Okay, so we know Eukaryote-synthesis happened multiple times, because both Mitochondria and chloroplasts were independent simple cells as one point.
So maybe in the early says of Eukaryote-synthesis when the earth was being poisoned by oxygen and snowball earth was happening the selective pressures caused dozens of Eukaryote like cells to evolve, and for even a tiny span of time maybe 30,000 years or less or even 100,000 years privative tiny multicellular formed with endless forms and systems that would have looked like dozens of alien linages unrecognizable, and that by the time of the macro life explosion of the Cambrian only our linage survived.
Untraceable but the most biodiversity in the planets history which is why the current domain with Plants Animals and Fungi are so successful at avoiding total annellation.