Another possibility is that there is no language of thought (LOT) at all. Maybe we just use our ordinary languages to think; if we don’t have language, we can’t think very well.
So maybe early humans were just a bit cleverer than other animals. They made noises to try to communicate — just like other animals do — and slowly developed a sophisticated language ability. They also started using their language to think and this helped them to develop mentally.
By this view, language is just one of the tools that humans used in order to develop advanced mental capacities.