UPDATE: On December 2, 2010, scientists at NASA announced that they'd indeed identified Shadow Life here on Earth. They found a new type of naturally-occurring bacteria that uses Arsenic instead of Phosphorous in it's DNA.
Shadow life is a theory which states that life already exists on Earth which has no evolutionary connection with any form of life currently known to science. It was proposed by Astrobiologist Paul Davies. The idea is that, regardless of whether life first came here from elsewhere (exogenesis) or formed here out of non-living components (abiogenesis), it only makes sense that this may have happened more than once. If so, the second wave will have evolved in different directions, and will probably exhibit radically different genetics than other life on the planet.
The first such Shadow Life detected was GFAJ-1, an extremophile bacteria found in the very hot Mono Lake in California. There may be some small amount of debate about just how genetically different GFAJ-1 is from other common terrestrial bacteria, as it seems to be able to make DNA from either Phosphorous or Arsenic (with the later producing cells 50% larger than the former). Detailed genetic sequencing may be needed to say for certain whether it represents a new branch of our existing and known tree of life, or the first detected example from a whole new separate tree. (See arsenic-based life and alien amino acids for more ideas of what this could mean.)
Further searches for additional examples of Shadow Life may help expand our understanding as well. Other oddball entities may be living in other remote ares, such as the upper atmosphere or deep sea volcanic vents. Regardless of what we discover about GFAJ-1, we may find other Shadow Life whose genetic structure is very unlike the majority of earth life. The search continues.
However, it's also possible that Shadow Life is known and documented, but no one's ever noticed it's unusual origin - we just think it's normal life. Significant painstaking genetic sequencing would have to be done to determine this, but if so, we may find that some species of microbes have no genetic commonality with the rest of life on the planet. Might. Again, see the Alien Amino Acids page, as chirality or obscure aminos are likely telltale signs of the exotic source and evolution of the shadow life.
Lastly, it's possible that the shadow life is so bizarre that it has it's own Shadow Biosphere that doesn't interact with our own biosphere. Again this might be spatially distant (such as at the earth's planetary core), or it just might be a matter of having incompatible biochemistry. And it may even be so unlike our life, that we've failed to identify it as even being life. Also possible is a combination of all these factors - say "right-handed" microbial silicon-based life existing deep underground. If we aren't actively looking for it, we'll never find it.
The finding of shadow life speaks volumes about the universe. If life started (or arrived) here twice, then it would be very likely that the universe is teeming with life. If significant research had instead shown that there was no evidence of Shadow Life here, then that would have strengthened the Rare-Earth Hypothesis instead.
Game and Story Use
- Shadow Life may also prove to be a Living Fossil.
- For more ideas on how shadow life might differ from traditional Life As We Know It, see Alien Biochemistry.
- The existence of Shadow Life here at home suggests a more densely populated universe. We may find that Jupiter, Venus or Mars are teeming with lifeforms we hadn't previously expected. A hard science game can now more believably feature alien life much closer to home.
- A team of NASA astrobiologists goes to a remote corner of the world to collect bacteriological samples, and while there discovers (or is attacked by) a form of Shadow Life far more advanced than they'd ever imagined.