Humans began to rapidly accumulate technological knowledge 600k years ago

You have to think that there were breakthroughs in communication technology — not just language in general but possibly also one individual who happened to be good at explaining things, either before or after language, who both taught more people, but also taught them how to teach — that led to step changes in technology.

Theory: there are no humans without language. Consider: what language do you think in?

Quite a lot of humans don't think in language, or do only some of the time, see e.g. https://www.iflscience.com/people-with-no-internal-monologue..., https://www.livescience.com/does-everyone-have-inner-monolog..., https://www.bustle.com/wellness/does-everyone-have-an-intern....

There are also humans who can’t conjure up an image in their head. Mozilla cofounder wrote a fairly famous piece about his own experience.

https://www.theverge.com/2016/4/25/11501230/blake-ross-cant-...

If there are people who can’t picture and people who don’t have an inner dialogue, I think it lends more credence to the idea that we don’t have free will and are just a bunch of chemicals controlling our behavior. It also makes you think about consciousness and whether it’s even real.

I find it hard to believe that they can't at all imagine what a tree looks like or imagine a face of a friend. I can understand some difficulty in a perfect image, but nothing?

I feel like I kinda have this, and I'd describe it like this:

With actual vision, there's a pipeline of steps: light hits actual cone/rod cells -> optic nerve fires -> brain stage 1 -> brain stage 2 -> brain stage 3... where each of those stages also have various side effects associated with the experience of "seeing"

I can't synthesize "brain stage 1" at all I don't think, I need my optic nerve to send some signals to "see". But I think I might be synthesizing "brain stage 2" when I imagine seeing e.g. a red apple in a pretty similar way to actually seeing it - I can feel "red apple vibes" but there is no image of a red apple my field of view. My brain state certainly contains some data about the color of the imaginary apple and the shape of it, but it's not nearly the same as actually seeing it.

This is all astonishingly hard to explain in a way that communicates accurately between two people though.

Absolutely same. I wonder how much of it is most people being like this and many of those saying "well yes of course I see the apple" without really thinking about it. If someone actually sees the apple, v.s. imagining what it would be like if they were seeing it, they're hallucinating. Not that it wouldn't be cool to be able to do that on command, I just wonder how many people really can.

I really see it. It's not hallucinating as the imagined image is not in the real world. It's like a different input to the visual processing part of my brain (but both inputs work at once, a bit like looking at a different thing with each eye). I have visual memories (snapshots I can call up) of things I never saw with my eyes (e.g. scenes from novels) that are as strong as any that came from my eyes.