Boy, Lemmy sucks donkey dick. For every one legitimate answer there are two or three edgelord answers like "capitalism" and "the internet".

Here's an answer that hasn't come up yet: cooperation among mono cellular organisms. I don't mean the development of polyp analogues or colonies of single celled organisms; I mean getting down to mitochondria. Brace for wild oversimplification.

Before mitochondria, life had a hard time creating enough energy to do much more than barely stay alive. The current line of thinking is that one organism ate another and didn't digest it. The two organisms worked symbiotically, one handled energy production and the other handled getting food and staying alive.

Just about every living thing utilizes mitochondria and if the current idea that mitochondria were actually symbiotic organisms is true, that means that what was likely a chance "sparing" of prey is the underpinning of all complex life.

The odds of that happening are ridiculously low. There could be simple life in tons of places even within our own star system, but if the mitochondria-like symbiotic capture never happens for those extraterrestrial organisms, then complex life is probably unlikely to develop.