 What is NOT a technological advancement involved in this story?
Obviously there was no contact throughout the ages before space travel— and yet each planetary race can interbreed with a minimum of ten others ! "Outside of your own world, there's nothing like your species, except superficially, and that's true of all other creatures, intelligent or not, with the sole exception of mankind. Actually, the four of us here, though it's accidental, very nearly represent the biological spectrum of human development. It's reasonable to assume that, if men were set down on a hundred such worlds, they'd seem to fit in with native life-forms on a few of them. That's what happened on Earth; when Man arrived, there was actually a manlike creature there. "But there are other worlds in which humans who were there before the Stone Age aren't related to anything else there. "Something had to distribute one species so widely and it's not the result of parallel evolution—not when a hundred human races are involved, and only the human race." Man was the most numerous though not always the most advanced—Ribboneers had a civilization as high as anything in the known section of the Milky Way, and there were others—and humans were more than a little feared. The theory is that humans who can mate with each other were once physically close. When we project back into time those star systems on which humans existed prior to space travel, we get a certain pattern.
 Advanced weaponry
 Rapid healing abilities  Advanced space travel  Rapid mutations  Advanced weaponry