Until Jeff Spock says Earth is part of the Endless Universe, Earth is not part of the Endless Universe. So no matter how many theories are out there, it just isn't as a fact of the canon. 

The planet Sobra has many similar textures to Earth, however, it is not the same planet. 

There's no confirmed direct connection between the Mezari peoples and the Nakalim peoples. They could be cousin species (similar to the Hissho and Yuusho) but until it is otherwise stated by the DEVs, they are not the same species. As with many of the Humanoid species in the game (especially those that bear a great deal of resemblance to Earth's Homo-Sapiens), many happen to be offshoots of other species and so with this in mind, it is likely that neither the Nakalim nor the Mezari were the first "human-looking humanoids" within the Endless Universe. 

As for your questions;

1. The various human-like factions on Auriga are descendants of both the Concrete Endless and the many different Mezari colonization efforts that occurred on the planet. Some are also the product of human experimentation (the Forgotten) and others are the result of cross-species breeding (some Vaulters and Sisters of Mercy). You can find more information in the EL Artbook. None have been stated to be the ancestors of the Nakalim due to the latter not existing (wasn't created) when EL came out. 

2. There is no official statement regarding the origin of the human-like humanoids present within the Endless Universe. 

Suis3i wrote:

Until Jeff Spock says Earth is part of the Endless Universe, Earth is not part of the Endless Universe. So no matter how many theories are out there, it just isn't as a fact of the canon. 

The planet Sobra has many similar textures to Earth, however, it is not the same planet. 

Ok, I get that we don't know for sure, if Sobra is Earth. But do we know for sure that it isn't? Did Jeff say at some point that Earth is not part of his universe or something like that? I don't know.

There's no confirmed direct connection between the Mezari peoples and the Nakalim peoples.

Huh, I guess the Nakalim were never called actual humans yet, weren't they...

Well I hope there is at least some connection there, because, if not, the Nakalim make no sense to me :(

There was a post about this I saw yesterday about this same topic that quoted the developers several times saying that the Endless Universe is not supposed to be an alternate version of ours, they are completely separate.

Until proven otherwise I'm going with placeholder art. 

Relevant to this discussion: The texture and art of Sobra will change in an upcoming patch.

Nakalim ARENT Humans, they cannot find Mezan in Celestial quest. 

Or they forgot to rectroactively enable "Mezan" path for Nakalim in Celestial Worlds.

Let's hope for the best - that they aren't hypocrites and cheap ones at that. Their homeworld is being changed, you know?

When I get the chance, I'll see with Slowhands if there was a lore reason for them not being able to discover Mezan, or if this was just an oversight.

Well, change everything that Slowhands said before then

https://www.games2gether.com/amplitude-studios/endless-space-2/forums/65-general/threads/34687-dehumanize-nakalim-they-appearence-is-a-mistake-solid-sources-and-proofs-proposals?page=1#post-287739 

Further proofs:

Slowhands about humans in space:

https://www.games2gether.com/amplitude-studios/endless-space-2/forums/65-general/threads/33285-attempt-to-trace-the-history-and-timeline-of-the-united-empire?page=1#post-279803

Slowhands wrote:

(...)  Officially, the Endless Universe is what is referrred to as a "Second World" setting; Earth does not exist in the timeline.(...)

- The Mezari civilization has been around a long time (though not as old as the Endless), but its history is full of erratic ups and downs (civil war, invasion, over-expansion, etc.) so it never gained the power of the Endless. (...)

and 

https://www.games2gether.com/amplitude-studios/endless-space-2/forums/76-creations/threads/33206-my-mspaint-lore-chart-of-humans#post-279368

Slowhands wrote:

Okay, here we go with a few clarifications:

(...)
- The Mezari are the root of all spacefaring humanoid races in the galaxy. I use "spacefaring" to distinguish them from those native to Auriga (ed. mine - that are speculated to be Endless-derivative)

 (...)
 - To clarify the Mezan / Raian question:

The ancestral origin of all spacefaring ES2 humanoid races is indeed Mezan. However, there were several major exoduses... exodusses... exodi... whatever. The Vaulters were part of an earlier one, and the UE part of a later one.(...)

