The rise of the mammals, primates such as humans included, was something of an evolutionary fluke. The world was dominated by proto-avian, likely feathered behemoths before the rise of the mammals. What if humans were the only mammalian species to evolve "cogniscience", and mammalian species are otherwise quite rare? Reptilian, avian, cephalopodic, or insectoid life might be far more common. Aliens must find humans quite odd. -- Nani
It's almost natural for newly spacefaring species to be egocentric. They are, after all, the only intelligent life they know. It's natural to assume that all other lifeforms in the universe evolved on the same path. Insectoids assume that all other lifeforms will be insectoids, cephalopodii likewise, and so on.
The most populous species form is the saurian/avian kind. Dinosaurs in space, but not how you think. They are all intelligent, naturally. Far-flung from the ancient beasts that you might be thinking of. Those are Deathworlder dinosaurs. The multiple disasters that shaped life on Earth as we know it simply don't occur on other worlds. Evolution is kinder there, than here.
That said, the anthropic[1] life pattern is handy for any species living close to a one-Standard-Gravity environment. Translated, you find an awful lot of human-shaped dinosaurs and birds. All of which are easily confused by mammals in general and humans in particular. Especially in the early years of Galactic Acceptance.
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