Consciousness in organisms without brains?

Epistemic status: very speculative

As far as I know, there are basically four mainstream theories out there as to what things posses consciousness:

1. Only humans
2. Only organisms with brains (or at least nervous systems)
3. Everything (panpsychism)
4. Any system fulfilling certain informational or physical properties (IIT, Tegmark, etc.)

I've been inclined toward an intermediate position, that consciousness is a property of all (or many) biological organisms, not necessarily limited to the ones with nervous systems.The central problem of defining what is 'in' and what is 'out' with respect to consciousness is that we are always generalizing from ourselves; we know that we are conscious because of cogito ergo sum, other humans look and behave sufficiently similar to ourselves so we assume that they are conscious even though they might be philosophical zombies. Dogs also seem to behave autonomously and respond to stimuli in a similar manner to how we do, so we (everyone in group 2 and below) add them to the 'in' group as well. As we move further out the taxonomy of organisms, things start to look less like us, so we doubt whether they have consciousness. Plants don't move, so we tend not to think they're conscious, even though there's no a priori reason to believe that movement is a criteria for conscious experience. At the same time, we can look at a single-cell microorganism which uses chemical sensing to find food, move toward it, and consume it, and it's easy to attribute human-like thought to it just like we would to a dog.

I don't know what it's like to be plankton or an amoeba or a cactus, but it's not inconceivable that it might be feel like something to be like them. I don't think that organisms without brains and organisms with brains are that different from each other. In other words, while the brain has a particular way of going about computing and whatever else it does, biology had mechanisms of sensing, locomoting, processing information, making decisions, and so on long before brains existed, using various sorts of molecular machinery. Our brains still rely on much of this molecular machinery, and while our brains may be more 'advanced' in some sense, brains are just one solution that evolution found to surviving and reproducing in a changing environment. People who study cell and molecular biology will tell you that even a single cell has an extremely intricate and complex stucture; cells are basically cities of molecules, complete with a government, transportation of products, an economy, specialized professions, and so on. So if the complexity of the brain is what drives people to a brain-centric view of consciousness, I think the complexity of the cell could inspire a biology-centric view of consciousness. It could also inspire a cell-centric view of consciousness, as some have argued, but I think this runs into the same combination\binding problem as panpsychism -- in other words, if each of our individual cells are conscious, what accounts for our unified conscious experience? My intuition is thus that while complexity exists at the level of the single cell, somehow the organism as a biological whole - whether it be a single-celled amoeba or a tens-of-billions-of-cells human - comprises a complete entity with a unified conscious experience. The nature of that experience can vary widely, depending on the sensory, motor, and information processing capabilities that the organism has, but we already know that consciousness is diverse; there are blind people, people with synesthesia, people with autism, and so on. Extrapolating from our experience to that of rocks is difficult because rocks don't have sensing, processing, or decision-making capabilities, but extrapolating to a sniper plankton isn't inconceivable. My preference for this approach is also guided by the idea that there seems to be some kind of phase transition between non-living matter and biological organisms that doesn't seem to exist between organisms with brains and those that don't. The brain seems to have evolved rather smoothly via normal evolutionary processes, whereas the emergence of life from inorganic matter remains one of the great mysteries of science despite the Miller-Urey experiments. I am thus inclined to speculate that whatever mysterious process sparked the emergence of life may have also sparked consciousness, and the brain, while important for certain kinds of fancy computation, is not necessary for conscious experience. (In organisms which already have brains, of course, the brain is an integrated and necessary part of our conscious experience, but brainless organisms may implement their conscious experience differently.) If this view is correct, the scientific side of the hard problem of consciousness is no longer strictly a neuroscience question but rather a general systems biology question and deeply tied to questions regarding the origin of life on Earth. Addendum: There are some interesting questions on this view pertaining to the definition of a 'complete organism' which I'm not sure are easy to answer. For example are viruses conscious? Mitochondria? Are individual ants conscious, or perhaps the entire colony can be considered conscious? What about human societies? To the latter question I'm inclined to say 'no' because my strong feeling is that consciousness really is only present at the single-organism level, but for some reason organisms that are very colony-like such as ants or colony-forming bacteria make me suspect that the boundaries between organisms are not trivial to define. Humans may also be able to form a collective conscious experience, which Haidt discusses a bit in The Righteous Mind; I'm not sure how far I want to take that though.