Proto-Indo-European isn't even close to the earliest form of language. He was referring to what you asked- what language "we" would have been speaking "thousands of years ago" and he's most likely right about that. There are a large number of languages still spoken today that aren't even descendants of PIE; Basque, for example, which is surrounded by Indo-European relatives is a total linguistic isolate and apparently unconnected to any of them.
The "earliest form of language" by our modern definition, what you might be looking for, is Proto-Human Language, which, while impossible to learn or speak, we are able to define a remarkable number of words from.
Yeah. When I said 'we' I was referring to the human species in general though.
Many linguists reject the methods used to determine these forms
In other words, who the fuck knows?