I think you're basing this on a false premise. There's no rule that states that every life form that ever existed must have any descendants. In fact the opposite is true, the vast majority of life forms were exctinct.

Famous case in point are the Ediacarean fossils in Newfoundland, the first known multicellular organisms. No descendants live today, they are completely extinct. Without these fossils we wouldn't even be aware of this branch of the evolutionary tree.

Maybe we can't find it because we don't know how it looks like? Maybe it's everywhere? If you dig up some dirt and sequence the genome, you find tons of DNA and RNA but for most we have no idea about what it does. Likely once we find the first pre-cellular life (if it exists), we'll figure out what to look for and then suddenly it'll appear everywhere.