What is most ancestral: isogamy or anisogamy?
As stated here the first sexual reproduction seems to be isogamy, which's purpose was likely to repair the DNA, as a damaged cell could repair its' (DNA) strands via recombination with a "healthy" one. 

As for lineages, the only thing I could find was also in this book at the very beginning, but there doesn't seem to be a difference as for which is the most ancestral.