You may already have wandered off, irritated or incensed that I put the Fermi paradox on equal footing with the beloved crocodile paradox. Perhaps you’re now thumbing through a dog-eared copy of Chariots of the Gods? or watching YouTube clips of that “alien autopsy” TV special that Fox aired in the 1990s.
Indeed, one possible resolution of the Fermi paradox is that it’s no paradox at all, because ET has already journeyed to Earth. Adherents of this explanation often point to UFO sightings and alien abduction stories, topics that you can read about in chapter 10. For our purposes here, suffice it to say that scientists generally don’t regard any of these reports as convincing evidence of alien life. (If they did, you definitely would have heard about it.)
There are more subtle possibilities in play as well. For example, what if ET came to our planet long ago, before people were around to be probed? Unless the voyaging aliens were particularly interested in us, this is much more likely than a documented visit, given that our species has existed for just the last 200,000 years of Earth’s 4.5-billion-year history and has been capable of capturing encounters on blurry, low-light video for only a few decades.
Let’s indulge in some wild speculation, because it’s fun! Say Earth has been colonized many times over the eons by greedy, grabby alien civilizations, each of which ground the planet’s native species into the dust in the process. (Don’t get too high and mighty: pioneering humans have tended to wreak ecological havoc as we’ve explored the globe.) As astrophysicist and sci-fi author David Brin has pointed out, a history of such oppression could explain why it took intelligent life so long to arise on our planet as well as the radio silence in our galactic neighborhood. Maybe Earth is the only planet for light-years around to have recovered from the ravages of invasion.
If you squint a little, this scenario lines up with the five mass extinctions that scientists have identified in the fossil record. These great purges occurred about 450 million years ago, 375 million years ago, 251 million years ago, 200 million years ago, and, most famously, 66 million years ago, when an asteroid strike wiped out three-quarters of all Earth’s species, including the dinosaurs. “It may not be preposterous,” Brin wrote in a seminal 1983 paper, to compare the intervals between these extinction events and the time it might take for different waves of invasion to wash over Earth. The dino-killing asteroid could even have been a weapon of war, slung by a space-dwelling alien faction with a beef against their brethren on Earth.
Brin didn’t mean to suggest that any of this actually happened, and neither do I. There’s no evidence that it did — no spacecraft entombed in ancient amber, no ruins of a 200-million-year-old city — and I certainly wouldn’t put any money on it. But it’s possible.