If I recall correctly, when The Solus Project was first announced, it was billed as a "spiritual successor" to The Ball, another Teotl Studios game from 2010. Once I played it though, I began to consider it more as a direct sequel.

The Ball portrays the end of one chapter of humanity's ("Race d9's") interaction with the "Caretakers" ("Sky Ones"), and the Solus Project tells the beginning of the next chapter. They may be separated by a couple of centuries and have different protagonists, but they have at least one character in common, and the "Tracking of Time" stories from the Underground levels make the connections even clearer.

I will attempt to elucidate all of these connections here, tracing the full history of the interaction between the Sky Ones and humanity.

The short answer to this question is in their "Odyssey Codex":

The long answer is: I don't know for sure. But what is clear is that some time after they left their home planet of Ouroboros, they got some kind of signal from the "Ancients' Secret" / "Gravity Sphere", and have been trying to decode it ever since. Unfortunately, the game's stories are mutually contradictory on how long this "some time" was. According to the codex, it's 441 "years". According to "Tracking of Time", it's 781 "years". Perhaps there are different definitions of "year" at play here, given that they have no planetary orbit to reference (and, as noted below, these "years" are definitely not equal in length to Earth years).

Anyway, to decode this signal, they have been colonizing other intelligent races in the universe and searching for patterns in their genetic material. Somehow these patterns act as a key that allows them to decrypt the original signal and find out how to make all sorts of contraptions, including the Balls. Apparently they have colonized six so far, and the ones we are most familiar with are the third (us) and sixth (the dead long-headed aliens in this game).

Again, this is the Sky Ones' side of the story:

There's a lot to unpack here. Apparently humanity, assigned the number "Race d9" by the Sky Ones, showed an "unprecedented match to Ancients' waveform". As a result, they spent 466 of their "years" constructing a human colony in a volcano. Of course, this colony is the "Tépétl Nation" from The Ball, and the "nomad" referred to here is none other than the "Ancient King" encountered in that game. The volcano itself is none other than Pico del Miedo, Mexico.

To understand humanity's perspective on the same events, it's best to read the secrets from The Ball itself: https://sp.zhabite.com/sharedfiles/filedetails/?id=166585167
So the 466 "years" spent constructing the colony are reflected in the secrets found (at least) in the level "Pehua" in The Ball. These secrets are told by the people of the Tépétl Nation itself, as they learn to build and use the resources shared with them by the Sky Ones (or "Caretakers" as they are called here). So far so good.

The people who speak in Pehua allude to being challenged by the Sky Ones to figure out how to move water. I'm guessing that these kinds of tests helped the Sky Ones figure out how useful humanity was for decoding the secret. Apparently they spent a further 35 "years" doing this, before deciding they needed more subjects. This is presumably why the Tépétl Nation began subjugating other people as "slaves", who begin speaking from the second level of The Ball, "Oztoc".

Of course, this situation was not sustainable. The Tépétl Nation attempted to wield the power given to them by the Caretakers, but the sheer numbers among the subjugated meant that they would eventually be overpowered. The resonance of the "sub-waveforms" with humanity meant that all people, not just the Tépétl Nation, were influenced by The Ball, having dreams about it and realizing that its power "should" (in some sense) be available to all of them, not just a chosen few – "The gift belongs to all the people under the banner of fire!"

It is not clear if the ensuing "offerings" and eventually "executions" played a role in the extraction of secrets from humanity's genetic material. Clearly there was only ever one Ball on Earth, unlike on Galea, where it is hinted in The Solus Project that children were sacrificed to the Smoke Monster to make several more. Presumably, the executions were carried out by the many Tlaloque (singular Tlaloc) encountered in The Ball – recall that one is also encountered (and specifically depicted beheading rebels) in the Great Mill in The Solus Project.

