We often imagine aliens as they are presented in Close Encounters of the Third Kind; large head, white body, fundamentally humanoid. At our most creative, we imagine hostile giant squids or amoebas. At any rate, we almost always perceive extraterrestrial life as homologous to what we consider animals.

However, the eukaryotic life with which we are familiar evolved on Earth independently from the rest of the universe. The earliest ancestor they could possibly share with us would be prokaryotic; we have indeed found evidence of prokaryotic life outside our own biosphere. But the evolution of the eukaryotic cell, as far as we know, was unique to Earth. Extraterrestrial life must be based on a completely different type of cell, if based on cells at all (What works on Earth may not work on other planets.).

What can we possibly predict about such visitors? How can we conceptualize the path of evolution in a different, unknown biosphere? The fundamentals on which life as we know it is based are shattered here. We cannot even apply our own senses to this; how can these aliens understand the significance of eyesight or hearing when they are equipped with something we ourselves could never comprehend.

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