HOW TO INVENT ALIENS - THE RULES - WHAT DO REAL ALIENS LOOK LIKE?
Arguments and evidence about alien life has been based in school chemistry and naive biological philosophies. We can do better: evolutionary biology can tell us the likelihoods of biological innovations on this planet, and we can argue to other evolutions. Life, and indeed intelligence, is common - but the peculiarities of our evolutionary history, like airway-crossing-foodway, and our kind of excretory/reproductive mix-up, will not be found elsewhere. Knees and elbows are also "parochial" in (pentadactyl) detail, but joints-in-the-limbs are "universal"; so are photosynthesis, life on land, flight and many other multiple inventions here. The argument to intelligence as a Universal is interesting and has ethical consequences. Building credible aliens is fun, too.