Hi Conrad,
Thank you for your very interesting comments. If I may add a little bit more to your discussion on language:
"As to natural language, I fully agree that "natural language may have the same informational importance for human intelligence as genetics has for life." I would say that language was certainly necessary for the emergence of reflective thought in any form, and was definitely the key factor in bringing about "qualitatively distinct behavior" in our early human ancestors."
In a way, language plays a major role in combating the problem of the Landauer's limit in higher intelligence such as humans. We circumvent a limit on our physical possible brain memory by downloading it as information encoded in language, both spoken and written.
As you mention in your comment: "That is, it's not so much self-consciousness that makes us human as our consciousness of each other's consciousness, which also reflects us back to ourselves.", language and this interaction with each other's consciousness certainly developed hand in hand along with passing down of knowledge through generations, enabling us to start from information gathered from time = t = t1 rather than t = 0. We did not have much time or scope to look at other intelligent species which also develop languages such as penguins, if you may, this may help us in understanding that maybe the difference of the development of the written language plays a major role in a crucial phase transition.
Thank you once again for such detailed and interesting insights.