Mitochondrial symbiosis may only have happened once, but I'm not sure I buy that it's such a leap - mitochondria aren't the only symbiotic organelles. There are also chloroplasts. So unless chloroplasts are modified mitochondria, it seems that the eukaryotic leap happened at least twice. That tells me it's not all that improbable, or so I imagine.
Exactly, the existence of chloroplasts (in the language of the the article: the "power generators" in every plant cell) is an obvious proof that "complex life probably evolved only once" can't be worse title.
