Humanity has always yearned to uncover the mysteries of the past, and advanced civilizations that preceded us continue to be a topic of great interest. However, clues about them seem to be limited.
At the beginning of this century, climate scientist Gavin Schmidt and others became fascinated by a period in geological history that occurred 56 million years ago: The Paleocene-Eocene Thermal Maximum, also known as “Eocene Thermal Maximum 1.”
What intrigued them the most were the similarities to our current era: rising carbon concentrations, soaring temperatures, and disrupted ecosystems. At specialized workshops, experts have attempted to speculate on what natural processes could have caused such severe global warming.
Gavin A. Schmidt is a climate scientist, climate modeler, and director of NASA’s Goddard Institute for Space Studies in New York, as well as co-founder of the award-winning climate science blog RealClimate. (Image: NASA).
At one such event, Schmidt, currently the director of NASA’s Goddard Institute for Space Studies, pondered the issue deeply. If modern climate change—certainly a product of human industrial activity—occurred, what could have led to climate change during the Paleocene-Eocene Thermal Maximum? Accordingly, Schmidt suggested: “Wouldn’t it be interesting if the causes of climate change in both eras were similar?”
One day in 2017, Schmidt hosted astrophysicist Adam Frank from the University of Rochester. Together, they sought to understand whether civilizations on other planets might alter their climates in the same way we do.
Schmidt then startled Frank by proposing an unusual idea he had been contemplating for years: “What makes you think we are the first civilization on this planet?”
All Human Creations Will Disappear
A common thread among most human creations is that, geologically speaking, they will disappear over time. Pyramids, pathways, temples, and other structures will gradually erode and quickly become buried, turning to dust beneath shifting tectonic plates. The Negev Desert in southern Israel is the oldest vast area on Earth, traceable back to 1.8 million years ago. Once we disappear, it won’t be long before the Earth erases the traces left by human civilization from its surface. The fossil record is so fragmented that short-lived species like ours (which have existed on Earth for a brief period, at least until now) may never find a place within it.
So how might observers in the distant future know that we once existed? If direct evidence of our existence is forgotten, could there still be anything hinting at us? What if we are present-day observers overlooking some prehistoric ancestors who ruled the world long ago?
All human creations will disappear over time. (Illustration: Zhihu).
Advanced Civilizations Have Limited Lifespans
For example, if we compress Earth’s entire history into a single day: complex life emerged about 3 hours ago; the industrial age lasted just a few milliseconds. Some researchers believe the average lifespan of advanced civilizations might only extend for a few centuries. If this is true, then any industrial period could be hidden within the past few hundred million years.
In the months following that conversation, Frank and Schmidt seemed to arrive at what could be the first comprehensive academic answer to the question of the existence of civilizations on Earth before humans. Even science fiction has largely overlooked this idea.
However, in the 1970s, an intelligent reptile first appeared in the British science fiction series “Doctor Who,” awakened by nuclear tests after hibernating for 400 million years. To pay homage to these fictional predecessors, the two scientists named their thought experiment the “Silurian Hypothesis.”
Intelligent reptiles in the science fiction series. (Illustration: Zhihu).
Although no concrete evidence has yet been found for this issue, Frank argues that “this question is important and deserves to be answered delicately,” rather than being dismissed outright.
The seemingly indelible mark we leave on this planet will one day be reduced to a thin layer of rock, composed of various materials we used to build our world. Together, they will form our “technological signature,” a unique imprint accompanied by each technology type. For instance, sediments from the current geological epoch – the Anthropocene – may contain unusual amounts of nitrogen and rare earth elements, with the former from fertilizers and the latter from electronics. More notably, these sediments could contain substances that do not occur naturally, such as chlorofluorocarbons, plastics, and synthetic steroids.
Even aliens cannot violate the fundamental laws of physics in the universe. (Illustration: Zhihu)
Of course, there is no reason to think that all civilizations must behave in the same way, but they must share certain universal characteristics. Schmidt points out that even aliens cannot violate the fundamental laws of physics in the universe.
We humans conquer the Earth through combustion. Therefore, it seems reasonable to conclude that dominant life forms everywhere do the same. For a long time, we have built industries based on fossil fuels and ignored climate impacts, leaving behind a geological footprint. Carbon comes in many types, known as isotopes. By burning long-dead biological material, we alter the ratio of carbon isotopes in the atmosphere, a change known as the Seuss effect. Scientists have noted that these ratios are similar in events like the Paleocene-Eocene Thermal Maximum. If someone were to look back 50 million years from now, it would not be difficult for them to see similar ratios in the Anthropocene Epoch.
Humans may not coincide in time with many extraterrestrial civilizations. (Illustration: Zhihu).
The Fermi Paradox Stimulates Thought
So what happened during the Paleocene-Eocene Thermal Maximum? Did those plumes originate from the engines of early transportation? The increase in carbon during that time occurred much more gradually than the period starting from our Industrial Revolution. The same holds true for similar events in distant history.
This suggests that determining short timeframes in the rock record, as well as at the astronomical level, can be extremely challenging. This brings us to the Fermi Paradox.
If the universe is vast and many habitable planets exist, why have we not found any signs of intelligent life? This is what puzzled Italian physicist Enrico Fermi. The answer is that many civilizations exist, but they end so quickly that hardly any civilization is alive at any given time. Just like space, time is infinite. Humans may not coincide in time with many extraterrestrial species, which reduces the likelihood of us discovering them. There is a more optimistic conjecture: “They may evade our notice not because they have disappeared but because they have mastered sustainable development skills, making their ‘technological characteristics’ less conspicuous.”
Research on “Early Earth Humans” is a captivating prospect. (Illustration: Zhihu).
Since the publication of “The Silurian Hypothesis,” these two scientists have attracted the attention of several scholars and eccentrics. Both Schmidt and Frank recognize that the study of “Early Earth Humans” presents a fascinating perspective.
This hypothesis is also related to the famous Drake Equation. The Drake Equation is used to estimate the number of active civilizations in the Milky Way galaxy. The equation assumes that there is at most one civilization on each habitable planet; enhancing that estimate could completely change the output results or the probability that intelligent neighbors exist in the galaxy.
Both potential answers to the Fermi Paradox (extinction and technological transcendence) could occur. Schmidt suggests: “Whether we will live sustainably or continue to create chaos. The more we flaunt ourselves in the universe, the shorter our existence will be.”
The Silurian Hypothesis is a thought experiment assessing the capability of modern science to detect evidence of advanced civilizations that may have existed millions of years ago. In a 2018 paper, Adam Frank, an astrophysicist at the University of Rochester, and Gavin Schmidt, director of NASA’s Goddard Space Flight Center, imagined an advanced civilization that existed before humans and pondered whether “an industrial civilization could be detected in the geological record?”
According to Frank and Schmidt, since fossils are relatively rare and the least exposed surfaces of Earth are from before the Quaternary period, the chances of finding direct evidence of a civilization, such as technological artifacts, are minimal. After a long period, researchers concluded that we are more likely to find indirect evidence, such as anomalies in chemical composition or isotope ratios in sediment. Objects that could indicate plausible evidence of past civilizations include buried plastic waste and nuclear waste deep underground or at the bottom of the ocean.
Previous civilizations may have ventured into space and left artifacts on other celestial bodies, such as the Moon and Mars. Evidence of artifacts on these two worlds would be easier to find on Earth, where erosion and tectonic activity would erase most of these relics.
