After the Asgard had trapped the Replicators on the planet Hala in a Time dilation field, the machines eventually found out how to use it, allowing them to speed up the time inside the field. In this time, they studied their Android creator Reese and eventually created the Human-form Replicators, with the first being appropriately named First. He in turn created more Human Forms, with one of the being Sixth.
When SG-1 entered the planet to repair the Time dilation device after being asked by Thor, the first five Human-form Replicators greeted them. After some talk, the Replicators probed the mind of the team to learn more about them. Afterwards SG-1 was "invited" to dinner with the Human Forms, during which they got more information about their creation. When SG-1 worked together with Fifth to reactivate the Time dilation device, Sixth, together with all other Replicators was captured inside it. (SG1: "Unnatural Selection")