I'm not an Operating Thetan either, nor do I want to be. Fortunately, those of us who aren't permitted to be in the know do have access to some documents, including the Fishman Affidavit. I worked off of the transcript/analysis here.
Quoting an analysis of OT III material, from the Affidavit,
Let's turn to Incident II, which allegedly happened 75 million years ago and occurred on earth:
Incident II is over 36 days long. Capture on other planets was weeks or months before the implant. Those on Teegeeack (Earth) were just blown up except for Loyal officers who were (shortly before the explosion on Earth) rounded up.
L. Ron Hubbard, OT III, p. 79.
So Xenu captured everybody who did not live on earth, held them imprisoned for a few weeks or sometimes even months, and then transported them to earth and mixed them with the rest, that is: the people who did live there. He then proceeded to nuke the lot. Just a few loyal officers escaped this major hydrogen bombing.
Not everyone was killed. That leaves a loophole for the direct survival of some non-thetan beings.
Also important, however, is the relationship between thetans and death. Quoting from the linked article,
Hubbard said that when a person dies, his or her thetan goes to a "landing station" on Venus, where it is programmed with lies about its past life and its next life.
A better explanation of the relationship between thetans and humans is quoted in the OT III analysis:
One's body is a mass of individual thetans stuck to oneself or to the body.
Margery Wakefield, The Road to Xenu, p. 77.
If we go back to Wikipedia,
Eventually, officers of the Galactic Confederacy launched a rebellion against Xenu, which continued six years before capturing him and placing him in an electrified prison in the center of a mountain. Hubbard taught that the thetans brainwashed by Xenu's forces remained on Earth, where the "body thetans", attached to human psyches, contribute to human problems; and that individuals could be freed from these brainwashed thetans and thus attain a type of salvation.
Better text (from the OT III analysis) is
When through with his crime loyal officers (to the people) captured him after six years of battle and put him in an electronic mountain trap where he still is. "They" are gone. The place (Confederation) has since been a desert. The length and brutality of it all was such that this Confederation never recovered. The implant is calculated to kill (by pneumonia etc) anyone who attempts to solve it. This liability has been dispensed with by my tech development.
Margery Wakefield, The Road to Xenu, p. 76.
Note that this mountain is not on Earth, but another planet.
So, there's no explicit description of the "rebirth of humankind" aside from what is given in the Fishman Affidavit.