Have you ever noticed that footprints in the mud end up being erased by the next rain? Yet, somehow there are places in the world where footprints from thousands of years ago are perfectly preserved? In some places in the world, we can walk around preserved footprints, just stroll down the beach and marvel at them. Specifically, in my case, there is a protected area, right at the beach, where we can find footprints as old as 12000 years old. These footprints are close to a tiny summer village called Pehuen Co in Argentina and very close to where Charles Darwin first started pondering about his Theory of Evolution. There are both human and animal footprints in the reservation. The human ones, younger at 6000 years of age, however, are located closer to the more urban end of the area, near the city of Monte Hermoso.
It is funny how the tendency to be more urbanized on that side of the reservation started that long ago. That tendency is so pervasive that there is a distinct difference between the people who go to Monte Hermoso and those who go to Pehuen Co. Those who vacation at Monte Hermoso want to get the beach experience without leaving the comfort of the city. In contrast, the ones who choose Pehuen Co are the opposite: they’d rather forget they ever lived in a city and get lost on the beach. You hardly find anyone who likes both of them. It is either one or the other! I definitely prefer Pehuen Co. Still, no matter which one you like, there is a fantastic advantage to both: from the beach at both sites, you can see both the sunset and the sunrise. No matter if you’re an early riser or a night owl, you’ll get the best views.
Yet, before there was a beach in the south of the Buenos Aires province, the now protected area used to house a small coastal lagoon. Large mammals walked and fed in the area. There were sable-tooth tigers, 3 m-tall bear-like Megatheriums, and weird-looking camel-like animals (which may or may not have had a short elephant- or moose-like trunk) called Macrauchenia, among others.
As the lagoon started to dry, conditions developed for the colonization of the sand, silt, and clay of the sediments. Who colonized them? Different microorganisms, like bacteria, fungi, algae, and even some stranger ones called archaea. When there is enough humidity with prolonged periods of tranquillity, microorganisms which can do photosynthesis start to grow as thin films –creatively named, biofilms–, producing a lot of substances called extracellular polymers (EPS for short). These EPS substances are a bunch of organic molecules that stick the microorganisms and the sediments together. With continued calm conditions, these biofilms can grow and evolve to be up to 1-cm thick, turning into what we know as microbial mats.
By tranquillity, I don’t mean quieting down your neighbours’ heavy metal blasting speakers. I mean hours and maybe even days where no water currents or other events move the sediments.
Microbial mats are named that way because, when a water current comes in, the mats roll themselves as if they were a yoga mat (you know, just like the one you bought and put away in the closet without using). The reason for that is the elasticity that the EPS confers to the sediments, which allows the mat the flexibility it needs not to break.
For the same reason these microbial mats don’t break when they roll up, they can withstand a lot of pressure, like a person standing on them, without breaking. If the pressure is enough, the mat will be deformed, making a cast of the footprint. However, the mats do have a certain point when the pressure is too much, and they break. For the most part, though, if you were to walk over microbial mats, you’d be able to leave your own footprint. My colleagues and I started to study microbial mats because we would walk over a place that looked like mud, but that didn’t always behave like regular mud. Our footprints, and of the local fauna, would still be there months and even years after they had been left.
These microbial mats also encourage the formation of carbon carbonate and other minerals, which, over time, form rocks. So, in summary: first, conditions develop for the growth of microbial mats, then the animals and humans walked over them in the right conditions, and last, time let the mats turn into rock (mineralize). Thus, the main difference between the preserved footprints from 12000 years ago and our more recent ones is that our footprints are still not old enough!
This article is the first in the Mad Mud Love series. Keep tuned to this website or subscribe to my mailing list in order to keep yourself updated. The list is sent on a monthly basis with a digest of new posts and sometimes updates on my fiction writing or new scientific papers published.
Also, you can find the research paper here.