Proto-human language may have evolved over many million years on Bioko. Large crowds of people entertained themselves by standing up to their necks in water and singing to each other. It was not so important what was said or sung, but how it was said or sung.

When individuals and small groups of people left Bioko from time to time, they took advanced language with them. That is, they took the experience of advanced language using grammar, syntax, rhythm, and rhyme. But they emigrated without much in the way of vocabulary and even lost most of the vocabulary they had. They invented new words for new situations in new places where they found themselves. This could explain how the world's thousands of sophisticated languages started.

The Denisovans and Neanderthals left Bioko well before the Sapiens. They may have come away with a poorer understanding of language, and did not communicate and cooperate as well as the Sapiens, who mainly left in a large exodus -- the Recent-out-of-Africa events — around 60,000 years ago. 

To think about the evolution of language, I am reading the book The Singing Neanderthals, by Steven Mithen. 
Here is a figure from a review of that book.