Humans have long been fascinated by the possibility of finding technologically advanced beings elsewhere in the Universe, but what if such a civilisation had lived right here on Earth, long before humans evolved?
In 2018, physicist Prof Adam Frank and Dr Gavin Schmidt, a climate modeller, published a paper exploring whether modern scientists would be able to detect evidence of an advanced industrial civilisation that ended millions of years ago.
They named this possibility the ‘Silurian hypothesis’, after an advanced reptilian race from the BBC’s long-running sci-fi show Doctor Who. They concluded that while it's extremely unlikely, evidence for such a civilisation might be hard to find.
Focusing on a period between 400 million and 4 million years ago, the researchers pondered what evidence might be left behind by this hypothetical society.
In just a few hundred years, our industries have made a huge impact on the world’s climate and ecosystems. But if the human race vanished, within a few hundred million years, any direct evidence of our society – or a previous one like it – would almost certainly be gone.
Our biggest cities would disappear in a geological blink of an eye, due to erosion and tectonic movements.
Instead, scientists searching for a long-lost civilisation would need to look for the geological fingerprints of their activities.
Industrialised civilisation would likely have many of the same needs as modern humans, including producing large amounts of energy and food. So, we might expect them to leave similar markers in Earth’s rocks and sediments, such as evidence of massive carbon emissions, global climatic changes and sea-level rise.
The problem is that in the geological record, climate change caused by a fossil-fuel burning civilisation is hard to distinguish from climate change caused by natural forces.
In fact, there's an eerie similarity between modern climate change and events in Earth's history known as 'hyperthermals'. One such even occurred around 55 million years ago, when global temperatures rose by up to 8°C (14.4°F), but hyperthermals tend to coincide with intense tectonic activity.
Another problem: the longer an advanced civilisation lasts, the more evidence it’ll leave behind. But to survive a long time, a civilisation would need to become sustainable, which would leave less of a geological mark.
A society powered by wind and solar energy, for example, wouldn’t leave as much evidence as a fossil-fuel powered one. This paradox explains why we would be unlikely to find any signs of a past civilisation, if one existed.
The Silurian hypothesis pushes us to consider what traces of evidence humanity will leave behind. Answering these questions might help refine our search for advanced civilisations on other planets.
This article is an answer to the question (asked by Joshua Stuckey, Exeter) 'If an advanced civilisation lived on Earth millions of years ago, would we be able to tell?'
To submit your questions, email us at questions@sciencefocus.com, or message our Facebook, X, or Instagram pages (don't forget to include your name and location).
Check out our ultimate fun facts page for more mind-blowing science
Read more:
