Was there an intelligent civilization on Earth before us? There almost certainly was not an intelligent civilization on Earth before us. They would have left signs that could not be mistaken. The thought that an ancient civilization might have existed on Earth is called the Silurian hypothesis, named after such a species that exist in the Doctor Who world.
The thought behind this hypothesis is that there is more than enough time for another civilization to have evolved and disappeared. Humans have only been on this planet for 300,000 years, and we have only had technology that could be found after we disappear for the last few centuries. That means humans have only been here for 0.0066% of Earth’s life. If we believe that we can last a million years before we go extinct, that is only 0.022% of the length that the Earth has been here. There is certainly enough time for another civilization to have done the same.
One of the problems with this hypothesis is that it might be quite hard to find physical evidence. We can see fossils of dinosaurs in most science museums, and we might be forgiven for thinking that fossils are very common, but, in fact, they are incredibly rare. It takes an enormous amount of luck for something to become a fossil. It would be very likely that an ancient civilization wouldn’t leave physical fossils. There is also the problem of our tectonic plates. Unlike the moon, the tectonic plates on Earth are constantly shifting and disappearing under each other. Any fossils from a few billion years ago could have been pulled back into the Earth and melted. So, if we were looking for traces of an ancient civilization, what would we expect to see? Well, we can look at our civilization and the things we produce to answer that question.
In a thousand years, or even ten thousand years, there will still be signs of us. Some of our buildings and monuments will survive. The mines we have dug will still be there. The giant underground caverns built to house our spent nuclear fuel will still be there. The faces on Mount Rushmore will likely still be recognizable as faces. If you go to a million years into the future, there will be fewer signs of us left. All of our buildings and structures will have gone. We will have made very few fossils, but there might be a few lying around. Metal things will have gone, but there may be bits of plastic buried in the ground. If you go forward a billion years, almost all traces of us will have gone. The mines we dug, the coastlines we made, even the mountains we reshaped, will all have gone. Yet, there will still be traces of us.
If humans go extinct and a new intelligent species appears in a billion years, they will be able to find possibly four traces of us. The first would be huge gaps in the fossil record that show there was a mass extinction in the past, but no natural disaster to peg it to. That alone doesn’t mean an intelligent species wiped out all of the animals, but, if taken with other information, it is a clue. The second would be faint geometric shapes in the Anthropocene layer of rocks. The Anthropocene is the era we are currently living through, and those faint geometric shapes would have been made by the foundations of our buildings. That is another clue. The third thing that would show we were here would be a layer of unnatural radioactive elements, such as plutonium-239, all over the world at the same depth. As an intelligent species, we are able to alter elements and then scatter them over the world using nuclear bombs and power plant meltdowns. This would not happen naturally, and being all over the world at the same depth would show it all happened at the same time. And lastly would be signs of climate change, which would also be at the same level, on the land and in the seabed. There would be excess carbon, which would acidify the layer of rock. Future civilizations would find these, and they would probably be able to put them together and work out that an intelligent species that could alter the planet had once existed.
This is why it is unlikely that an intelligent species has existed in Earth’s past. If they had, the traces of their buildings and the things they made would have gone, but the effects they had on the planet would still be visible if we dug down far enough. And there are none of those signs. This is also a method astronomers can use to find if life has existed on other planets as well. And this is what I learned today.
Sources
https://www.discovery.com/exploration/Advanced-Civilization-Silurian-Hypothesis
https://en.wikipedia.org/wiki/Silurian_hypothesis
Photo by Boris Ulzibat: https://www.pexels.com/photo/white-concrete-pillars-under-blue-sky-3610377/