The Ice Ages caused tropical rainforest around the world to retreat. Although debated, it is believed that much of the Amazon reverted to savanna and montane forest. Savanna divided patches of rainforest into "islands" and separated existing species for periods long enough to allow genetic differentiation (a similar rainforest retreat took place in Africa. Delta core samples suggest that even the mighty Congo watershed was void of rainforest at this time). When the ice ages ended, the forest was again joined, and the species that were once one, had diverged significantly enough to be designated as separate species, adding to the tremendous diversity of the region. About 6,000 years ago, sea levels rose about 130 meters, once again causing the river to be inundated like a long, giant freshwater lake.