One of the many reasons we are doing a lot of ice age hole boring. Get a clear picture of the hot/cold and wet/dry periods. The problem is that it can only peer into that biom.

The planet has been hotter and there was still wildlife. I am not going to rule out climate change as a problem.

But I am probably going to point out increased commercial farming in Africa as a potential culprit. The use of fertilizers are an issue with algae and bacteria growth.