It is estimated that anatomically modern humans appeared on Earth some 200,000 to 300,000 years ago, but if we're talking about humanity as an industrial civilization, that's only about two centuries old. With so many unknowns and vast gaps in our history, how far-fetched is it to imagine an advanced species living on this planet millions of years ago? That's the Silurian Hypothesis, proposed by scientists Gavin A. Schmidt of NASA's Goddard Institute and Adam Frank of the University of Rochester.
Earth has an average age of 4.5 billion years. For part of that time, it wasn't compatible with life, and even the oxygen we depend on today once functioned as a global killer. From a certain viewpoint, human presence on Earth is less than a blink.
The idea that we aren't the only intelligent beings in the universe is very popular... but what if we weren't the only ones on the planet? Of course, we share the Earth with a huge number of species, but I'm referring to an entire advanced civilization, with broad industrial and technological capacity, living just as we do now, millions of years in the past.
The Silurian Hypothesis
That doubt brings us to the so-called Silurian Hypothesis, a reference to the Silurians from Doctor Who (and not the Silurian period). Scientists Gavin A. Schmidt of NASA's Goddard Institute and Adam Frank of the University of Rochester basically ask, through a 28-page study, what kind of traces a civilization with those characteristics would leave, and whether they would be detectable today.
The study explores the relevance of these questions in light of the famous Drake Equation, and regarding the rest of the planets in the Solar System (it's believed that Venus and Mars were more habitable before), but it also highlights the most important barriers to answering them, starting with the poor geological record beyond the Quaternary period.
The oldest surface ever found is in the Negev desert, and it's only 1.8 million years old. In other words, anything older than about 2.5 million years is probably dust.
Another problem is fossils. Schmidt points out that fossilization is extremely rare and partial (very low in tropical environments, more abundant in arid zones), so there's a great chance of losing even an industrial civilization of 100,000 years along the way, which is 500 times longer than ours.
In the case of artificial objects, the situation is just as bad. Despite our intervention on a global scale, urbanization of the Earth's surface is less than 1 percent, and unless there's relatively active maintenance, nature tends to reclaim abandoned spaces within decades (Pripyat is a good example). The chances of finding direct physical evidence are negligible.
Fortunately, not all is lost. Schmidt and Frank propose focusing on the chemical traces that an advanced industrial civilization might leave behind, and for that there's no better guide than us. Our human footprint will be detectable millions of years from now. Since the mid-18th century, we've released 500 billion tons of fossil fuels by burning oil, coal, and natural gas.
The increase in nitrogen presence in certain sediments (thanks to fertilizers used in agriculture) is very significant. The same goes for the so-called rare earths, increasingly used in manufacturing devices and high-tech in general. And of course, we shouldn't forget plastic, with its very high potential for persistence and detection on the ocean floor.
In summary, if an ancient industrial civilization did things in a more or less similar way to humanity, any trace of its existence might be found in its chemical impact on the planet, and not in fossils or ruins.
Building a civilization depends on a huge number of factors, but one of the most critical is the extraction and consumption of energy. Over time, that civilization must adopt more sustainable and efficient practices to survive, which leads to a reduction of that impact, but it doesn't completely disappear.
And as a finishing touch, what better way to end than with an excellent Kurzgesagt video that explores, precisely, the possibility of a civilization that preceded ours. Enjoy!
Access the study (PDF): Silurian Hypothesis paper