Anyway, 550 "years" after the beginning of slavery, the one and only Ball on Earth was stolen by the rebellious humans (the "coloti"), leaving the spacecraft ("Phoenix 8882") unpowered. Judging by the secrets beginning from the level "Hueca", the Caretakers used what they had learned about humanity's genome to craft some kind of bioweapon ("a plague"), in the hopes of quelling the rebellion and retrieving the Ball. However, the humans who "worshipped the Ball" used their own ingenuity to seal up the entire colony with a series of puzzles and traps – a "mechanical tomb" – and get the Ball as far away as possible from its original owners. (Before doing this, they apparently carved a myriad of symbols all over it, representing matters of religion known only to them. The Balls in The Solus Project do not have any symbols on them, apart from the models of skulls.)

Anyway, apparently all the ingenuity of the coloti was no match for a curious 20th-century archaeologist when separated from his crew. In The Ball, Harchier Spebbington falls into a hole, finds the Ball, and (presumably under the influence of the "sub-waveforms") solves all the puzzles created by the coloti, restoring the Ball to the power socket on Phoenix 8882. In Earth years, this took place in 1940, and in the Sky Ones' timeline, it took place 5611 "years" after the rebellion. Rumours and stories of this event survived on Earth, and even Sken remembers them in The Solus Project, albeit dismissing them as "wartime paranoia".

Seemingly, these 5611 "years" were not wasted, as further analysis of humanity's genome was carried out. Presumably this was done on all the mummified remains strewn about the caves (before Spebbington's reintroducion of the Ball caused them to reanimate!).

In the course of The Ball, the player interacts with the eponymous object using a handheld contraption that works somewhat like the Gravity Gun from Half-Life 2. The handheld device can attract the Ball towards its user. Based on what we learn in The Solus Project, presumably it does this by awakening the smoke monster inside the Ball, just enough to make it move, not enough for it to break out (as it does when Sken pulls out the Ball's "eye").

What is interesting is how the Ball's "eyes" react to this device. Normally, they glow light yellow, as they also do in The Solus Project. When the Ball is being pulled from afar by the device, however, they turn blue. And when the Ball is fully connected to the device, they turn green. In this sense, the "eyes" seem to do the opposite of what the smoke monster inside is doing. In The Solus Project, we see that these monsters normally glow blue, but turn yellow when chasing a target.

It's not clear who built the device that Spebbington uses to move the Ball around. Given that he uses it to solve puzzles created by the coloti, it's possible that they made it. However, it seems unlikely that they would have such a deep understanding of how to manipulate the internal workings of the Ball, even if they were under the influence of the sub-waveform.

In addition, there's a very slight hint that the same device was used by the rebels on Galea: the tablets describing the theft of the Ball from the windmill on Highpoint mention that its eye glowed blue!

The remainder of "Tracking of Time" helps fill in the blanks here:

Seemingly, 1004 "years" after Phoenix 8882 was released from the volcano in Mexico, the Sky Ones found another species, "Race c7", apparently again with "unprecedented" quality of the preserved key in their genetic code. These of course are the dead long-headed aliens encountered in The Solus Project itself. (Note that there's another apparent contradiction with the "Odyssey Codex" here – according to "Tracking of Time", the code in Race c7 is of even higher quality than in humanity; but according to the Codex, Race c7 has "significant corruption" whereas humanity has only "moderate corruption" :/)

Anyway, it seems they hadn't really learned from their mistakes here. It's true that they only took a very small colony to a new planet, thus ensuring their isolation (in contrast to the Tépétl Nation, who were surrounded by all the other peoples of Earth). However, they still attempted to impose their authority via a "social elite", which had failed spectacularly on Earth. Predictably, the outcome was almost exactly the same, but at least their "new experiments" seem to have allowed them to make many more Balls this time. Apparently, unlike humanity, Race c7 died off completely. This is because the colony was made up of the remainder of their species after their home planet was "doomed".

