OK, I'll try to explain what Hubbard's theory was, but it can be confusing, even to Scientologists.
When Hubbard wrote Dianetics: The Modern Science of Mental Health, and other early Dianetic books, he did not believe in nor espouse any theory of past lives.
He thought we only lived once.
Part of the Dianetic theory is that a person's mental or physical problems are caused by "chains" of related, harmful events. If a "preclear" (person getting Dianetic therapy) has, let's say, trouble hearing, it will be found, according to Hubbard, that there is a chain of harmful incidents related to his ears or hearing. As part of Hubbard's theory, this "chain" is held in place by one "basic" incident: the earliest incident related to ears/hearing. Once that basic incident was relieved, the person's hearing would be recovered.
I'm not saying this is true, just that's the theory. I bring up the theory because of this: As part of Dianetic therapy, the auditor keeps asking for an "earlier, similar incident" until they get to this basic incident.
With the idea that we only live once, the search for "earlier, similar" would often end up with "memories" from the womb. There is quite a bit from Hubbard about prenatal memories of harmful incidents.
And then a number of preclears, upon being asked for an "earlier, similar incident" started "recovering memories" from before this life.
Initially, Hubbard's reaction to this was to posit a "genetic memory". He decided that an entity he called a "genetic entity" (or "GE"), was remembering evolutionary information. Hubbard even wrote a book, A History of Man, about this. In that book Hubbard claimed that this genetic memory not only went back through evolutionary life forms to the earliest amoeba, but even further back to the earliest formation of atoms.
This book, The History of Man, is where the derogatory term "clam" for Scientologist comes from, although, technically, this is not part of Scientology and is not "whole track" (a thetan's complete life-after-life memory) or "past lives".
All this didn't last long. Rather quickly, Hubbard changed his mind and started recognizing and talking about past lives, not "genetic memory" but actual memories from previous incarnations. This was not a popular decision amongst many Dianeticists, who broke with Hubbard at that time.
But that was OK with Hubbard. He had discovered the "thetan", the soul, the spirit, the "being himself, not his body or his mind" -- and Scientology was born.
Hubbard's explanation of the life-death-birth cycle goes roughly like this: Everyone is a thetan. Scientologist or wog, everyone is a thetan, inhabiting a body. As part of arriving on this planet, almost all the people here (the thetans), have been programmed to "report to the implant station" when they die.
When and where did this programming occur? Primarily, that was the OT III "Xenu" incident, 75 million years ago.
So, when anyone dies, they immediately "report to the implant station" for a refresh of their programming.
Hubbard claimed that there were implant stations on the far side of the moon and on Mars. I don't recall him ever saying Venus had an implant station (just trains, apparently).
The implant stations are all automatic, no people there. The thetans receive a refresh of their programming ("implanting") and then are given orders to "go pick up a new body on Earth".
So the thetan just goes to Earth and picks up a body.
Exactly when they inhabit the baby body is not set. They might inhabit the body before or after birth, but they will hover nearby in any case. Thetans will fight each other for a body, since there are more thetans than bodies.
Some thetans will take an adult body that is in a coma or has been seriously injured which, Hubbard said, explained amnesia and drastic personality changes at those times.
That's the dogma, according to Hubbard, as near as I can recall. I may have missed a few minor details.
By the way, Hubbard got quite upset if you called previous lives "reincarnation" since, in his version, the birth-death-rebirth cycle is not tied to spiritual progress towards Nirvana. In his version, it's all a horrible trap that leads, in a dwindling spiral, down to total degradation. It is a Bad Thing that "Scientology can help you with".
In Scientology's system of belief, people's past lives can contain, literally, anything, including scientifically impossible things. No "memory" is doubted, all is accepted no matter what. Some Scientologists are quite enamored with who they were and what they did in all these past lives.
Hubbard, of course, more than anyone else. For a pulp science fiction and adventure writer, it was perfect. Here comes the space opera! For many, many years, Hubbard loved to tell audiences his "whole track" experiences -- usually space opera and usually with himself as the brave, wise, powerful hero. And his yarns told in private could, apparently, be even wilder.
Unfortunately, Hubbard forgot to take scientific progress into account. He made up stuff that he thought could never be disproven. But science does progress and science could discover what was previously unknowable.
I'm sure there are hundreds of statements we could mention, but let's limit ourselves to a few well-known claims.
- Hubbard claimed that the physical universe was over "four quadrillion years old". Scientists have estimated the actual age of the universe to be closer to 12 billion years old.
- Hubbard declared the location of the OT III "Xenu" events "75 million years ago" to be the current major volcanoes of Earth. He very specifically named them. The problem is that geologists all know that those volcanoes didn't even exist that long ago.
- Most, if not all, ancient Earth civilizations used super-advanced technology.
- The ancient gods and goddesses were real. They were actually "OTs" visiting Earth -- but have since been "trapped and degraded".
If anyone else is seriously wondering if Hubbard's version of the whole track is true, you really need to stay far, far away from Scientology -- you are just the kind of gullible person they are looking for.