Adam Frank said it began with the strangest question he had ever asked someone in an office. In 2017, he went to see NASA climate scientist Gavin Schmidt to try to figure out if alien civilizations would change the climate of their planets in the same way that humans change ours. He said that he was looking at other worlds because, well, there couldn’t have been any other civilizations on Earth. Schmidt stopped him almost right away. He asked, “How do you know that?”
The Silurian hypothesis, which was based on that one question, is one of the more unsettling scientific thought experiments of the past few years. The idea is named after a made-up race of smart reptiles from the British TV show Doctor Who. It asks if it would be possible to find proof of a complex civilization that existed before humans in the rocks that make up Earth’s crust. In 2018, Frank and Schmidt wrote a formal paper on the topic. Since then, they have both been very clear that they don’t think there was a previous industrial civilization. They just couldn’t show that it didn’t happen.
The problem is time and the damage it does to things. The Earth is about 4.5 billion years old. Life with many parts has been around for about 400 million years. It’s possible that an intelligent species could have come into being during that time, built something that looked like an industrial society, and then disappeared, leaving the planet to slowly destroy what was left behind. The Negev Desert in southern Israel is the oldest large area of open land on Earth. It is thought to be about 1.8 million years old. Tectonic plates move around. Land is resurfaced by volcanoes. Everything gets worn down by erosion. If no one took care of a city, it would be unrecognizable in a few thousand years and pretty much gone in a few million.
So, it’s likely that direct artifacts are not the answer. Already, fossils are hard to find—dinosaurs lived on Earth for 180 million years and only left behind a few thousand nearly complete bodies. A civilization that only existed for 100,000 years and then vanished would be a very small part of geological time that would be easy to miss. More than anything else, Frank and Schmidt were interested in chemical signals, which were not found in objects but in the chemistry of rock layers. The Grand Canyon is a good way to picture this. Each visible stripe in the canyon wall shows a different time period in Earth’s history and stores chemical information about what the oceans and atmosphere were doing at that time. If a civilization in the past used up fossil fuels and released a lot of carbon, like we do, that could show up as a strange spike in the isotope record.
One example that Frank and Schmidt found interesting was the Paleocene-Eocene Thermal Maximum, which happened about 55 million years ago and caused global temperatures to rise sharply. Ecosystems collapsed as large amounts of carbon entered the atmosphere. Its cause is still not completely clear. An eruption of a volcano, the release of methane, or something else. Most likely something natural. But because it was so much like what’s going on now, the scientists wondered if anyone had really looked into these kinds of events. There are also less common chemical signatures to think about. For example, plutonium-244 doesn’t naturally happen on Earth unless there is a supernova. It would be really hard to explain away if it showed up in the rock record in a way that didn’t make sense.
It’s hard not to notice how quickly this way of thinking brings us back to the present. Frank has made it clear that the hypothesis raised a deeper question about the longevity of civilizations like ours, which have figured out how to use energy on a global scale but may not have figured out how to handle the effects. If advanced civilizations tend to make the climate unstable and eventually fall apart, the fact that there is no evidence in Earth’s rocks could mean something specific. Not a friendly one.
Jan Zalasiewicz is a geologist at the University of Leicester and co-author of a new book about what technofossils (like plastics, heavy metals, and artificial hormones) will mean for the geological record. He thinks that Frank and Schmidt didn’t think that a truly industrial civilization would leave behind so much evidence. According to him, it would be easy to see how much trash people are making, the chemicals that don’t break down, and the way land is being changed on a global scale. He doesn’t think there was a civilization like ours in the past—the kind that makes huge amounts of trash, changes the shape of landscapes, and smelts metals on a large scale. He says that the fact that those signatures are missing is meaningful in and of itself.
In fact, that’s what makes it so beautiful. There was never going to be an answer to the Silurian hypothesis. To make it clear what we would even look for. And that brings up a question that is important no matter what the answer is: are we doing an experiment that the planet has already done?
