Advanced civilizations should leave a mark after their disappearance. But are we able to detect it after millions of years?
Could we tell if an advanced civilization lived on Earth millions of years ago? - science news - society and civilization

Humans appeared and evolved into the current shape in the last 2.5 million years ago. However, our planet is 4.5 billion years old. The first life appeared 3.8 billion years ago, so in theory, there was plenty of time for advanced species to evolve, develop a civilization, disappear and be replaced by others. This is the scenario that researchers imagined, in an effort to understand if we would be able to trace down the existence of a previous civilization.

Two scientists, Gavin Schmidt, the director of NASA’s Goddard Institute for Space Studies and Adam Frank, professor of physics and astronomy at the University of Rochester, explored the possibility to find traces left behind by a past civilization, that existed millions of years ago. The results of their investigation have been published in the International Journal of Astrobiology and it is probably the first time this subject is seriously and scientifically approached.

The Silurian hypothesis

The researchers hypothesized that the existence of a past industrial civilization is possible, and named this the “Silurian hypothesis”, as a reminder of the advanced reptiles from the Dr. Who TV show. The Silurians are a prehistoric race of advanced reptiles that appears in the above-named science fiction TV show. But, could they be for real? Could it be that another advanced species existed before us? Is the idea of an advanced intelligent species that lived on Earth before humans so farfetched?

Not much time is required for an intelligent species to grow into an industrial civilization. For humans, this happened only in the last 300 years. This is a tiny fraction of the time we existed as a species and an even tinier since life existed on Earth. The question is, if we accept the possibility that another advances civilization could have existed, would we be able to find traces of it after millions of years?

What traces could a past industrial civilization leave behind?

The first thing that comes to mind is the geological records. If we are able to find dinosaur bones in the ground, we could maybe also find traces of a past civilization. For the Quaternary era (the last 2.5 million years ago) we found physical evidence of the existence of non-human species (like the Neanderthals). I am not talking only about fossils, but also changes in climate, soil and archeological evidence that prove they existed. However, older evidence for life is far sparser, existing only in a few places. The proportion of life that was preserved fossilized is extremely small. Only a small number of plants, animals and other life have been fossilized and the rates vary widely with geographical region, time passed and the type of tissue. From all dinosaurs that have ever existed, only a few thousands have been fossilized and discovered as almost intact specimens. And there were thousands of species that inhabited Earth for a very long time! Thus, it is possible that species similar to us, that lived millions of years ago, could be completely absent from the existing fossil records.

Fossil records most likely will not contain proof of a past civilization
Fossil records most likely will not contain proof of a past civilization

How about the objects that a civilization could develop? Could they be preserved and discovered after millions of years? Unfortunately, science suggests that such objects are extremely difficult to find. Think about this: currently only 1% of Earth is covered by urban areas. If this was true for a past civilization, in order to find some objects from them, we should dig in the ground in an urban area from those times. But, if only 1% of Earth contains proof of that civilization, the chances to find this place are extremely small, comparing the size of an archeological or paleontological site with the size of the entire planet. It is enough to think how difficult it is to find artifacts and objects from very recent human cultures, like the ancient Greeks or similar. Complex objects from early human technology are extremely rare.

“Despite impressive recent gains in the ability to detect the wider impacts of civilization on landscapes and ecosystems, we conclude that for potential civilizations older than about 4 million years, the chances of finding direct evidence of their existence via objects or fossilized examples of their population is small”, said the researchers in their paper.

Geological records could hold the key

The good news is that there are other ways to track down the existence of a past industrial civilization. For example, we are leaving a mark on our planet through our industrial activity and this will probably be recorded in the geological sediments. The same could be true for a past advanced species.

Since mid-18th-century humans have released 0.5 trillion tonnes of fossil carbon through the utilization of coal, oil and natural gas. This triggers a complex chain of events that leads to changes in the proportion of carbon isotopes in nature. Moreover, it contributes to global warming and this, in turn, will have an impact on future sediment formation. The nitrogen cycle is also affected by agriculture and other human activities and these anomalies are already detectable in sediments. These are the clues we should look for if we want proof of a past civilization, similar to ours.

Another indication might come from the sedimentological records. Human activities increase sediment flow in rivers and soil erosion. Canalization of rivers has increased the oceanic deposition of sediments. In addition, the composition of sediments is also changed as a consequence of human activities.

A past industrial civilization would most likely have had an impact on the other species living at that time. One could look at the fauna and extinction rates to infer its existence. If we think about us, humans, in the past few centuries we greatly influenced the abundance and distribution of animal species. Some invasive species have exploded, such as rats and mice that accompany human agglomerations. The fossil record could reflect this.

How about science and technology, could it leave a mark? We have developed many non-natural chemicals and materials that are manufactured industrially and used in many daily applications. Some of them can persist for a very long time. Some of these chemicals could leave traces in sediments, atmosphere, and oceans and could potentially be discovered after many thousands of years, maybe even millions. And, let’s not forget about plastic. It is slowly accumulating in sediments at the bottom of the ocean. Although it is not clear for how long plastic could persist in such environment, it is not excluded that this time could be long enough to allow tracing back the existence of a civilization.

Finally, the study suggests we should consider the radioactive elements. Many radioactive isotopes are the result of human intervention. They are created using fission or by nuclear arms and their lives are very long, in the range of millions of years. For example, Plutonium-244 has a half-life of 80.8 million years and Curium-247 has a half-life of 15 million years. Discovering them and calculating when they were produced, could point out the existence of an advanced species capable of producing nuclear isotopes.

Is there proof that an advanced civilization existed before us on Earth?

Analyzing the evidence from the past of our planet reveals some geological changes that have similarities to what we could consider signatures of a past civilization. However, the evidence is not convincing for the moment and in most cases, they can be also attributed to natural causes. About 56 million years ago, there was a rise in global warming, much like the changes induced now by humans. Called the Paleocene - Eocene Thermal Maximum, this event was not alone as during the history of our planet other changes in climate have been recorded. Several mass extinction events have existed, some of them induced by clear causes (like the asteroid that wiped out the dinosaurs), but others more mysterious, accompanied by global warming, wildfires, and deforestation.

The implications of this research study go beyond our planet as we could pose the same question about other planets. Was there intelligent life on Mars in the past? We know that Mars was more hospitable in the past and if there was an advanced civilization there, could we find traces of it? By analyzing this problem, we will know what to look for when we arrive on Mars, or on other planets.

“These questions make us think about the future and the past in a much different way, including how any planetary-scale civilization might rise and fall,” the authors of the study said, according to While it is largely accepted that no advanced civilization existed before us, asking this question has important consequences for both understanding the impact of our current civilization on the planet and for astrobiology.

Original article by ScienceBriefss

Scientific publication: International Journal of Astrobiology

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