I, somehow (and I do NOT know how) unlearned earlier steps in the technology tree. In this screenshot, pay close attention to the white bar animation for technologies that I can select to research next. I've added red arrows to the earlier technologies that I can now research next, and I've added green arrows to the later technologies that I've already researched. Notice the lack of the animated white bar on Launch Facility and the technology below it. I have several planets with Launch Facilities, already.
No, I didn't skip over intermediate technology. I researched the intermediate technology (the middle red arrow in this case), got the planet, researched later technologies, and then at some later point that "get another planet" technology became available for researching again.
It wasn't the only case in that game, either. Do you see that magnifying glass in the third column? "Get an extra slot on a planet"? In-between Mental Scheduling and Genetic Labs? I researched that twice in total. That is, I researched it once, then researched later technologies, and then the magnifying glass became available again, so I researched it a second time.