Aliens among us?

Aliens among us?

If life emerges from non-life as often as some astrobiologists hope, then maybe it happened more than once on Earth? I have been pondering the idea that alternative life forms might once have inhabited Earth, and may even be around today. Of course, I am referring to mere microbes, but it would be hard to tell from looking whether they were "our" sort of life, of another sort entirely. If there are "aliens" under our very noses, how would we be able to tell? What sort of search strategies might we employ? (See my paper for more about this topic).

1.‘Aliens under our noses,’ Scientific American, 12 December 2007