The most important factor of Tom's question of "low-hanging fruit" is how we consider what is low-hanging. As humans become better and more comfortable in more hostile environments, as greater masses of people have more access to the technology and techniques of exploration and the education needed to know what they're looking at when they see it, the tree of knowledge steadily bends toward us, and things that now seem "low-hanging" were out of reach for most not that long ago. It is immensely comforting for me to know that many of the things that are only just attainable by the most expert of researchers today will be "basic high-school" science in a generation or two.,(reply>), discovery thanks fully to something awful like deforestation.