Leah's origin is a mystery; she is an Early Human found in Gaea's Navel by Serge, and states that she faced "land-anger" before finding herself in that place. She notes that she came from a village. After the Time Devourer is defeated, she states that she will return home and name her daughter Ayla someday. What is Leah's origin?
Reptite Dimension Key
Perhaps Leah was removed from the Reptite Dimension to prevent Ayla from coming into existence, thereby perhaps preventing the destruction of the Reptites through her powerful leadership. The planet would potentially do this in contemplation of Project Kid (similar to the removal of Dinopolis). By allowing the Reptites to evolve and grow to Dragonians, the planet effects the creation of Dinopolis and the Dragon God, which are integral components in Project Kid and the Chrono Cross's creation. The Chrono Cross, in turn, ultimately saves the planet, which thematically puts the planet on the same level as species and animals fighting to survive in the contest of evolution alluded to by Chronopolis and Schala. Whether Lavos fell or not in the Reptite dimension may be immaterial. Since Reptites did exist outside the Tyrano Lair, it is possible that Ayla was required to fend off the rest for search of shelter after Lavos fell. On the other hand, if Lavos did not fall in the Dragonian dimension, removing Leah (and thus her daughter Ayla) would deprive the humans of their strongest warrior and perhaps only salvation. The Reptites easily win.
But there's a big problem with this idea. Gaea's Navel existed in El Nido as early as in the Post-Time Crash era (Polis War circa 12,000 B.C.), yet Leah seems to have appeared only some time before Serge meets her in 1020 A.D.! The Planet would have teleported a patch of land containing Leah from another dimension, but would then have proceeded to a second warp: time-warping Leah from 12,000 B.C. to Serge's era, apparently only in Home World (Leah being 6 years old, she couldn't have been time-warped to a year before the 1010 A.D. dimensional split). This makes the Planet appear really caring and friendly to Serge's cause... But this also begs the question of why she would remove a whole patch of land from another dimension and effect two space-time warps, instead of removing just Leah and effect a unique warp.
Mystic Mts Gate (Really Shoddy Theory)
Leah might have slipped into a strange Gate or temporal distortion in the years before 65000000 B.C. The veracity of her status as Ayla's mother is debatable, but it is possible that Mystic Mts of prehistory was temporally active (Kino also strangely appeared there one day; in his case, he could have simply fallen into some kind of distortion in another period of prehistory). The Gate took her to Gaea's Navel (which existed even before Chronopolis terraformed El Nido, which provides for her ability to return and mother Ayla before El Nido would have been created). After Chrono Cross, she presumably returned to her home time, but this would require a Gate on Gaea's Navel, a claim which cannot be verified.
The only problem is that if Leah were to travel to the future, Ayla would never be born and the world's history could change drastically. However, since Leah persumably went back and birthed Ayla long before the events of Chrono Trigger or Chrono Cross, she would be protected by Time Traveler's Immunity. Regardless, it is a stretch, as this bout of time traveling would have had to predate the entire Lavos / Entity manipulation of time. Even worse is that there is no basis for temporal activity, which renders this theory unlikely.
From: Theory (People)