There's also a chance that past visits to Earth by intelligent aliens left signs much closer to home. But probability and the length of the universe's age suggest that any such alien visit would have taken place before humans ever emerged on Earth, says Paul Davies, astrophysicist with Arizona State University.
That means any traces of an alien visitation would have had to survive for hundreds of millions or billions of years of erosion and plate tectonics for humans to still find them today.
Non-human deposits of nuclear waste consisting of plutonium would point to artificial origins, because natural deposits would have long since decayed, Davies said. Scars of mining or quarrying could remain buried beneath the Earth or on its moon.
Perhaps the most fascinating possibility is if aliens used bioengineering to leave behind unintentional or intentional traces or messages in the DNA of life on Earth. The self-perpetuating nature of life forms could help ensure survival of any such biologically-embedded messages.
Citizen scientists and school students could pitch in to run genomic versions of SETI programs to find any such traces, Davies said. Data-mining software programs could do much of the heavy lifting as just a small part of the usual genomic analyses going on in everyday research.
Alien bioengineering might have also created a "shadow biosphere" of life built upon biochemistry separate from that of Earth life forms. Examples would be life forms that don’t use DNA or proteins, or incorporate different elements in their biochemistry than all other known life forms on Earth do. Scientists have already begun major efforts to find shadow biospheres, but of natural rather than artificial origins.
If scientists find "weird" shadow organisms that arose separately from the Earth life forms we know, that won't necessarily suggest intelligent alien involvement. But such a find could give more credibility to the idea that life has a good chance of arising when given the right circumstances, rather than simply being a one-time freak accident, Davies said. And that might make everyone feel a little less alone.
The Daily Galaxy via Astrobio.net