Astronomer Seth Shostak's reevaluation of the Drake Equation ["Drake's Brave Guess," May], which attempts to estimate the probability of finding extraterrestrial civilizations via radio, is too optimistic with regard to at least two of the equation's terms. Concerning the term fi, the fraction of inhabited worlds that develop intelligent life, Shostak concludes that "the path of evolution may often lead to the development of intelligence." However, on our only known example, Earth, simple life developed quickly, whereas intelligent life has taken 4.5 billion years. This was possible only through a rare set of factors, including a moon of a particular size and orbit and a radiation-protective magnetic field. Far more frequently, otherwise-habitable planets are likely to undergo cosmic disasters that inhibit the evolution of complex life. As to fc, the fraction of intelligent species that develop radio technology, Shostak says that it is "close to 100 percent." However, many intelligent species might occupy aquatic environments, perhaps with miles of ice overhead and no land.

Donald G. Davis
Denver, Colorado

The fraction of intelligent species assumed likely to develop radio technology, virtually 100 percent, strikes me as being indefensible as legitimate science. Supposedly, "it seems obvious that if a species has the brainpower for speech, along with the sort of appendages that can manipulate a pair of pliers, it will eventually blunder into science, technology, and radio." This hardly seems obvious to me. The conviction that nature will invariably produce something very much like Homo sapiens introduces a sort of anthropocentric teleology into evolution, buying into the 1950s/Star Trek sci-fi convention of portraying every intelligent species as being just like us, possibly with a big head or pointy ears or green skin, but invariably a hominid with something functionally indistinguishable from human hands and producing a technologically advanced material culture. The Search for Extraterrestrial Intelligence project can be real science, but it can also be a kind of quasi-religious conviction that if we look far enough into space, we will find another, more perfect version of ourselves.

Robert Smith
Atlanta, Georgia

Seth Shostak responds: The idea that Earth is a habitat of an extraordinarily rare kind, which encourages the thought that Homo sapiens is the smartest species in the galaxy, relies too much on an ex post facto explanation. There's no doubt that our large moon stabilizes Earth's spin and strengthens the tides. Neither is clearly necessary for the development of life, however. And while magnetic fields keep harmful radiation from Earth's surface, many planets and moons have magnetic fields; such fields are not even unusual, let alone rare. As for "cosmic disasters," if Jupiter had not cleaned out the inner solar system of many of the small asteroids that could hit our planet, such devastating impacts would have been more frequent. As to Robert Smith's concerns, all we ask is that the aliens are able to build radio transmitters or lasers. Good looks are optional.

To Debate . . . or Not?

The May interview with John McCarter of the Field Museum was disappointing. The notion that scientists are willing to debate evolution is disturbing. We don't doubt that Earth revolves around the sun, so we don't debate it; yet there is as much evidence of the development of life over eons as there is of solar movement. When is someone going to say what we know to be true: that we are the natural, inevitable result of cosmic evolution and that creation by a deity is pure myth?

Sergio Alarcon
New York, New York

McCarter's summation of intelligent design theory—"the human eye is too complicated to understand, so a supernatural intrusion must have enabled it"—is an inaccurate generalization. It is an understanding of all there is to know regarding an eye that strengthens the argument for intelligent design. From a scientific standpoint, it seems highly improbable that the diversity of life on Earth could have originated from a common ancestor that arose by chance. True, evolution is a real phenomenon. But it could be argued that an intelligent designer is behind it.

Cary R. Woodson
Mount Juliet, Tennessee