Scaling: An Attempt to Find a Common Denominator
What makes toxicologists believe that experimental findings in laboratory animals can predict what is going to happen in humans under the same conditions? Probably the fact that the human is as much an animal as the laboratory model and that both humans and laboratory animals had a common ancestor in their respective evolutionary pasts. While it is easy enough to see that both the human and mouse are animals and that evolutionary theory provides a reasonably good explanation for the occurrence of life on earth, this is only part of the issue. Evolutionary theory certainly explains the continuity of life on earth, but it also explains the diversity as well. The observer of nature may not know what is the most impressive phenomenon—the marvelous unity and similarity of life as tied together by a common genetic code or the striking diversity of sizes, shapes, forms, and functions. For while the predictive toxicologist may want to emphasize the phylogenetic continuity of life as the basis of animal extrapolation, he or she must also successfully deal with the diversity issue as well. Thus how is it possible to predict a human response from a mouse, when the human is about 3500 times larger or the mouse takes 14 breaths for every 1 human breath or the metabolism 500of the mouse is about seven times faster than the human’s (Jones and Grendon, 1976). While there are similarities between humans and mice, are the differences so great that any predictions are of little practical value?