The most interesting aspect of this story is that the subjects were transported in Phoenix 8882 – that is, the same spacecraft that was powered up by Harchier Spebbington in The Ball! Presumably, this also means that the "Ancient King" encountered by Spebbington is none other than A'Daar Suum, the "great Sky One" mentioned several times in The Solus Project (and ultimately encountered by Sken). This ties in intriguingly with Varsa's Account, where she comments that A'Daar Suum seems not to age, unlike his "brothers". Clearly, this ability helped him out when he was stranded in a volcano for centuries. Did he obtain it from his study of humanity's genome, perhaps?

Anyway, 61 "years" after the failure of the Galea / FC87 operation, Sken arrives, and the Sky Ones decide to study them, and see if more can be gleaned from humanity after the previous unpleasant experience. In the end, as we see in the closing cutscenes of the game, they decide to try colonizing humanity again, albeit with the latter race now in a much more technologically advanced state. It remains to be seen how well this plan works out for all parties involved…

First of all, it is clear that almost all of the events on Earth took place over the course of a single human lifetime. This is evident from the fact that there was only one king (Ixtlilton, the "Son of Quetzalcoatl") of the Tépétl Nation during all this time. This means that the Sky Ones' "years" are at least an order of magnitude shorter than Earth years.

In fact, by checking the length of time between humanity's two encounters with the Sky Ones, we can get an accurate conversion factor. The Ball takes place in AD 1940 on Earth, Anno 11483 on the Sky Ones' calendar. The Solus Project begins in AD 2166 (Earth destroyed in 2151, ships arrive to Galea 15 years later), or Anno 15003 on the Sky Ones' calendar. So the conversion factor is:

(15003 - 11483) / (2166 - 1940) = 15.58 Sky One years per Earth year

Then the 5611 "years" that A'Daar Suum was stranded in the volcano in Mexico are actually only 360 Earth years. This places the end of the Tépétl Nation in 1580, i.e. almost a century after the voyages of Columbus. The plot thickens…

Or maybe there's some time dilation involved in the timing of Phoenix 8882's journey from Earth to Galea. This could place the Tépétl Nation in the Aztec era, which would make a lot more sense – and on that note…

One of the central plot devices of The Solus Project is the fact that Sken's PDA, WILSON, can read and decode (almost) everything written on Galea, despite it being in some strange alien script. Apparently there's a one-to-one correspondence between characters in this script and letters in the Latin alphabet. The in-universe justification is that our histories intersect, which of course they do. Nevertheless, it is highly illogical that these aliens would all be speaking English, and writing it with all the same orthographical oddities that have developed here on Earth. Unless…

My thinking is that the Sky Ones originally used the Náhuatl language. It is present in at least one inscription in the game: the mechanical eyes that fall out of the sky in Flashpoint are marked "Ixtelolotl", which simply means "eye". They would have imposed this language on the Tépétl Nation, and the other peoples under its jurisdiction. In reality, this language was indeed used as a lingua franca in Mexico for some time, even after the Aztec era.

However, with all the time A'Daar Suum spent alone with mummies and Tlaloque in that volcano, perhaps he began to forget Náhuatl, or doubt its usefulness. Perhaps the "sub-waveforms" carried the sounds of other human languages to him, as they were brought into the vicinity and evolved. Eventually, after he escaped, he decided that one of these languages would be more useful for future endeavours. Thus, the Sky Ones used this new language in their dealings with Race c7, and for all the re-transcription of their records on the new planet (as in the aforementioned "Codex" and "Tracking of Time" tablets).

This language might have been English, as depicted in the game, but it could also have been Spanish. After all, Spanish would have been the main language spoken in Mexico during much of A'Daar Suum's captivity. It's also one of the six ♥♥♥♥♥♥♥♥ languages of the UN, so it could easily have been adopted by the Prolus and Solus ships. Its orthography is practically phonetic, so it makes much more sense that there would be a one-to-one correspondence between the human and alien ways of writing it.

So, my headcanon is that, while everything in the game is written and spoken in English, in-universe it is actually all in Spanish!