All we know is that, if a planet was formed similar to Earth in the distant past, there are three big steps that must have occurred in order to get a recognizably advanced civilization like our own.

1. Life must have somehow arisen from non-life. This is the problem of abiogenesis, or the origin of life from nonliving precursor molecules. To go from the raw ingredients associated with organic processes to something that's classified as life, which means it has a metabolism, responds to external stimuli, grows, adapts, evolves, and reproduces, is the first big step.

It occurred at least once, more than 4 billion years ago, on our world. Has it occurred elsewhere in our Solar System? In our galaxy? In the Universe? We have no idea how frequently, out of the multibillion planetary candidates in our galaxy or out of the 1022 candidates in the visible Universe, this may have occurred.

2. Life must have thrived and evolved to become multicellular, complex, and differentiated. For billions of years, life on Earth was single-celled and relatively simple, with copying errors from one generation to the next providing the overwhelming amount of variation in organisms. Wherever resources abound, the simplest organisms to first make use of them fill that ecological niche. Under most circumstances, they find a way to persist.

It's only when something changes, such as resource availability, the survivability of the environment, or from competition, that extinctions occur, leaving open the possibility for a new organism to rise to prominence. Extinction events and selection pressures gave rise to many critical evolutionary steps on Earth: DNA absorption, eukaryotic organisms, multicellularity, and sexual reproduction, among others. This could be an inevitable occurrence on a planet with life, or it could be an ultra-rare event that happened to take place many times on Earth. We don't know.

3. Intelligent life must have evolved, with the right traits to also become a technologically advanced civilization. This may be the step with the greatest uncertainty of all. It's been over 500 million years since the Cambrian explosion, and it's only over the past few hundred years that life on Earth has achieved the technologically advanced state that an extraterrestrial observer would recognize as a sign of intelligent life.

We can broadcast our presence to the Universe; we can reach out beyond our home world with space probes and crewed space programs; we can look and listen for other forms of intelligence in the Universe. But we have no known instances of success on this front in our Universe beyond our own planet. Life like us could be common, or we could be the only example within the limits of our observable Universe.

The notion that we can quantify the odds that a form of intelligent life arises in our Universe based on the scientific knowledge we have today is old: it goes back to the mid-20th century at least. Enrico Fermi, whom the famous Fermi Paradox is named after, posited that such estimates led to the notion that intelligent life in the Universe should be common, so, then, where is everyone?

The Drake equation was a famous way to parameterize our ignorance, but we still remain ignorant about the presence of alien life and alien intelligence. Hypothesized solutions have included:

that they're there, but we aren't listening properly,
that intelligent life self-destructs too quickly to maintain a technologically advanced state for very long,
that intelligent life is common but usually chooses isolation,
that Earth is purposely excluded,
that interstellar transmission or travel is too hard,
or that aliens are already here, but choose to remain hidden from us.
These proposed solutions usually leave out the most obvious option: that one or more of the three big steps is hard, and that when it comes to intelligent life in all the Universe, it's just us.

Our scientific discoveries have led us to a remarkable point in the quest for knowledge about our Universe. We know how big the Universe is, how many stars and galaxies are in it, and what fraction of stars are Sun-like, possess Earth-sized planets, and have planets in orbits that are potentially habitable. We know the ingredients for life are everywhere, and we know how life evolved, thrived, and gave rise to us here on Earth.

But how did life arise to begin with, and how likely is a planet to develop life from non-life? If life does arise, how likely is it to become complex, differentiated, and intelligent? And if life achieves all of those milestones, how likely is it that it becomes spacefaring or otherwise technologically advanced, and how long does such life survive if it arises? The answers may be out there, but we must remember the most conservative possibility of all. In all the Universe, until we have evidence to the contrary, the only example of life might be us.