Matt Williams has an interesting article at Universe Today on the Aurora hypothesis, a part of a long running series on the Fermi Paradox: if alien civilizations are numerous, where are they? The Aurora hypothesis is that the reason we don’t see signs of alien colonization throughout the galaxy is that most biospheres are not compatible with each other. In other words, just because planets are habitable by some lifeforms, doesn’t mean they’re settleable by foreign lifeforms. The hypothesis is named after Kim Stanley Robinson’s novel about human settlers on an exoplanet discovering that their new home has serious problems.
It does seem like the possibility of biosphere incompatibility is pretty high. As living systems, we are an integrated part of our own biosphere. Our systems depend not only on environmental conditions, such as the atmosphere and Earth’s magnetic field, but also on an entire ecosystem of other life, including a food chain, as well as many symbiotic relationships. Even the oxygen in the atmosphere is a result of other life.
A lot of science fiction pictures colonization working similarly to how European colonization worked in the Age of Discovery. But this was Europeans exploring and colonizing (and conquering) new regions within the same biosphere. Colonists of an alien biosphere might find themselves always having to wear environmental suits.
Colonizing space may be more analogous to life moving from the oceans to land. It’s not something that’s likely to happen without a change in the systems doing the colonizing. We see this already in the fact that most space exploration is done with robots. When scaling to interstellar exploration, the difficulties increase exponentially, which seems to make it inevitable it will be primarily, if not exclusively, a machine endeavor.
An advanced civilization could probably have self replicating probes, seeking out construction materials in a destination solar system to make new copies. These probes, as intelligent energy seeking reproducing agents, could be viewed as a form of life. (Over long enough time fames, we might even expect they would undergo evolution.) And I see no reasons in principle that what they produce couldn’t include engineered life in a discovered biosphere, life built out of material from that biosphere and tailored to function within it.
If so, then the Aurora issue would have been bypassed. Of course, this does assume that self reproducing machines are possible, but we already have such machines. Indeed we are such machines, just evolved ones rather than engineered ones. And we’re already starting to play with our design via genetic engineering. It also assumes that interstellar travel, even by machines, is possible. But the Fermi Paradox seems moot if it isn’t.
It does raise the possibility that maybe we were colonized long ago, but the colonizers just added new lifeforms based on our existing biosphere. These engineered lifeforms could have devolved over time while blending into the fossil record. But I think we’d still see at least some remnants of their technology, and a much larger discontinuous jump in evolutionary history than currently shows up. Although we can’t completely rule out the possibility we were uplifted by a Monolith or similar alien probe.
Anyway, while Aurora definitely raises an important issue, I don’t see it as a definitive answer to Fermi. But maybe I’m missing something?