Our brains evolved over hundreds of millions of years to react to rapid changes in our environment. One apparently built-in feature is the tendency to keep the environmental backdrop against which our lives play out relatively constant. That makes it easier to detect rapid changes to which an organisms must react. One of our fish ancestors in a Devonian lake 400 million years ago was not paying attention to .01 degree Celsius temperature changes in the water occurring as clouds passed the sun - it was looking out for dangerous predators in that water that could end its life in a second. An australopithecine ape on an African savanna 3 million years ago was not tuned into a gradual change in the climate - its nervous system tended to keep the environmental background constant so stalking leopards would stand out. If the climate was changing, australopithecines weren't causing it, and there was nothing they could do about it, so natural selection wouldn't put a premium on detecting changes that occurred on a decadal time scale. This evolved tendency to keep the background constant, to "tune out" changes in the background is still with us today. When the air conditioner switches on we notice the change, but soon are unaware of the noise. When you sat down at your computer you noticed the signal from your butt that you were in the chair. The signal is still coming, but until you read this, you were unaware of it. Can you think why this characteristic of our nervous systems - promoting "habituation," ignoring more or less continuous stimuli - may give us difficulties in dealing with environmental problems? What background trends are people tuning out, for example, when they urge more drilling for oil?? ———- Paul R. Ehrlich is Bing Professor of Population Studies and Professor of Biological Sciences at Stanford University. He is the author of hundreds of scientific papers, and numerous books including The Population Bomb and Betrayal of Science and Reason (Island Press, 1997). His latest book is The Dominant Animal: Human Evolution and the Environment, which he co-authored with his wife Anne.