"I think therefore I am" - Descartes

It’s awareness.

Personally I see consciousness as an emergent property of sufficiently capable brains (processing power and both long and short term abstract memory) and the development of language and self awareness. This produces a sort of "observer" (as a computer programmer I tend to think of it as a background task, even though there are limitations to that metaphor as the brain is NOT a fancy computer, it's more of a pattern [mis]matching engine). This observer remembers the past, sees the present, and can predict the future based on extrapolation and comparison to other similar beings who are older, and it sees something it does not like. At all. And that is mortality. Hence, the "human condition", existential angst, and the various immortality projects that most people are obsessed with.

There's a hypothesis (untestable, unfortunately) that at one time this "observer" was a quasi-separate personality, that would speak to us and we would literally hear. Jayne's hypothesis of the bicameral mind says that we all heard voices in our heads and assumed that was the gods talking to us. Later (in the past 4 to 6 thousand years) we have become unicameral (well almost all of us; there are some schizophrenics and other people with psychoses who still hear the voices at least some of the time) and while we still have an inner thought life we don't hallucinate or personify it. The spiritist / animist religion of the bicameral mind has become the polytheism or monotheism of the unicameral mind.

Regardless of the mechanism of evolution, I think consciousness as we know it just needs sufficient thinking resources and some tipping point of that plus need to bring it forth.

I believe many species are conscious at a lower amplitude or in more specialized ways -- dolphins, orcas, elephants, the great apes and others all have the ability to love, hate, mourn, laugh, etc. They lack our sophistication only because they haven't evolved language to support more abstract thinking, and/or lack the physiology to readily handle tools. This suggests to me that consciousness tries to emerge in multiple species in a thriving biosphere like ours; on some planets, two or more species may have achieved sentience. Indeed ... sibling species to ours, like the Neanderthals, have existed and died out.