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248 APPENDIX all aspects of our various cultures in the form of radio for most of this century and in the form of television for the last forty years, so that most of the parameters about our planet and our civilization can be readily acquired by unobtrusive, remote technical means. The collecting of physical samples would require landing, but it could also be accom- plished unobtrusively with a few carefully-targeted missions of the type of our own Viking experiments on Mars. All these considerations appear to contradict the ETH. PHYSIOLOGY The vast majority of reported "aliens" have a humanoid shape that is characterized by two legs, two arms, and a head supporting the same organs of perception we have, in the same number and general appear- ance. Their speech uses the same frequency range as ours, and their eyes are adapted to the same general segment of the electromagnetic spec- trum. This indicates a genetic formulation that does not appear to differ from the human genome by more than a few percent. Such an observation, if the entities were in fact the product of independent evolution on another planetary body as stated by the ETH, would stretch our understanding of biology. Humans share the unique combination of gravity, solar radiation, atmospheric density, and chemi- cal composition known on earth with an array of creatures closely related to us through evolution, yet deprived of legs and arms like the dolphins or endowed with multiple eyes like the spiders. It should also be kept in mind that the human shape has evolved in response to an extremely narrow set of constraints. For example, it would not exist as it does today if the earth had started out with twice its present mass, giving a surface gravity of 1.38 times earth normal. Such an environment would have forced the development of a stronger skeleton and might have precluded bipeds altogether. Similarly, a planet with half its present mass and a surface gravity of 0.73 times what it is now would have radically affected our shape. As pointed out by Stephen Dole (8), if the inclination of the equator had been 60 degrees