Looking down at the Amazon River from above, an observer cannot help noticing that water dominates the landscape even beyond the sinewy main channel. The river, which extends from the Pacific highlands of Peru some 6,500 kilometers to Brazil's Atlantic coast, swells out of its banks and inundates vast swaths of forest during the rainy seasons, and myriad lakes sprawl across its floodplains year-round.

All told, the river nurtures 2.5 million square kilometers of the most diverse rain forest on earth. Until recently, however, researchers had no idea how long this intimate relation between river and forest has actually existed. The inaccessibility of this remote region, now called Amazonia, meant that long-held theories about the early days of the river and surrounding forest were speculative at best.