The apex of countless years of Replicator evolution, created in mere moments (relatively speaking) after accelerating a time dilation device on the Asgard world of Halla. The Milky Way Galaxy's Replicators are a later re-emergence of beings first engineered in the Pegasus Galaxy by the Lanteans (the Ancients), created as a weapon against the Wraith but which would eventually grow into their own independent civilization: the Asurans.
After accessing Asgard memory banks the Replicators discovered data on the android Reese and realized she was their creator. They studied her design and form and found aspects of her physiology superior to their own. They composed in millions of cell units (similar to nanites) which combined to create human form. First was forged in her image. Soon he was joined by Second, Third, Fourth, Fifth, and Sixth who also took on human form. They successfully reversed the Asgard time dilation device to work in their favor and give them thousands of years to propagate the species before spreading from Halla like a horrible plague.
Instead of kerons, humanoid Replicators are based on the rare element neutronium (itself of fundamental importance to the Asgard civilization). They also possess an arrogance fitting of "higher" life forms. They are impervious to projectile weaponry, their bodies taking on a consistency that allows all ammunition to pass entirely through. They have the ability to "meld" with the minds of human beings by literally shoving a hand into the forehead, recalling memories and interacting with the violated individual in real-time. They are also capable of "merging" with keron-based Replicators, be it for communal or information and strategizing purposes.
One humanoid Replicator, Fifth, was created without the "flaw" in the Replicator programming, in existence since the very beginning with Reese. Consequently he became far more human in behavior than his counterparts, who believed he was a mistake. When Sixth was created they reverted to their original line of development.
SG-1 used Fifth's humanity against him, convincing him to help them in exchange for safe passage out of Halla and beyond the range of the time dilation field. He agreed to stay behind and fend off his Replicator brethren from the device until time for him to join them. Not realizing the device would go off two minutes too soon, he became trapped with the rest who declared the humans had made a fool of him.
In real time it only took two years for the Replicators to reach the dilation device and deactivate it. SG-1 was with Thor in the Halla system when the Asgard collapsed Halla'sstar, hoping the singularity would absorb all of Halla and every Replicator in existence. Unfortunately Fifth and his Replicator siblings found a way to escape the gravity field, captured Major Samantha Carter and programmed a new course for the new Asgard planet, Orilla.
Fifth interrogated Carter intently en route. Upon arriving at Orilla, he led a detachment of Replicators to the surface while their ship was destroyed by Asgard firepower in orbit. On the ground, they located a large vein of neutronium where Fifth recreated Carter in humanoid Replicator form, released the actual Carter, and took off into the unknown with a new ship.
The Replicator Carter later turned on Fifth and destroyed him with the Ancient disruptor technology. She then led the Replicators in an all-out invasion of the Milky Way Galaxy, and brought the Goa'uld empire to its knees before being destroyed -- along with all known Replicators -- by the Ancient weapon on the planet Dakara.
The Pegasus Galaxy Replicators turned against the Atlantis expedition from Earth after learning that the ancient home of their makers had survived the war with the Wraith. But Dr. Rodney McKay managed to reactivate the Asurans original base code and send them to war with the Wraith. Following a protracted conflict that raged across the galaxy, a coalition of forces joined Atlantis in attacking the Replicator homeworld of Asuras and wiping out the species ... or so they believed.
Unnatural Selection - At the request of Thor, SG-1 enters a time-dilated field to battle humanoid Replicators until they can isolate them with a corrected time dilation device.
New Order, Part 1 - SG-1 watches in horror as the Asgard's latest attempt to destroy the humanoid replicators by collapsing Halla's star fails; Fifth pilots a ship away and captures Major Carter.
New Order, Part 2 - After repeatedly torturing Carter, fifth enters her mind in the form of Pete Shanahan, but fails in creating a convincing facade; He eventually creates a duplicate of Carter and takes off into deep space.
Gemini - Replicator Carter tricks the S.G.C. into letting her study the Ancient disruptor technology, using it to destroy Fifth and take control of the Replicator army.
Reckoning, Part 1 - Replicator Carter brings her army to the Milky Way Galaxy and begins to systematically attack the Goa'uld.
Reckoning, Part 2 - With defeat at the hands of the Replicators imminent, Sam and Jacob Carter activate the Ancient weapon on Dakara and destroy all Replicators in the galaxy, including Carter's doppelganger.
Progeny - John Sheppard and his team discover the Asurans -- a 10,000-year-old civilization of humanoid Replicators, created by the Ancients, living in the Pegasus Galaxy.
Be All My Sins Remember'd - At the climax of a war with Atlantis and the Wraith, Asuras is destroyed and the artificial species wiped out -- with one ship of survivors secretly escaping.
Ghost In the Machine - The last surviving humanoid Replicators have their minds reconstituted in technologically limited bodies, and are tricked into walking through a space gate.