There is the simple observation that one has no conscious experience during dreamless sleep. (A panpsychist could respond that maybe one merely lacks memory of one's sleeping experience, but that would be epicyclic).
I didn't think much of your comment at the time, but I think it's extremely central to the whole thing now. We go from unconscious to conscious almost all at once.
I like the idea! If you have it, the question is when does it start? Let's look at it for different aspects of consciousness:
Sentience (Bentham, Singer): Behavioral responses to pleasure or pain stimuli and physiological measures. This is observable across animal species, from mammals to some invertebrates and it should be known when responses to such stimuli start in the embryo.
Wakefulness: Measureable in virtually all animals with a central nervous system by physiological indicators such as EEG, REM, and muscle tone. The fetus is known to have a sleep wake rhythm, but I don't know when it starts.
Dennet's Intentionality: Treating living beings as if they have beliefs and desires makes good predictions for many animal species, esp. social, like primates, cetaceans, and birds. Infants show goal directed behaviors right after birth. I remember ultrasound photos that show babies suckling their thumb. I think we can identify when the nervous system is first capable of goal direction.
Dehaene's Phenomenal Consciousness: A perception or thought is conscious if you can report on it. As this requires language or measuring neural patterns that are similar to humans during comparable reports, I think this starts when communicable representstions of perceptions first form, for toddlers around age one at the earliest with baby sign language.
Gallup's Self-Consciousness: Recognition of oneself e.g. in a mirror. Requires sufficient sensual resolution and intelligence for a self-model. Dito.
Rosenthal's Meta-Consciousness: This is investigated through introspective reports on self-awareness of cognitive processes or self-reflective behaviors. Requires more abstraction. Maybe at age five?
I'd like to explore these in more depth, but for now I'll just reduce all the angles you provided to the helpful summaries/applications you provided. I'll call the perspective of going from adult human to zygote the "physical history" and the perspective of going up the ancestral tree as the "information history" (for simplicity, maybe we stop as soon as we hit a single-celled organism).
Note that "continuous" does not need to be "linear" or similar to that. Maybe the qualia decrease exponentially with the complexity of the structure that experiences them, so maybe each particle has a technically non-zero quality, but still all particles in the universe together have less of an experience than a single human. Numbers can be technically non-zero, and yet zero-ish for most practical purposes.
Continuous does not imply non-zero either. There is no reason to rule out consciousness being present in humans and not at all — not merely negligibly present, but absolutely not present — in a single atom, or an entire insect.
This was actually what I meant by a version of panpsychism that seemed to be the natural conclusion of humans having subjective experiences, but a conclusion I want to see if I can avoid.
Some advanced meditators report that they do perceive experience as being basically discrete, flickering in and out of existence at a very high frequency(which is why it might appear continuous without sufficient attention). See e.g. https://www.mctb.org/mctb2/table-of-contents/part-i-the-fundamentals/5-the-three-characteristics/
It really helps if we just taboo the word "consciousness" because people have too many implicit associations wrapped up in what they want that word to mean.
On a day to day level, we want "conscious" be to a stand-in for something like "things that have subjective experiences like mine". This is unfortunately not very useful, as the world is not carved up into thing that are like this and not, other than for other humans.
On the other, if we try to get technical about what we mean for things to be conscious, we either end up at panpsychism by deflating the notion of consciousness (I'm personally supportive of this and think in many cases we should use "consciousness" to refer to negative-feedback control systems because these are the smallest unit of organization that has subjective information), or we end up with convoluted definitions of consciousness to add on enough qualifiers to avoid deflation.
"Consciousness" is a word people are really confused about and have lots of different competing intuitions about what it should mean and I really wish we'd just stop saying it and talk about what we mean directly instead.
I tried some different definitions of consciousness while writing this point, until settling on "able have subjective experiences that transcend the 'easy problems of consciousness'"
Do you have any suggestions for making this more precise?
Panpsychism is correct about genuineness and subjectivity of experiences, but you can quantize your caring about other differences between experiences of human and zygote however you want.
That's a bit of a long read, and both your endorsement and the title seem too strong to be believable. If a few more people endorse that it's worth reading, I'll give it a go!
This question is not very well-posed, but I've done my best to make it as well-posed as I can.
Suppose that humans with sufficiently functional brains are able have subjective experiences that transcend the "easy problems of consciousness".
I'm interested in understanding if this can be reasonably accepted without also concluding a theory of some sort of "panpsychism". For instance, taking a single conscious human and working backwards in time to conception: is it natural to believe a non-zero level of consciousness or subjective experience emerges at some time in this process, or is it more natural to believe the zygote has a level of consciousness?
From another perspective, start with the same human, and keep going up the ancestral tree (applying some system to pick a parent when there are multiple). If consciousness isn't quantized and keeps decreasing as we go back, and we further assume life emerged from inorganic processes, do we eventually arrive at some theory of panpsychism?
My intuition is that panpsychism seems false and I have genuine subjective experiences. To reconcile this, I think I would need consciousness to be quantized in some way. Is there a natural reason to believe consciousness is quantized? Is there any recommended reading on this?