In a new paper, Gavin Schmidt of the NASA Goddard Institute for Space Studies and Adam Frank from the University of Rochester ask a provocative question: Could there have been an industrial civilization on Earth millions of years ago? And if so, what evidence of it would we be able to find today?
The authors first considered what signs of industrial civilization would be expected to survive in the geological record. In our own time, these include plastics, synthetic pollutants, increased metal concentrations, and evidence of large-scale energy use, such as carbon-based fossil fuels. Taken together, they mark what some scientists call the Anthropocene era, in which humans are having a significant and measurable impact on our planet.
The authors conclude, however, that it would be very difficult after tens of millions of years to distinguish these industrial byproducts from the natural background. Even plastic, which was previously thought to be quite resistant, can be degraded by enzymes relatively quickly. Only radiation from nuclear power plants—or from a nuclear war—would be discernible in the geological rock record after such a long time.
This thought experiment is not outlandish. In our book The Cosmic Zoo: Complex Life on Many Worlds, William Bains and I devoted a whole section to the question. Think about it: Intelligent dinosaurs such as the troodonts were around in the Cretaceous period, more than 65 million years ago. If it is confirmed that they had bird-type brains—where the neurons are much more compact than in our brains—they would have been quite smart, much smarter than we previously thought. Perhaps some of them build cities, or flew around the world, or pumped oil. Maybe a heroic dinosaur even landed on the moon. Okay, that seems unlikely, because we would have seen the artifacts by now. But even if we rule out space travel, how technologically advanced could such a civilization have become?
That, of course, begs a deeper question. There should have been enough time for dinosaurs to become smart enough to launch spaceships. Why didn’t they? For that matter, why haven’t octopi, dolphins, and some birds, which developed intelligence long ago? Kangaroos, for example, are large, social, have manipulative hands, and are reasonably intelligent. Why did they not advance further? What is so special about us humans?
Speculating about prior civilizations is important not only for Earth, but also for other planets and moons. After all, we might find the remnants of a civilization that does not exist anymore on an exoplanet, if we can recognize the signs. The soon-to-launch TESS mission is expected to discover thousands of new exoplanets, many of which will be Earth-size and relatively close to us (within 50 light years or so). Some of those we may even be able to visit and explore in the not-too-distant future.