I know it deserves an entry in some atlas obscura, it’s so poorly known outside very niche circles, but so help me I’ve always loved “Alien Planet” ever since the 2005 speculative documentary was first shown on the Discovery Channel (yes, I’m that old). It’s based on a book that’s even older than me, Wayne Barlowe’s “Expedition”, and while I’m much less well acquainted with the book and so will be using the TV show as my primary reference. If you’re not familiar with it, do peruse the helpful fan wiki.
While well-known and well-loved inside “speculative evolution” circles (yes, there’s a genre of worldbuilding and speculative fiction even more obscure and marginalized than what I gravitate towards), the creatures of “Darwin IV” have been challenged for being unrealistic in several aspects. Heck, some of them were even touched upon in the TV adaptation, where the talking heads (who were real scientists, by the way; you could tell some were much more “into” the whole scenario than others, hehehe) point out that the sheer size of these creatures, especially with them being primarily bipedal, makes it mandatory that their bodily density is just much lower than what we’re familiar with from Earth life, even with the gravity being modestly lower and the air being denser than Earth baseline. Maybe so. But more on the bipedalism in a bit.
Consider also another much-criticized point about the creatures of “Expedition”: they don’t use vision as a sense much, instead relying primarily on a sonar-like system. Which is found in cave creatures and in lineages like bats in our world, but on a planet with a transparent atmosphere with plenty of sunlight it’s downright bizarre that eyes wouldn’t evolve. Or maybe they did?
Consider that Darwin IV is a rather arid planet, in fact very reminiscent of the American West in its climate, its landscapes, and presumably its geology. Only unlike the Earth there are no global oceans, just a small sea roughly “the size of Texas”, along presumably with groundwater, and the odd stream or two, which is what feeds “pocket forests” that dot the landscape, despite the lack of rainfall or snowfall anywhere on the planet (we do see clouds and windstorms, so it probably does rain or even snow occasionally, but obviously precipitation is not a dominant feature of the water cycle here; the probes in the show never observed any precipitation during the months they were there).
It’s outright stated in the TV show that the oceans “evaporated” at one point, and there’s the mile-high evaporite salt deposits to prove it near the shores of the “Amoebic Sea” (similar to Mono Lake’s tufa, but far bigger). So named because the sea is covered with cellular material that forms a kind of gelatinous seal, keeping the remaining seawater in and usable for life (the sea itself functions as a giant algal colony of some kind).
Consider it’s also outright stated in the TV program that the air is so thick and oxygen-rich precisely because the “oceans evaporated”. Which is highly suggestive of a process at some point in the geological past of Darwin IV whereby water (H2O, two parts hydrogen and one part oxygen) is split into its components, H2 and O, the light H2 escaping into space but the heavier O being retained in the atmosphere. This is a fairly well-known process in astrobiology at this point, through interestingly was slightly obscure when the program was made: the usual cause of such a dissolution of the molecules in the ocean would be high levels of radiation from the parent star.
In astrobiological literature it’s sometimes supposed that red dwarf stars especially early in their formation put out high levels of radiation which would tend to split water into hydrogen and oxygen; on an ocean-dominated planet, this would have the net result of accumulating large oxygen atmospheres…which would be generated entirely without life needing to be involved! To wit, a “false positive” biosignature.
Obviously Darwin IV was an Earth-like planet originally, so, strangely in the astronomical literature, this process of ocean water dissociation would have to have happened late in the planet’s evolution, rather than right after the parent star’s original formation. But the key to all this might be what in the show comes off as a background detail that’s there purely for atmosphere: Darwin is a binary star system. It has not one sun, but two. Always appearing close together in the sky, so Darwin IV would seem to be in a “circumbinary” orbit. A la Tattooine, it orbits both suns’ barycenter, and the two stars are in a close orbit about each other.
It’s never stated at least in the TV show what sort of stars these are supposed to be, but in the program the suns come off as somewhat bluer and brighter than our sun (i.e. they appear small in the sky but still put out more or less Earth levels of luminosity). Which is interesting, because if you suppose that one of Darwin’s suns went off the main sequence, radiation levels may indeed have been elevated…and the oceans would be vulnerable to radiolysis: the splitting of water into hydrogen and oxygen, with the oxygen accumulating. This would normally be presumed to have annihilated all life on the planet or at least have caused a mass extinction…but consider that maybe it did.
Consider the rather interesting detail about sonar. What sort of life uses sonar on our world? Life that exists in the dark, which includes nocturnal creatures but also animal forms that inhabit deep caves. And where on land would be the safest from a sudden spike in radiation? Deep caves. Miles underground, even radiation from a star going off the main sequence would not annihilate animal life. As all but self-contained ecologies, cave-based biospheres could be sustained even through a mass extinction on the surface. So when the surface life died back, the sun went off the main sequence, and it perhaps stabilized into the white dwarf part of its life cycle (which is consistent with the spectrum we see on Darwin IV in the show, by the way…), the surface was somewhat desiccated but not extinguished to the point of uninhabitability. Water was still there, atmosphere was still there, microbial life was still there, and oxygen was even more abundant than before. Whatever biosphere was on the surface probably mostly died, in terms of species of complex life, but consider if Darwin IV was recolonized from cave-dwelling species that primarily used sonar, it would explain a lot.
Initially you’d see bat-like forms, but over tens of millions of years, that’s plenty enough time for them to have evolved to fill all the niches and have diversified to the point you see in the program. What it wouldn’t be enough time to have done is to have evolved eyes; while the environment veritably mandates eyes re-evolve eventually, the fact is the eye is one of the most complex organs there are, and it’s thought on Earth it took a long time to evolve. If the creatures that recolonized the planet had entirely lacked eyes for a long time, then they’d probably have stuck with and adapted what they already had, and which worked enough even on the surface: sonar. From an evolutionary point of view, it’s easier for a bat-like form to attain T-rex-style proportions than for an eyeless creature to re-evolve fully functioning eyes.
It’s also worth noting that if this lineage that re-spawned the world was bipedal (like…well, Earth bats), then the path of least resistance would be to optimize those two remaining legs, rather than re-evolving tetrapodism. So, bipedal creatures that use sonar, on a planet where the oceans were evaporated away by stellar radiation? Plausible!
So we see Darwin IV in the intermediate state where there’s been enough time for niches to repopulate and for the surviving lineages to have radiated out, but not enough time for eyes to have evolved again to a fully functional level. Heck, in an eerie bit of symmetry, the timing even works for Darwin IV’s catastrophe to have struck around the same time as our “K-T extinction” on Earth, which ended the dominance of the dinosaurs at the hands of the Chicxulub impactor.
So you have this symmetry where turn back the clock 65 million years, and Earth and Darwin IV might have actually been very similar…but one happened to have an additional sun that went off the main sequence, whereas the other had a stable sun. So the path diverges. And in evolutionarily plausible fashion, oddly enough.
Is that what the creators of the show or of “Expedition” intended? Maybe not. Heck I wouldn’t be surprised if they didn’t even think it through quite that much. But as a fan theory? A head-canon? I love it.
And it really drives home, that planets are not stable: they might be Earth-like at some snapshot in time, but a factor as simple as what kind of star it orbits can easily cause them to diverge. And in marked fashion. Now, aside from a catastrophe like temporarily elevated stellar radiation, how else could a more or less Earth-like planet devolve into…well, an Alien Planet? I leave that as an exercise for the reader, but if you ask me, it’s a fun exercise indeed…keep worldbuilding, everybody.