The crucial point is that these two domains, the bacteria and the archaea, are extremely different in their genetics and in their biochemistry, but almost indistinguishable in their morphology. Both types are small simple cells that lack a nucleus and all the other eukaryotic traits that define complex life. The fact that both groups failed to evolve complex morphology, despite their extraordinary genetic diversity and biochemical ingenuity, makes it look as if an intrinsic physical constraint precludes the evolution of complexity in prokaryotes, a constraint that was somehow released in the
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