Re: Show me the trees...
But if Mars died when life was at a primitive stage we might want to look in unobvious places. Examining Hadean and early Archaean rocks here on Earth shows some evidence that isn't the usual fossils like stromatolites. There are some unusual carbon isotope excursions in graphite in the Isua Complex of Greenland (3.8Ga) which *may* be the product of carbon isotope fractionation that occurs in living organisms. And then, a bit later we have the colossal Banded Iron Formations that appear all over the globe as a by-product of primitive life oxidising iron for energy which resulted in changes in the neodymium and europium isotope ratios in oceanic sediments and eventually the release of oxygen in the atmosphere.
So if we want to look for life, we're going to be doing odd things like checking isotope ratios and examining trace metal deposits.
And smashing rocks, lots of smashing rocks.