I really like the idea proposed here that we are supposedly composed of animals due to the way atoms work, and how it carries from one state of life to the next as atoms never truly die off. Though with this I have my own counter to the theory, and I'd love to not only go off of this theory but also try to show evidence supporting it could or couldn't work out. Be prepared for a lengthy reply birds and boys
Concerning the theory itself
The idea that an atom will never die is theoretically possible once one has achieved the understanding of how the atom works. As stated in the article here used for mainly reference (Chemistry class knowledge wears off after time after all) (Are Atoms Immortal?
), an atom sustains itself with the use of balancing protons and electrons so that both quantities are at an equal amount. However, atoms have the ability to break down when the number of neutrons is incorrect (meaning Helium has two neutrons, and if one were to be lost it would be the incorrect number of neutrons within the element) then the atom will shoot off a subatomic particle in an attempt to stabilize itself from the unstable state. We cannot determine when breakdowns will occur precisely, but we can theorize such using the concept of "half life".
Though interestingly enough, atoms have an infinite number of protons and neutrons to the point where, theoretically, it could improve upon itself to make it's half life so long that it is essentially eternal, thus supporting the idea of how an atom will never die. But, this creates conflict as the elements we are aware of in this day and age all have a half life ranging from a mere couple of days to as long as 18 sextillion years, though this incredible feat of elements having incredible half lives is also possible due to radioactive materials. Basically, the half life of elements that are not radioactive are not as long as those that are radioactive (though Bismuth destroys this theory if I'm not mistaken) (this theory could also be wrong, but for the sake of the argument we will argue it is). This means that, theoretically, our atoms will breakdown overtime but not quick enough so that they cannot convert over to another state of life or be picked up upon, but this raises the question of can atoms convert to another state of matter that isn't their own?
Going off of the conclusion that atoms within ourselves and within animals are the same, we could technically say yes but I argue our atoms vary as, while although we are all mammals, there is a different structure set up within animals that allowed them to stay walking on all fours and acting as they do as opposed to our structure which has allowed us to become bipedal and evolve as we have as a species. Meaning, with my argument, that atoms cannot convert over to another state of life after we pass, and this could especially be the case concerning converting to plants as vegetation and human life are very different structures.
Now, let's assume that my argument is incorrect, and our atoms could indeed convert over without issue or variance. Technically speaking those atoms would at one point or another cease to exist if we're continuing to follow the law of half life, meaning that while the atoms would still be there to be picked up and carried over to somewhere, they will eventually inevitably divide and cease to exist. BUT, for an atom to stop existing entirely this presumably is a tremendous duration that expands over several decades, perhaps even centuries, which allows the carrying over of atoms from one who has passed to an animal that has passed to a new being that has not passed yet, meaning that technically speaking the atoms of that previous individual and that animal are within our bodies that we are not aware of.
If this is all the case, we would certainly have within ourselves the smallest of percentages of animal (whichever species it may be), and could explain as to why there are so many four legged Therians and Otherkin walking around as these animals would come into direct contact with the atoms. BUT, I argue that we would not get memories from this as atoms do not have the ability to store information as we do with memory, meaning they cannot remember. So how does one explain past lives and memories? I'm uncertain of this, but I'm welcome to anyone who might want to add upon what I already have stated concerning the convergence of atoms from one state of life to another, and how this could carry over memories.
Going off of this theory
Going off of what has already been stated, it could be argued that everyone does indeed have some animal within themselves to a degree of which they may not be aware of, but the question arises as to how do Therians awaken as a Therian and why don't others do the same? Couldn't everyone be a Therian?
I'd like to refer back to my theory on social conditioning that has resulted in the emergence of non-human feelings within the human conscious. Whether one is disordered or not (meaning one is diagnosed with let's say ADHD or Schizophrenia), there will be a point in everyone's life that one learns what is acceptable and unacceptable behavior to have both in public and at home. If a child were to begin barking, running on fours, eating out of bowl with their mouths, the instinct of the parent is to correct that behavior and teach them that it is inherently wrong. Some children don't display this behavior at all while others display behaviors that are indeed animalistic but not stereotypical (meaning one could show behavior of being a bird like myself but not showcase stereotypical animal behavior as a child), and this is where the differentiation of who is a Therian and who isn't comes into play.
Because we have these behaviors rejected, we have a sense of something is wrong within ourselves and naturally, as human beings, we desire to seek an answer as we fear the unknown within and outside of ourselves. This is where the feeling of non-humanness is created and dwells, due to the rejection of behaviors and the formulating of the idea that something is wrong within the child and must be corrected immediately. When one comes across Therianthropy they are relieved to have an explanation, and thus begin exploring the topic to then perhaps later identify as a Therian themselves.
These are just my theories and I'm very open to having anyone suggest anything or even add on!