 What is NOT true of the crew?
Man's family tree was awesome enough to give every galactic race an inferiority complex—but then he tried to climb it! "There are at least a hundred kinds of humans, each supposedly originating in strict seclusion on as many widely scattered planets. 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. "But I thought it was proved that some humans did originate on one planet, that there was an unbroken line of evolution that could be traced back a billion years." 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. "And whereas there are humans who are relatively near and not fertile, they can always mate with those they were adjacent to two hundred thousand years ago !" On some worlds on which there are no other mammals, we're able to place the first human fossils chronologically. And, of course, if it were a fast-moving star, it might be completely out of the field of our exploration. Anyway, non-humans weren't trained in the fine distinctions that men made among themselves. We have men who could do the job in a makeshift fashion, but the region we're heading for, while mapped, is largely unknown.
 They could all mate with each other (it was explicitly discussed as a possibility in the story)
 They are all different species  They are all focused on finding evidence to support an important theory  They are all respected in their field  They could all mate with each other (it was explicitly discussed as a possibility in the story)