The book pretty much presents all of the artifacts and fossils that we have for the past 2 million years. The premise is that cognitive skills went hand in hand with culture, tool making industry, hunting and language. He looks at the advances in tool making which follows the same evolution path of the vocal system and the hearing system in various species of hominids. He concludes that language was part of culture which went hand in hand with advancing cognitive skills. Culture, language, cognitive skills all advanced together over time.
This seems a dubious assumption. It may be true, but the evidence you suggest is weak.
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We’ll see how dubious it is as new studies are done. One thing is certain, a million years ago Homo Erectus was the most cognitively advanced species the planet has ever seen.
Don’t you mean “had ever seen”? Or are you suggesting that we have regressed from that peak?
Note that plenty of animals make tools and have various sorts of learned material culture but lack language.