I have said this before, but since fetal cells can linger in mothers for a long time, while maternal cells can be transmitted to offspring, there is a niche for a kind of cell that is transmitted indefinitely through the female line, and that would almost certainly have beneficial effects, at least on females. This might show up as a growing mtdna haplotype.
It’s a niche, and niches end up inhabited – like the cell line infections in marine filter feeders, which phenom was predicted well in advance of detection.
If you found something like this it would make your career. Especially if it were found in humans.