There is plenty of evidence that prehistoric peoples did not necessarily live in total harmony with their environment, but rather despoiled it in various ways. But it seems that in one instance at least steps were taken to alleviate the damage. The place was the central Peruvian Andes; the time was the early part of this millennium; and the clues are pollen strata, which on one interpretation show that during a certain period the Incas planted alder trees to prevent soil erosion on the terraces used for crops.