“The First Ones” is episode eight of season four of Stargate SG-1.
Daniel is at an archaeological dig with SG-11 on a planet, which he believes was the world the Goa’uld originated on, digging up remains of primordial Goa’uld, when the team is attacked and he is taken by an Unas – one of the early Goa’uld hosts first seen in “Thor’s Hammer” – although this one looks as if it may not be a host (and the impression seemed to be given in that episode that the Unas was a person, not a race). SG-1 and SG-2 go after him. The world is also still host to Goa’uld symbiotes, adding to the problem.
This episode looks to be the first one in which the SG teams are equipped with P90s. I’d thought they had those from the beginning; I must have got confused with the later episodes I saw several years after the first couple of seasons.