 Why is it significant that the aliens only differ from humans in one physical characteristic?
A gentle fancy for the Christmas Season—an oft-told tale with a wistful twistful of Something that left the Earth with a wing and a prayer. Earth was so far away that it wasn't visible. But this vast distance did not mean that isolation could endure forever. Instruments within the ship intercepted radio broadcasts and, within the hour, early TV signals. Machines compiled dictionaries and grammars and began translating the major languages. The history of the planet was tabulated as facts became available. The course of the ship changed slightly; it was not much out of the way to swing nearer Earth. "We've got to make or break," said the first alien. "I can guess," said Ethaniel, who had spoken first. "It's not what they've done," said Bal, the second alien. "The big bomb can destroy them. "I may remind you that in two months twenty-nine days we're due in Willafours," said Bal. "Without looking at the charts I can tell you we still have more than a hundred light-years to go." They've had two world wars in one generation and that the third and final one is coming up you can't help feeling in everything they do." "The wrong diplomatic move, or a trigger-happy soldier could set it off. A meteor shower could pass over and their clumsy instruments could interpret it as an all-out enemy attack." "We'll just have to forget there ever was such a planet as Earth." "Very much like ourselves," he said at last. "A bit shorter perhaps, and most certainly incomplete. Except for the one thing they lack, and that's quite odd, they seem exactly like us. "It is. The fact that they are an incomplete version of ourselves touches me. "There is. "In a week we can't negate their entire history. "You can't tell," said Ethaniel. "And then what? "Very little," conceded Ethaniel. "And when we get to Willafours we'll be busy. There's nothing in this region of space our people want," said Ethaniel. "And how long can Earth last? We're not committing ourselves by looking." They went much closer to Earth, not intending to commit themselves. For a day they circled the planet, avoiding radar detection, which for them was not difficult, testing, and sampling. Atmospheric analysis showed that as far away as we were."
 The aliens happened to look like certain beings from stories of Earth's history.
 The aliens happened to look like certain beings from stories of Earth's history.  The fact that humans are shorter makes the aliens more imposing.  It proves that the aliens and humans are actually distant relatives.  It means the aliens will not be trusted when they land on earth.