During season one of the show, Lt. Aiden Ford (Rainbow Francks) was the fourth member of Sheppard's expeditionary team, but infection from a Wraith attack altered his DNA and left him borderline insane.
Battles with the Replicators and the Wraith continued to plague the Expeditionary Force through the series conclusion, when the Ancient City was relocated to Earth.
Based on the creatures intiially introduced during SG-1's second season, these human form Replicators were actually created by the Ancients thousands of years before to destroy the Wraith, but something went wrong.
Just when a race begian to develop, along came huge Wraith hive ships to cull the herd before becoming dormant until the next time they needed to feed.
The Wraith were the reason the human races spread throughout the Pegasus Galaxy remained at such a low level of technological advancement.