The suggestion that artifacts from another intelligent species may be lying around the solar system is an old one, Wright said, first considered in the literature in the the 1890s. “Once it felt like we had good maps of everything, once we went to Mars and mapped mars and mapped the moons of Jupiter, it all became a lot less unfamiliar,” Wright said. It makes sense that astronomers now look elsewhere, studying the subsurface oceans of Europa and Enceladus and listening for radio pings around stars light-years away. But the existence of technosignatures from an ancient species somewhere in time, Wright said, remains plausible.

Wright suggests a few places to look, and the first is pretty surprising: Earth. He suggests there could have been an explosion in life around the time of or after the Cambrian period, when a sudden wave of complex animals appeared, according to fossil records. A cosmic catastrophe may have destroyed this early species, erasing all signs it ever existed and “forcing the biosphere to ‘start over’ with the few single-celled species that survived,” Wright writes. We may have already seen technosignatures in geological record, but mistaken them for natural phenomena, Wright said. Or, the evidence may be long gone, erased from the surface by shifting tectonic plates.

“The Earth is quite efficient, on cosmic timescales, at destroying evidence of technology on its surface,” he writes in the paper.

There’s no evidence another intelligent species with technological capabilities has ever existed on Earth. At least one paleontologist has already shut down his idea, Wright said. But Wright “correctly points out that there has existed ample opportunity for this to have occurred,” says Andrew Siemion, the director of Berkeley’s SETI Research Center. Earth is the only place known to host intelligent life, which makes it a prime target for this kind of search. Life, after all, develops on planets with suitable environmental conditions, and Earth has provided just that. The same processes that give rise to tiny, single-cell bacteria are the same ones that eventually lead to the evolution of intelligent beings. It wouldn’t take long for an intelligent species to leave its mark on a planet, in much the same way humans have. “As we improve our understanding of ancient Earth and the history of our solar system, perhaps we may someday uncover evidence that suggests the activity of another technological civilization right here in our neighborhood,” Siemion said.

Other targets include Mars and Venus, but not as they are now. Intelligent species could have arisen when water flowed on the Mars, or before warming temperatures boiled away any possible oceans on Venus. The Red Planet in particular may be well mapped by orbiters and rovers, but technological artifacts could be buried underneath its surface, Wright said. “For all we know, maybe Venus had cities all over it a billion years ago and now they’re gone,” he said.