Well-timed synchronicity is a wonderful thing. There’s nothing quite like the intellectual zing one experiences when one scientific presentation casts an unexpected light on a seemingly unrelated theory or idea. I was the beneficiary of this phenomenon last week, when scarcely twenty-eight years¹ after reading Geoffrey Landis’ “The Fermi Paradox: An Approach Based on Percolation Theory,” a Lindsay Nikole video essay on an entirely unrelated matter suggested an interesting embellishment.

But first! A quick background on the Fermi Paradox and on Percolation Theory as applied to it, for those of you who can’t be bothered to click the relevant links above.

The Fermi Paradox isn’t really a paradox so much as a question for which we currently lack a compelling, satisfactory answer. Back of the envelope calculations suggest that even modest propulsion technology should be sufficient for a single technological species to spread throughout the galaxy in a geological instant. However, when we look around, there is no evidence that this has ever happened². Thus, Enrico Fermi’s “But where is everybody?”

Landis’ proposed explanation centred on the fact that even for technologically sophisticated species, interstellar colonization is likely to be difficult and expensive. Not every civilization will bother to invest scarce resources in projects that by the nature of light-speed lag and distance can do them no direct good. Furthermore, light-speed lag and distance mean that child civilizations must be functionally independent of parent cultures. That could be a full stop to further expansions, as there’s no guarantee that colony worlds will want to invest in spawning their own colonies.

If the likelihood that colonies will be colonizers is low, then the effort peters out very quickly. Even when there is a civilizational commitment to colonization, the vagaries of chance will produce a patchy network of settled systems. Large swathes of space would remain unsettled.

Perhaps an example is in order. For convenience, consider the Stellar Database entry for Sol. Suppose for the sake of argument that the longest practical distance for colonization is 6 light-years. There are at present two stellar systems within 6 light-years of Sol: Alpha Centauri and Barnard’s Star. The only stellar system within 6 light-years of Alpha Centauri is Sol. The only stellar systems within 6 light-years of Barnard’s Star are Sol and Ross 154. The only star within 6 light-years of Ross 154 is Barnard’s Star. Thus, the poor Solarians can only reach four systems before their technology is no longer up to the task³.

Fans of the venerable tabletop roleplaying game 2300 AD know that increasing the maximum practical distance slightly (to 7.7 light-years) produces an interesting stellar road map where stars that are comparatively close to Sol while being farther than the maximum practical distance can only be reached via a circuitous chain of intermediary systems, if they can be reached at all. Tau Ceti, for example, is only about 12 light-years away but a ship limited to 7.7 light-year legs ends up covering 60 light-years as it dog-legs from system to system.

This is merely a plot-enabling inconvenience in 2300 AD, which features faster-than-light travel. In a world where there is no FTL, where every intermediate system needs to be settled and developed before a flotilla can be dispatched further down the line, the chain of colonies might peter out well before reaching Tau Ceti.

OK, so what does that have to do with Lindsay Nikole, who as you know is a zoologist and author with a colorful vocabulary whose interests, while expansive, appear to be entirely terrestrial? (That we know of.)

Among Nikole’s interests: cats, big and otherwise. Thus, videos discussing various feline species. Thus, a video about cheetahs. Thus, a video titled “How cheetahs became genetically f***ed.”

How did cheetahs become genetically f***ed? Long version, watch Nikole’s video. Short version: bottlenecks. Cheetahs experienced diverse catastrophes (see what I did there?) that left remnant populations with much smaller genetic diversity. Repeated catastrophes led to new bottlenecks, each of which further reduced overall species genetic diversity. This lack of diversity makes cheetahs vulnerable. Any disease that kills one cheetah will likely kill them all.

We can apply Landis’ argument to interstellar colonization here: Each colonization is a genetic bottleneck. The first colony is a subset of the whole population of the mother world. The second one is limited to a subset of the first colony. The third… well, you can connect the dots.

Let’s do some math. We’ll assume the colonizers aren’t idiots trying to emulate the Ptolemaic Dynasty… IN SPACE!⁴. Say they manage to preserve 90% of the genetic diversity of the species available to them. What does this look like in practice?

Extend the sequence far enough and you probably get populations that are functionally so closely related you could do skin grafts between random individuals. While that would be tremendously convenient for the skin graft industry, it would leave the colonists in the same situation as cheetahs—so uniform that any circumstance that imperils one individual would imperil every member of their species on that planet. Which is bad.

Maybe there’s a fix. If there isn’t—if genetic bottlenecks are an intractable problem—then what Landis called “percolation probability” would be low. To quote Landis, “For P<Pc, colonization will always terminate after a finite number of colonies.” Which is consistent with what we see.