What is the past civilization
Were we the first industrial civilization on the planet?
The Silurians are a species of lizard-like creatures from the British cult TV series "Dr. Who", who, according to the screenwriters, achieved extensive industrial expertise around 450 million years ago - much earlier than humanity had evolved on the planet.
The Silurians are an invention. But the question of whether there were advanced prehistoric societies on earth remains an interesting one. Because: It is unclear whether traces of such an industrial society would have been preserved at all. So how could they possibly be proven?
The Silurian Hypothesis
Gavin Schmidt from the NASA Goddard Institute of Space Studies in New York and his colleague Adam Frank from the University of Rochester have now tried to find an answer to this question. In a new paper they appropriately refer to the idea that there could have been an industrial civilization before us as the "Silurian hypothesis".
In their work, Schmidt and Frank first investigated what traces our own society would leave behind, which would be preserved in millions of years. Their conclusion: The influence of an industrial civilization on the planet can be felt, but at least in part it may be difficult to distinguish from various other events in the geological history of the earth.
The study could have some interesting implications for measuring the impact of human activity on the planet - and helping astrobiologists detect life elsewhere in the universe.
Man is the blink of an eye in the history of the earth
Schmidt and Frank first began by investigating how much we actually know about early Earth. The oldest part of the earth's surface is said to be the Negev desert in southern Israel, it is 1.8 million years old. Older surface structures only exist in exposed areas of the planet or where mineral resources have been searched for. Because of these limitations, research assumes that we only have evidence of the activities of Homo sapiens for around 2.5 million years - little in the planet's geological history.
The sea floor is also relatively young because the oceanic crust is constantly renewing itself. As a result, the bottom sediment of the oceans is nowhere older than the Jurassic Age and therefore less than 170 million years old. In addition, only a fraction of the prehistoric life on the planet is in fossil form. Dinosaurs are said to have lived a good 180 million years and there are only a few thousand almost complete specimens. Modern man in the form of Homo Sapiens, on the other hand, is said to have emerged at the earliest 300,000 years ago. "Such short-lived species might not even be represented in existing fossil history," said Schmidt and Frank.
Streets, buildings and garbage are disappearing
And what about other human artifacts, such as streets, buildings, food cans, or silicon chips? They would hardly survive long either - or, if they were preserved, they would ever be found. "Currently the area of urbanized life on the planet is only one percent."
"We conclude that for potential civilizations older than approximately four million years, there is little direct evidence of their existence to be found from surviving objects or fossils," said Schmidt and Frank.
However, there is another methodology: the chemical footprint. The researchers were interested in industrial societies that can be defined by the fact that they succeed in extracting energy from their environment. According to this definition, mankind has been an industrial society for around 300 years. "Since the mid-18th century, humans have generated over 0.5 trillion tons of carbon from combustion through coal, oil and natural gas."
That has a visible impact on the planet. Since the carbon was originally biological, it contains less carbon-13 than the much larger pool of inorganic carbon. Correspondingly, there was a change in the ratio of C-13 and C-12 due to the combustion processes, which in turn should be proven in geo-history.
The temperature increase due to the carbon release is said to be around 1 degree Celsius. This should also have a visible signature: the isotopic proportion of oxygen-18 in carbonates changes. Agriculture and nitrogen processes in fertilizers also alter the isotopic signature of nitrogen. Furthermore, there are changes due to earth erosion, rain and other elements of climate change, which should also be detectable in the marine sediment. Metals such as lead, chromium, rhenium, platinum and gold are easier to detect due to mining activities - for example in sea water.
Human civilizations are also changing the existing fossils, such as the extinction of large mammals. Furthermore, there could also be a nuclear signature, which speaks for example of an earlier nuclear war. Geologically, however, according to Schmidt and Frank, this would hardly be noticed because the half-life of most elements on this time axis is too short - with the exception of plutonium-244 (80.8 million years) and curium-247 (15 million years).
Isotopes remain from the nuclear war
Schmidt and Frank conclude from all these results that the existence of an industrial civilization before ours must be geologically verifiable. However, this signature is ambiguous because a number of other events timed similar images. This includes, for example, a rapid change in the proportion of CO2 in the air due to events such as large volcanic eruptions or the Paleocene / Eocene temperature maximum, at which the temperature rose by 5 to 7 degrees for 200,000 years. Nobody knows how the latter came about.
But what about the Silurian hypothesis? Schmidt and Frank don't want to lean too far out of the window. They cannot be considered likely just because there is no other valid idea. The researchers recommend "further synthesis and investigation" in the area of industrial by-products, for example, which could be preserved over millions of years. "Are there other classes of compounds that leave unique traces in the geochemistry of sediments over millions of years?"
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