An estimated 10,000 Homo sapiens might have lived on Bioko Island for 5 My
Many female sea-turtles come to the Bioko beaches each night during much of the year, to bury their eggs in the sand (Butynski 1996, Tomas 1999). The turtles return immediately to the sea, leaving the eggs unguarded. The eggs are about the size of hen's eggs, and there are about 100 eggs in a typical nest.
There are several species of sea-turtle that lay eggs on Bioko. They are huge animals, weighing several hundred kilos each. As recently as 1990, about 10 sea-turtles were being caught each night, and the meat transported by boat to city markets for sale. That corresponds to over a kilogram of meat daily for 1,000 people. Local people told researchers that in the 1940s, as many as 100 turtles were taken each night. The turtles are only taken along the 19 km of beaches on the southern side of Bioko (see the map), where there are no roads. Other beaches of the island are more accessible, and sea-turtle eggs there were overexploited in previous times, so that few if any turtles visit those beaches now.
I think that these sea-turtles were important in human evolution. In my version of the aquatic ape hypothesis (Krill 2020) a few chimpanzees became trapped on Bioko, and their descendants evolved all the human traits. Those semiaquatic humans could have lived largely on sea-turtle meat, supplemented by shellfish and seaweed. Turtles are easy to kill without weapons, and the meat can be shared and eaten raw without tools or fire. If the Bioko humans were careful not to overexploit eggs, they could have taken as many full grown sea-turtles as they needed.
There could have been a stable population of 10,000 semiaquatic humans on Bioko. This estimate is based on the 34 km of suitable beaches on Bioko (as mentioned in Butynski 1996), and an average of 300 humans per kilometer of beach. This estimate is obviously only a guess. At times there could have been many fewer, or many more. By being spread around the 200 km long coastline of Bioko, the semiaquatic human population would have been robust for millions of years, able to survive local viruses and volcanic eruptions.
Based on molecular clocks, it is thought that human and chimpanzee lineages diverged about 6 my ago. If the founding chimpanzees became trapped on Proto-Bioko at that time, it might have taken a million years for them to evolve human traits and become modern Homo sapiens. After that, a stable population of 10,000 of these semiaquatic humans may have existed on Bioko coasts for 5 My, without needing to invent fire, weapons, clothing, or tools, and without leaving a fossil record. They would have spent the days in the water, singing and talking. They could have taught their children to respect turtle eggs and help turtle hatchlings reach the sea safely (something that sea-turtle mothers do not do.)
Some humans may have left Bioko early. But unaware that large predators existed on the mainland, and having no weapons, few of them would have survived the first week away from Bioko. As recently as 200,000 years ago, most of the humans on Earth may have been living in the sea on Bioko.