https://phys.org/news/2018-06-life-marsnew-chimneys-key-success.html

The Curiosity rover looks for certain sedimentary rocks deposited near water, as it did for the latest discovery. This is based on the latest geological advice about the best prospects. Yet which rocks to prioritise is still a matter of some debate – and it's a question that is just as relevant to geologists trying to unlock the secrets of our own ancient world. The Earth's rocks and fossils are the nearest thing we have to time machines.

For a century or so, geologists focused on a type of rock called a stromatolite – devoting long hours to crawling around in awkward spaces trying to find them. Stromatolites occur mainly in shallow water and are layered on a millimetre scale. Many of them are undoubtedly built by slimy microbial "biofilms", but to cut a long story short we now appreciate there is more than one way to make a stripy rock – and they don't all involve microbes.