First off, it would be hopeless to find evidence of constructed “buildings” from 65 million years ago. If they liked shiny things as much as humans do, maybe they manufactured jewelry from gold or another noble metal. Perhaps we might still find radioactively enriched areas if they were advanced enough to use nuclear fission. If they got as far as space travel, maybe we could find their abandoned spacecraft on the Moon or Mars, where weathering wouldn’t have erased the evidence.

Obviously, we have found nothing remotely like this, at least not yet. No 65-million-year-old necklaces. There are in fact uranium-enriched regions on Earth — one of which, the Oklo deposit in Gabon, undergoes natural fission — but they don’t fit the timeline.

Still, if we never look for such evidence, or rule out the possibility that it could even exist, we’ll never find it. For now, when we say humans are the only species on our planet that evolved technological intelligence, I maintain that we may want to add the word “probably.”