I had the privilege, once again, to participate in a unique field trip with fellow scientists and friends to visit a fascinating geologic site in southern Wyoming. To be sure, a geologic field trip is always about ancient time travel, but this was truly remarkable. Our excursion offered us an extremely distant target: 2 billion years in time yet only 2 hours on the highway from Fort Collins. Our destination were the Precambrian-aged stromatolites of the Medicine Bow Mountains which in simple terms are– well– rocks that contain fossil slime. Fossil slime? Yes but really, really old fossil slime that was the dominant life form on our planet for more than half of earth’s entire existence!Rock-bearing stromatolites of this age are uncommon and quite different than typical Precambrian rocks we find in Colorado (and in many other places). These stromatolites are essentially sedimentary in nature as they originally formed at or near the earth’s surface. Over time, they have been deeply buried and ‘squeezed’ into meta-sedimentary rocks and then uplifted -most likely during multiple mountain building events in this area. Finally, in a mere fraction of its 2 billion year old history, Ice Age glaciation (which ended about 10,000 years ago) uncovered the wonderful outcrops we visited today. So now, we have a 2 billion-year old snapshot of Earth Antiquus where nothing more than simple lifeforms occupied the young Earth. A human visitor would be staring at bare rock everywhere amidst seas whose shallow margins supported the growth of thinly layered microbial mats. Over time- perhaps days, weeks or months, the sticky surfaces trapped lime mud into delicate wavy strata resulting in undulating domes and troughs that are the tell-tale signs of once living stromatolites. How do we know this? Modern stromatolites exist today in a few places that offer a glimpse into the mechanics of primitive algae and bacteria that were life’s rulers of Planet Earth for nearly 3 billion years.
So why do we get so excited about 2 billion year-old microbes? It is a window of understanding into the origin of our modern oxygen-rich atmosphere and perhaps– a view of what Extraterrestrial Life may actually look like. After all, astronomers are routinely discovering sister earth “exoplanets” whose billions-year old histories almost certainly contain similar evolutionary story lines. Just maybe…So, we salute these extraordinarily ancient stromatolites as postcards of primitive Earth. Envision countless mats of slime yielding tiny bubbles of oxygen that in turn tempered Earth’s primordial atmosphere into one that could harbor more complex life forms. And maybe, just maybe these most ancient of fossils offer us a clue to our most likely alien encounter from the beyond. A self guided walking tour of the site can be downloaded from the Wyoming State Geologic Survey.