So there you have it - logically, if Mezan is the homeworld, and Sobra is the homeworld of Nakalim (as is stated expicitly in Awakening) - there's no relation. 

If Vaulters are the early goers, and they changed so-little through ages of genetic research and constant struggle, Nakalim would not change - when they are scientifically illiterate and there's no struggle in their evolution.

Their not only similarity to Mezari ie humans, but full-on human appearance is just cheap and/or an oversight that bids removing.

To be fair it'd be nice to see a more, well, inhuman approach to the Nakalim given that they are supposed to be a different species. Honestly if they just give them a third eye and pointy ears and call it a day I'd be up for it. Also, good catch on so many species havinf breasts on their females. 8 have to admit it's a bit weird, especially on the Lumeris 

Why is it weird on Lumeris? I never got this. Who told you they cant be mammals?

dobyk wrote:

mamarider wrote:

Why is it weird on Lumeris? I never got this. Who told you they cant be mammals?

Oh they totally can, but they were described as amphibian so I couldn't understand why they don't have actual amphibian features. Boobs look weird on them but I don't mind. I think the Sophons having boobs is definitely weirder ;D 

I think they  look ok ;)

I'd just like to remind everyone that there was at least one case of Earthlings teleporting to Auriga.

Orbotosh wrote:

I'd just like to remind everyone that there was at least one case of Earthlings teleporting to Auriga.

wut

Sublustris wrote:

Orbotosh wrote:

I'd just like to remind everyone that there was at least one case of Earthlings teleporting to Auriga.

wut

The Australium Update.

Orbotosh wrote:

Sublustris wrote:

Orbotosh wrote:

I'd just like to remind everyone that there was at least one case of Earthlings teleporting to Auriga.

wut

The Australium Update.

I wouldnt take a crossover as 100% canon, it technically is and it also isnt really.

Orbotosh wrote:

The Australium Update.

Whatever made you think they came from Earth - they didn't. Those aren't even the same people.

In Team Fortress they are called Misha, Dell Conagher, Dr. Ludwig and Pyro's name is unkown.

In Dungeon of the Endless they are prisoners from Success prison ship, and their names are Dell Bradford, Pat Bates, Misha Cherny and Kaspar Herab.

Sublustris wrote:

Orbotosh wrote:

The Australium Update.

Whatever made you think they came from Earth - they didn't. Those aren't even the same people.

In Team Fortress they are called Misha, Dell Conagher, Dr. Ludwig and Pyro's name is unkown.

In Dungeon of the Endless they are prisoners from Success prison ship, and their names are Dell Bradford, Pat Bates, Misha Cherny and Kaspar Herab.

Dell Bradford is explicitly described as being a copy of the ""real"" Engineer, one of his lines also mentions the city of Austin.

Salterius wrote:

Orbotosh wrote:

Sublustris wrote:

Orbotosh wrote:

I'd just like to remind everyone that there was at least one case of Earthlings teleporting to Auriga.

wut

The Australium Update.

I wouldnt take a crossover as 100% canon, it technically is and it also isnt really.

Frankly, apart from the basic permise of the ship crashing on Auriga and some people escaping, I would not take any of the details in Dungeon of the Endless as canon.

Let's just say that the relation of DotE to canon is tenuous, at best. Some elements could be considered canon, but many are not. In fact, it was originally envisioned as a light, humorous companion game to Endless Legend. However, in the making of it, we kind of fell in love with the gameplay and the art and the style so we let it run its course. The writing in there was pretty much me on an Endless Universe-inspired delirium; I was too busy having fun to stop and ask myself if I should really be putting some of that stuff in there...

But, for the record, I will reiterate that our lovely planet home planet of Earth is not, for better or for worse, a place you can or will find in the Endless U.

-Slow

Also, in one of the Endless Space 2 minor DLCs, we find the original, long forgotten homeworld of "humanoid bipeds." Mezan. It was rendered nearly uninhabitable at some point long ago, apparently so long that the Mezari's chief descendents, the Raians, knew little if anything of its existance.