’s Mercs for Money.  

Both iterations of X-51 have maintained a turbulent romance with the fellow synthetic being, Jocasta.  The android hero first appeared in the pages of 2001: A Space Odyssey Vol 2 #8 (1977).