We do not know all that was on Earth. We wouldn't have known about dinosaurs if we didn't chance upon their remains, and we only recently learned that they had feathers.
There is probably no way to know everything that has happened on this planet.
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And that brings another facet of the point in the GP:
It's not just the distances between different planets that hinders life from meeting other life; even life on the very same planet is prevented by vast periods of time from ever encountering each other.
The next solar system is only four lightyears away. Given the will to break nuclear test ban treaties we think we could develop nuclear pulse propulsion drives that achieve about 10% the speed of light. Even after accounting for acceleration and deceleration that gets us there in a human lifespan.
Granted, even at that speed generational ships make a lot of sense, and the selection of targets is still somewhat limited. But we can get to our neighbouring solar systems in "reasonable" time if we want to, with very doable development effort
Also plenty of water out there and sunshine, burning hydrogen as fuel should be possible.
Now you will have enough machinery in space to mine building stuff and enough fuel. Gradually build permanent colonies in space.
It won't get all humans out of earth soon or ever. But it will put some humans permanently out of earth. Then keep building colonies as population in space grows.
Once you have done this you have done two things:
1. Eliminate a existential threat to humanity more or less permanently.
2. A slow but steadily growing population of humans in space.
Several decades/centuries after that you could have humans permanently living and working in space. From there its a matter of population growth, resources are a plenty and you have a universe for the taking.
Once you have a massive presence in space interstellar travel by definition is a question of acquiring real estate to keep up with population growth.
But if you told them they could get unlimited clean fuel. There could be plenty of funding to extract water/ice from asteroids and send it back to earth.
It gets _something_ there in a human lifespan. The only really well though out models for that are Daedalus and Icarus, and they would involve flybys only of probes the size of a hockey puck, or a washing machine at most. The cost of which would be civilisation impoverishing.
I think it's a very reasonable chance that humanity will never, as a functioning human populated civilization, ever spread beyond this solar system. Now 'never' is a pretty long term statement, but no matter how advanced our technology becomes, it's still likely to be constrained by the physics we know. It looks like spreading beyond this system would cost a very large fraction of all the resources we have in the system to start with, if it ever becomes practical at all.
The basic idea is to combine two kinds of ship, neither of which we know how to make work individually, to form one that we can make work.
The first is the generation ship. The problems with those are that (1) we don't know for sure how to make a completely self-sustaining ecosystem that can keep a large enough population for viable colonization alive for the hundreds of generations it would take to reach their destination, and (2) such a population would require a freaking large ship, raising all kinds of challenges.
The second is the cryogenic suspension ship. The problem there is that we don't know how to unfreeze them without killing them.
Can we address the generation ship problems? Both problems stem from the need to have a large population. If we didn't need a large population, we could use some of the space that would have otherwise been consumed by colonist families to bring food to supplement the food produced by the ship's ecosystem, and eliminate the rest of the space they would have occupied.
Assume we can develop a food that can be stored indefinitely and that has the caloric density of rice (200 calories/cup). There are ~4 200 cups in a cubic meter, which gives 840 000 calories in a cubic meter. At 2000 calories per day, that's 420 days of food. A storage unit 10 meters on a side gives 1150 years worth of food for one person. And this is assuming that the person is getting all their calories out of storage, not depending on the ship's ecosystem.
If the storage food only needs to be a supplement designed to make up for things that we can't figure out how to get from the ship's ecosystem, then the numbers get even better.
This is where the cryogenic suspension comes in. We can't suspend and revive people--but we can do eggs and sperm indefinitely and revive them. Our cryonic generation ship will carry the colonists as frozen eggs and sperm. The only warm people on board would be the crew to operate the ship, and to raise the next generation crew.
Note that the next generation crew could come from the frozen eggs and sperm rather than from mating among the current generation crew, so we don't have to worry about a small crew becoming severely inbred during the trip.
If we can ever develop robots to the point that a robot can raise and educate a human baby without any human supervision, then we can leave out the crew and the food and the ecosystem, and just send robots plus frozen eggs and sperm.
The Orion project was designed for sending a city-sized space ship to Alpha Centaury in ~100 years.
The hard part seems to be how to build a city-sized ship, not how to move it.
https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...
Basically nothing of it was actually built (nuclear propulsion wasn't an well accepted idea - for good reasons that won't matter as soon as we start building in space). There was just some very small scale prototypes.
In comparison, project Daedalus moved much faster.
So life can apparently adapt to extra-Earth locations. The leap of faith is then, that it hasn't left Earth, or that life on Earth didn't come from somewhere else to begin with? Because either scenario becomes plausible. It'd be strange to believe it didn't happen.
> Looking at all of those galaxies forces you to come to terms with how little any of us really know for sure.
Only if you are forced to look at all that - in detail - and since that's not essential to a mundane life, there is no obligation to believe anything much.
I mean, I'm trying to make a point about believe in distinction to knowledge. There is yet a weaker modus operandi, which I'd call awareness for now, but that's a loaded term, too. If you hold two thoughts about contradicting possibilities in mind at the same time, that's not believe. Believe happens when externalities force a decision, also called leap of faith. Then you choose to believe in one possibility. Knowledge on the other hand means the possibility of round about 1.
Until we actually observe it, no, we can't be sure; the probability of it existing is high simply due to the sheer amount of dice rolls there are in the universe, but the likelihood of us observing it is low. We already know there is and was no intelligent life in our solar system besides what's on Earth; there's still hopes of finding traces of microbial life on Mars and maybe that ice moon. But unless we get a signal from far away (what the SETI project has been scanning for for decades now), we'll not find evidence of alien life in this generation.
I also doubt humanity will find a way to travel fast enough to get to another solar system within a human lifespan. If we do, it'll probably involve bending the fabric of space-time and the Great Old Ones will return from Beyond the Observable Universe and will start eating us.