 Why is it significant that the aliens only differ from humans in one physical characteristic?
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. The history of the planet was tabulated as facts became available. "We've got to make or break," said the first alien. "It's not what they've done," said Bal, the second alien. 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." Except for the one thing they lack, and that's quite odd, they seem exactly like us. "There is. "In a week we can't negate their entire history. There's nothing in this region of space our people want," said Ethaniel. "And how long can Earth last? 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." "What's worse is that I now find they also have missiles, range one thousand miles and upward. They either have or are near a primitive form of space travel."
 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.