When pondering the Fermi Paradox and related matters, a popular thought experiment is to run back the clock and try to determine (or speculate about) whether something akin to humans would rise up again.
But as I was thinking about it, I realized that macro life basically evolved into two distinct branches - plants and animals (I realize there are actually 5, but I don’t think eukaryotic organisms will be building any radio communication devices or rocket ships soon).
So, my question, if you could run the clock back far enough, would life follow that same course and develop two main forms of life? Or can you envision others, three or maybe only one?
Besides, there are three branches of multicellular life: plants, animals, and fungi. And no, there is no reason to think that there is anything inevitable about these divisions.
If by “macro life” you mean multicellular organisms, there are actually three kingdoms (Animalia, Plantae, and Fungi); in general, the “Six Kingdom” paradigm is what is taught in most basic taxonomy, with Eubacteria and Archaebacteria sometimes being divided into separate kingdoms under prokayrotes. Fungus are probably both the most primitive and most diverse kingdom of multicellular life, and while largely unseen except for the sporocarp, are as ubiquitous in the environment as bacteria and far more difficult to eradicate.
In response to the question, there is no reason to expect extraterrestrial life to follow anything like the same evolutionary path or taxonomic divisions as Earthlife, and indeed, given that there will likely be significant differences in environment, resources, energy sources and gradients, et cetera, it is entirely probable that the way a hypothetical extraterrestrial biome would develop could be entirely different. One could imagine, for instance, a ‘kingdom’ of collaborative heterotrophs more akin to fungi than animals which use enzymes to dissolve and reconstruct materials, or autotrophs that feed off of energy and materials released by tidal forces driving volcanic processes on a moon of a Jovian-like system, or some combination of both like some kind of colonial cyanobacteria that have developed sufficient complexity for emergent intelligence. In fact, I think anything we might imagine as derived from our experience with Earthlife is probably not creative enough to anticipate how extraterrestrial life may develop.
I guess what got me to thinking about it is my pet theory about human evolution. Primates, or precursors thereof, developed manual dexterity because of their need to climb trees. Without trees, that dexterity might not have happened?
It’s going to be hard to say what would happen if you went back in time and started life over. The way things worked-out are not just dependent on biology, but also climate and geology, and more universally, physics. There is also the unscheduled encounters with a life-threatening (literally - all life) asteroid tossed-in to make things interesting, by opening-up niches to some organisms that were previously occupied by another. There are a lot of variables that got us to this point, and, sadly, chance seems to have played the major role.
With that in mind I would guess (unsatisfyingly) we’d still end-up with aquatic and terrestrial life forms, and they would likely be similar to what we have today.
Sure we will. We are eukaryotes. As are plants and fungi and protists.
Other way around: The division between bacteria and archae is above the other divisions. Bacteria, archae, and eukaryotes are the three domains, and then the eukaryotes are subdivided into four kingdoms. And of the three domains, archae and eukaryotes are more closely related to each other than either is to bacteria.
You’re conflating the Woese “Three Domain” system (which eschews defined kingdoms entirely) with the more tradition “Two Empire” system that divides between prokaryotes and eukaryotes as separate “empires” and kingdoms below (confusingly, one of them also being developed by Woese, Balch, Fox, et all in 1977). What you will find in most introductory biology textbooks today is the system developed by Thomas Cavalier-Smith with various evolutions but dividing Protista into Archezoa, Chromista, and Protozoa. The distinction of kingdoms between Archaea and Eubacteria is somewhat arbitrary because both kingdoms are polyphyletic and could be further divided, while in Woese’s “Three Domain” system lumps all eukaryotes into a single domain without distinct subdivisions, which, while useful from a symbiogenetic standpoint of evolution is considered too convoluted for an introductory course.
I think while “plants” and “animals” and “fungi” are our arbitrary divisions, some sort of autotroph vs heterotroph split seems like it is more likely to arise than not, given the physics of it all : anti-entropy “as we know it, Jim” is going to require some energy input, and organisms are either going to use some “free” energy directly to make “life stuff”, whatever that is, or they’ll need to get their “life stuff” from the ones that do. That doesn’t have to work the way it does here with carbon, but some analogue is going to arise.
Now, somewhere life may be all exclusively autotrophic. All hererotrophic is harder to finagle, but I guess not impossible in some sort of Haldanesque “primordial soup but self-perpetuating” scenario. Which is why I said “more likely to arise”, not “inevitable”
