Aliens: Intelligent creatures from a distant star who have traveled to Earth.
Alien Life: A necessary precursor to those aliens, simple self-replicating molecules, such as cyanobacteria (blue-green algae).
Earth is one of eight planets with an assortment of smaller bodies in thrall to our star, the Sun. The Sun is one of about a third of a trillion stars bound by their mutual gravitation (and that of so-called dark matter) into a huge spiral galaxy that takes light 100,000 years to cross from one edge to another: the Milky Way. Which, in turn, is one of maybe 2 trillion galaxies in the observable universe ....
My point being, if there's intelligent life here, there must be intelligent life somewhere out there. Right? Let's just stick to our own galaxy, given the vast distances — millions of light years — to the closest neighboring galaxies of any size. With 300 billion stars (that's about 100 times the number of your lifetime heartbeats, if you're lucky), each star being host to maybe 10 planets (some of which we already know are in the liquid-water "habitable zone," you'd think the Milky Way would be teeming with aliens.
Well, it likely is teeming with alien life — simple molecules that replicate — but not with aliens in the usual sense of the word, as defined above. We know, of course, what aliens look like because we've seen the movies: a lot like us! Head on top, two eyes, weirdly shaped skulls, limbs, spines, probably bipedal — we'd know one when we saw one. Vulcan, Romulan, Xenomorph (for Sigourney Weaver to battle): They're bad guys. The good aliens are the outlines seen at the end of Close Encounters of the Third Kind, E.T., or the blue Navi on Avatar's Pandora — same basic idea. We've come a long way from Michael Rennie in The Day the Earth Stood Still (and his 8-foot-tall bodyguard, Gort) thanks to CGI, but I think it's fair to say aliens — malevolent or benevolent — are immediately recognizable as intelligent life.
But to get from alien life like algae to intelligent life that can zip around the galaxy (supposedly crash landing here about as often as self-driving cars) takes time. Much time. On Earth, it took nearly a third of the age of the Universe, about 4 billion years. Much is made of the fact that life started almost as soon as there was liquid water. We have "biotic" fossils possibly as old as 4.1 billion years in rocks found in Western Australia. But it took eons, nearly 4 billion years, to get from microbes to intelligent life, life with the capacity to build radio telescopes and interplanetary spacecraft.
Our Sun is exceptionally stable, we wouldn't be here if it wasn't. The vast majority of exoplanets don't have the luxury of 4 billion or 5 billion years of relative stability for intelligent life to evolve. And Earth — unlike other planets and moons in the solar system — is exceptional in that we have a moon (from when a Mars-sized body slammed into our young planet) to keep us in a balanced, non-oscillating orbit. In addition, mass extinctions — each one of which made way for novel species to evolve — may be essential to progress from microbial to intelligent. The most recent major extinction resulted from the not-too-big, not-too-small asteroid that collided with Earth 66 million years ago, wiping out non-avian dinosaurs and clearing the decks for our mammalian ancestors. "Lucky" doesn't even begin to describe it.
There's probably tons of microbial life out there, maybe even in our own solar system: My money's on Saturn's moon Titan and Jupiter's moon Europa. But aliens? The good-guy or bad-guy aliens we want to believe are Out There? Given the astronomical unlikeliness of us being here, I'd say we're SOL.
Barry Evans (he/him, firstname.lastname@example.org) would love to be wrong about aliens. His Humbook Two, which deals with topics somewhat closer to home, is at local bookstores.