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....

I'm highly suspicious that the whole "inner vs no-inner monologue" thing doesn't actually exist and has more to do with these things being extremely hard to explain using language, and people describing their subjective experience in subtly different ways.

I 100% do not think in words or sentences or language. It feels more like node-traversing a graph of concepts, but it's all completely abstract. While I am able to consciously play back my thoughts linguistically or visually with some effort (and at the cost of efficiency), by default there are neither words nor images.

French is my first language, I'm fluent in English, and I know a bit about many languages... But I've never had an inner voice or thought in language, even when I was very young.

Right, very like me. Language is the front end; I use it to express thoughts when needed (to record, communicate and check reasoning) The back end is working in some kind of abstract space.

I suspect people who say they think in language are not really thinking "in" language. I think we all have this abstract conceptual space in us. I'm not even sure what it means to think in language; where do the ideas and concepts come from that are being expressed? I guess some people have to internally "hear" a thought expressed before it is real to them.