And that's one of things that makes Mars fascinating. There aren't any other good examples of a solar system body where:

1) Conditions in the past plausibly would have allowed, even if only marginally, Earth-like life to exist
2) Geologic deposits from that era have been preserved and are accessible

Note the "plausible" and "marginal" in #1. I don't consider it certain at all that Mars was a nice place to live. But at this point the upper bound on the habitability of periods during the Noachian is not conclusively below than the lower bound of what Earth-like microorganisms could plausibly tolerate.

Unless we find 4 billion year old, unmetamorphised ocean sediments up in the Venusian tesserae -- WAY harder to identify remotely than Noachian clay -- Mars is it.