I have a little fan theory about why Corellians are pretty much literally humans. Bear with me here.

The first part isn't my own idea, I read it somewhere. But as far as I know, the second part might be original. Either that, or I have source amnesia.

How ET's Species Reveals the Identity of the Galaxy

It all starts with an unexpected character: ET.

As in, "ET, phone home!"

Now, you probably know that Lucas and Spielberg know each other and have worked together.

In the prequel films, ET's species made a cameo, as a sort of nod or in-joke.

So ET is from the Star Wars galaxy.

In some of the posters for ET, or so I have read, ET was described as being "3 million lightyears from home."

So Earth is around 3 million lightyears or so from ET's home planet.

This places said planet in a different galaxy.

Specifically, Triangulum, a spiral galaxy in the Local Group that's around 2.723 million light years away from the Milky Way.

ET's home galaxy, which is also the Star Wars galaxy, is none other than our neighbor, the Triangulum Galaxy.

So wait, why are there humans?

The Star Wars galaxy is described as being not only "far, far away," but "a long, long time ago."

How long ago, we're not sure. But we're talking about vast intergalactic distances, and probably about "deep time" as well.

So here's my fan theory: Corellians were originally humans from Earth. When they were able to travel to the Triangulum Galaxy from the Milky Way, the particular type of transportation they used carried them not only through space, but through time as well.

So from Earth's perspective, the events of the Star Wars galaxy are perceived to be in our past.

I do realize Star Wars doesn't really do "time travel" per se, but this idea involves moving simultaneously through time and through space via a wormhole or something. (It's Star Wars, so I don't feel the need to construct a genuinely plausible mechanism for this. It is not, and has never been, hard sci-fi.)

The journey from Earth to Triangulum was a one way trip. Once they arrived, it would no longer be possible for them to communicate with the humans on Earth in the Milky Way, nor would it be possible for them to ever return there.

In the Star Wars universe, hyperdrives and interstellar travel are a very old technology, whose exact origin isn't certain.

What if the human colonists from Earth were the ones to introduce that technology in the first place?

TL;DR: The Star Wars Galaxy is actually the Triangulum galaxy. At some point in hundreds of thousands of years in Earth's future, a brave group of colonists made a one-way trip to Triangulum.

This journey transported them not only in space, but in time, and they ended up in what would be perceived from Earth as the distant past.

This occurred so long ago that the knowledge of this event has been lost. These humans may also have been the ones to introduce hyperdrive technology to the Galaxy.

I have no idea if this would contradict anything in Star Wars Legends, or if there's been any info added to the "real" canon that would contradict it.