Ancient Domes Reveal 3.45-billion-year-old Life History
Stromatolites are very cool. They’re basically mounds of cyanobacteria (aka blue-green algae), built up over a long period of time like a microbial layer cake. They start out as a layer of happy little bacteria hanging out in warm, shallow water. The bacteria have wild parties and eventually build up a dense mat (you know, those kinds of parties) that sediment sticks to. Well, once sediment has started building up, the bacteria have to get back on top so they can get to the sunlight. Rinse and repeat. Over hundreds or thousands of years, a stromatolite can build up into an impressive hump rising from the sea floor. They stick out of the water entirely during low tide.
We have stromatolites building up even today, though they’re not quite as common as they used to be. They’re mostly found in warm, shallow waters, such as those off the coast of the Bahamas – modern carbonate platforms. There hasn’t really been a time in Earth’s history since life appeared that we haven’t had at least a few of these shallow marine platforms, though during some periods of history they were far more common than in others.
As fossils, stromatolites are very distinctive. Relatively young (young being in geologic age, here) have a lot of organic material left over in them, which creates the dark bands. Much older stromatolites, like the ones referred to in the article, will have had the organic material “cooked” out of them by depth or perhaps some other mechanism of metamorphism. The internal structure of the fossil remains, however, even if the material has been chemically altered.
Dr. Allwood has a fascinating point when she mentions stromatolites in connection with mars. If there was life on Mars at any point, bacteria is definitely the place to start looking, and stromatolites are some of the most ancient evidence we have for the existence of bacteria on our own planet. It’s currently impossible to scope out single bacteria with a rover, but evidence of entire colonies of them? I could definitely see that happening. Our stromatolites can get impressively big, as seen in the shark bay picture. There aren’t that many things that would cause internal structure like you see from stromatolites, though there are inorganic processes that can. So finding a bump on Mars and cutting it open to reveal a stromatolite-like structure wouldn’t be definitive proof without further analysis, but it would still be pretty exciting.