Densely populated aquatic humans on Bioko would evolve and self domesticate
AAT message #70636 Thanks for discussing perceived problems with Bioko. It helps me to consider these things.
The aquatic humans didn't have boats, so I'm not sure that a wide continental shelf would have been important. I think what is important that the aquatic habitat stayed unchanged and the food supply stayed reliable for the last few million years, while the sea level was rising and falling by as much as 120 meters.
The southwest side of Bioko currently has beaches with a steady supply of huge sea-turtles and turtle eggs. The sand on beaches typically stays in the surf zone, moving up and down as sea levels rise and fall over geologic time. The aquatic humans did not need much space. They could have been packed densely along the shoreline, like Emperor penguins, or Crabeater seals (there maybe 75 million of them!), or the Marine iguanas of Galapagos.
This is how densely the Marine iguanas can live:
On some shorelines they can be very numerous, with densities as high as 8,000 per kilometer, and their biomass compared to the area they occupy may surpass that of any known reptile. However, their distribution is patchy, and colonies are generally found within 100 m of the ocean, naturally limiting their range. The total population for the entire archipelago is estimated to be 200,000–300,000 individuals
With this kind of dense population and fat easy living on Bioko, the aquatic humans would self-domesticate. They would develop language and civilized behavior. They would share genes and evolve aquatic traits together, unlike clusters of undomesticated humans or hunter-gatherers spread around several continents.
Here is a map to show Bioko the last time it had a land bridge (from about 70,000 to 10,000 years ago). I think the last 'aquatic apes' (fully evolved Homo sapiens) may have left Bioko only about 50,000 years ago.