Science fiction, rather

As you will shortly see from this Informed Reader blurb (B10; December Scientific American), I used the "scientific method" label quite sarcastically and tongue-in-cheek. "Scientists," not content to believe/know that Earth is the only hospitable planet for life in at least the solar system, are trying to find proof of an "alien encounter" - i.e. "life has emerged twice on this planet." Physicist Paul Davies (AZ State University) says that the "most likely survivors from a first wave of life would be microbes, microscopic organisms like bacteria." Why? They have "no connection" to the currently accepted tree of life (accepted by the shrinking majority, at least) and they probably live in "extreme conditions...such as the bone-dry valleys of Antarctica, California's extremely alkaline Mono Lake or heavily polluted rivers such as Spain's Rio Tinto."

I've got one tiny problem with the last one: if these extreme microbes likely live in rivers that are polluted today, where would they have lived back when the planet was young and water sources were pristine?