It is impossible to figure this out and it's dumb to try to give a number. We don't think Neanderthals could speak? They also had languages.
Phylogenetic Analysis has been fairly successful already in analyzing our genetic history, so I'm not sure why you'd think it's impossible.
https://www.reddit.com/r/AskAnthropology/comments/78o048/did...
>As a quick point and this is quite a late response, having the same FOXP2 gene may not be enough. New evidence suggests
>Using statistical software that evaluates gene expression based on the type of gene, Vanderbilt graduate student Laura Colbran found that Neandertal versions of the gene would have pumped out much less FOXP2 protein than expressed in modern brains. In living people, a rare mutation that causes members of a family to produce half the usual amount of FOXP2 protein also triggers severe speech defects, notes Simon Fisher, director of the Max Planck Institute for Psycholinguistics in Nijmegen, the Netherlands, who discovered the gene.
But, as I could see, only humans have so significant language culture that lead to great number of manuscripts.
I will even accent - little number of humans write books, but near none other species do this.
We don’t know, as you note. But the genetic evidence combined with tool use and their large Supra-orbital indices (fairly big brains, if shaped differently than ours) all would make that the preferred prior I’d say.