Years ago. Evolution of life on Earth. Homo habilis in Africa , using stone tools for cleaving meat from bone. Homo antecessor in western Europe Atapuerca , Spain , closely related to the last common ancestor of Neanderthals, Denisovans and modern humans.
Homo sapiens enter Eurasia Greece : first of multiple dispersals out of Africa by humans with early modern traits, including globular braincase and descended larynx facilitating spoken language. Hunter-gatherer nomads. We never got back to aliens.
But once you roll the clock back to tens of millions or hundreds of millions of years, things get more complicated. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert.
Go back much further than the Quaternary, and everything has been turned over and crushed to dust. That means the question shifts to other species, which is why Gavin called the idea the Silurian hypothesis, after an old Doctor Who episode with intelligent reptiles. So could researchers find clear evidence that an ancient species built a relatively short-lived industrial civilization long before our own?
Perhaps, for example, some early mammal rose briefly to civilization building during the Paleocene epoch, about 60 million years ago. There are fossils, of course.
But the fraction of life that gets fossilized is always minuscule and varies a lot depending on time and habitat. It would be easy, therefore, to miss an industrial civilization that lasted only , years—which would be times longer than our industrial civilization has made it so far. Given that all direct evidence would be long gone after many millions of years, what kinds of evidence might then still exist? Future researchers should see this in characteristics of nitrogen showing up in sediments from our era.
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Did those fumes originate in the engines of primeval jalopies? The carbon surge of that period was far more gradual than the one that began with our Industrial Revolution. The same is true of other comparable events in the distant past; geologists have yet to find anything as abrupt as the Anthropocene. That said, the brevity may be the problem — it can be incredibly difficult to make out short intervals in the rock record, as well as at the astronomical level.
Which brings us to the Fermi paradox. One solution is that plenty of civilizations have arisen, but they fizzle out so quickly that few exist at any given moment.
Time, like space, is enormous, and humans may not overlap with many other extraterrestrial world-builders, reducing our chance of discovering any. That said, Frank is skeptical that a technological species could ever become undetectable — subtle, certainly, but not invisible.
To build solar panels, you need raw materials; to acquire those materials, you need some other form of energy. As for wind power, recent research suggests that even if we raised enough turbines to power the planet, they too would contribute to short-term warming.
Since publishing the Silurian hypothesis, the authors have predictably attracted as many eccentrics as academics. Both Schmidt and Frank realize the prospect of earlier earthlings is a seductive one. But regardless of who latches onto their hypothesis, they still see meaningful scientific lessons in their research. For one, they hope it will inspire geologists looking in and astrobiologists looking out to hone their methods of detection.
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