The Wall Street Journal last weekend ran an essay by physicist Paul Davies, who has a current book about the Search for Extraterrestrial Intelligence (SETI). The essay, “Is anybody out there” is a short precis of the book.
This paragraph got my attention:
Our bodies contain some genes that have remained little changed in 100 million years. An alien expedition to Earth might have used biotechnology to assist with mineral processing, agriculture or environmental projects. If they modified the genomes of some terrestrial organisms for this purpose, or created their own micro-organisms from scratch, the legacy of this tampering might endure to this day, hidden in the biological record.
A likely early adopter use of biotechnology is people implanting codes of various kinds into the junk DNA of their children. This idea of ancient ET is kind of a love child of that “implanted code” idea and intelligent design – if bacteria were actually products of alien engineering, they might have systems that could not have evolved by incremental processes.
So it’s highly significant that no such systems in bacteria have been identified.
The next paragraph:
Life on Earth stores genetic information in DNA. A lot of DNA seems to be junk, however. If aliens, or their robotic surrogates, long ago wanted to leave us a message, they need not have used radio waves. They could have uploaded the data into the junk DNA of terrestrial organisms. It would be the modern equivalent of a message in a bottle, with the message being encoded digitally in nucleic acid and the bottle being a living, replicating cell. (It is possiblescientists today have successfully implanted messages of as many as 100 words into the genome of bacteria.) A systematic search for gerrymandered genomes would be relatively cheap and simple. Incredibly, a handful of (unsuccessful) computer searches have already been made for the tell-tale signs of an alien greeting.
There’s a lot of potential for “Bible Code” -like delusion as people start looking for messages of ancient aliens in reconstructed ancestral genomes. In reality, implanting such a message would pose immense problems. Consider the difficulty of transmitting a message through DNA over 10 million years. If your DNA “message” is neutral to the organism’s fitness, then the chance it will eventually be fixed in that population is its initial frequency. So, if you want a 50% chance of survival in that population, you’ve got to find and tag 50% of the individuals. Then, you’ve got to pick which populations will survive. Possibly more abundant populations will be more likely to persist, but you’ll have to tag many more individuals in those cases.
This is an awful lot of genome tagging just to get your message to last 10 million years. Over a hundred million years, any one population has an infinitesimal chance of survival, so you’ve got to tag pretty much every one of them. The degradation of the message by mutations is the least of your problems – one mutated copy is unlikely to survive, but if any do, probably many lineages will carry altered copies that might be subjected to phylogenetic reconstruction. The problem is analogous to the preservation of classical literature – few things were copied widely enough to survive, but the things that were copied tend to have multiple slightly different versions that inform us about the “ancestral” text.
By far a simpler sign of ancient aliens would be a massive biogeographic transfer that proved to be inexplicable by the known pattern of plate tectonics or dispersal mechanisms. So far, we don’t know of any such events.
Or, of course, a subset of metazoans that differed in some radical way from the rest of life on Earth – different genetic code, lack of common cell machinery like ribosomal RNA, etc. Nothing like that has ever been found.