The diverse forms of life on earth have emerged probably from a common source, through a process of evolution that has the following characteristics:

1. The course of evolution does not always proceed along a straight path (for example, from simple to complex forms). Instead, it can meander like a stream, directed largely by environmental circumstances, although also affected by limitations on the capacities of organisms to respond to environmental challenges, and by unpredictable "chance" factors. Occasionally it reverses direction in certain respects, as when our own distant monkey-like ancestors became adapted to life in the trees, and our more recent ancestors readapted to living on the ground. When the course of evolution does reverse direction, it generally does so with respect to comparatively few features only, not all features. Thus, we humans retain various characteristics evolved earlier in connection with life in...