Flamsey

Somewhat early on, Olimar makes the following observation in his Voyage Log:

[ALT ID: Olimar's Voyage Log, entry 60, "Lost Civilization": Some of the objects I call treasure seem to be more complex than they initially appear. It makes me wonder... who made these? And where are they? I don't sense their presence here at all, but I suppose they could be hiding in the shadows, secretly watching me. END ID]

This vaguely threatening entry does confirm that the characters, or at least Olimar, are aware that there was indeed a civilization on PNF-404 at some point. It should also be noted that the Japanese website includes notes from another character who is also aware of the ancient civilization, though the canonicity of it is debatable. I'll get back to that "they're in the shadows, watching" line in another post, as it presents its own can of worms. For now, what matters is that the characters are aware that this planet was once inhabited and Nintendo is drawing attention towards that.

Where this starts to get weird and interesting is with Olimar's Treasure Hoard entries on the Heroic Shield, Memory Fragment (Center Right), and Buddy Display. I can't show all of their text in just one screenshot and the pikipedia hasn't added every treasure note to the Pikmin 4 treasures as of writing, so I'll only be including the segments most related to this conversation.

[ALT ID 1: Treasure Hoard, Olimar's Notes, Heroic Shield (segment): That would mean these living beings must have looked something like us... I wonder who they were and what their faces looked like. Could they have been our ancestors who migrated through space to our planet long ago? (the treasure is a pin showing the helmet of a human-style spacesuit) END ID 1]

[ALT ID 2: Treasure Hoard, Olimar's Notes, Memory Fragment (Center Right) (segment): I'm amazed that such a similar species could exist in the far reaches of outer space. I've heard that organic components can travel through space on meteorites and comets. Could there be a connection between my planet and this one? (treasure is of a puzzle piece with part of a dog's face on it) END ID 2]

[ALT ID 3: Treasure Hoard, Olimar's Notes, Buddy Display (segment): I can't find any evidence of them in any of the existing creature databases. But there's something familiar about them. It's almost as if I've met them before... I guess I'll chalk it up to an ancient memory that's been imprinted on my genome. (treasure is a photograph of two earth dogs) END ID 3]

There is so much to unpack here alone. Olimar straight up theorizes that there may be a connection between PNF-404 and Hocotate, even going to far as to propose if maybe the people of PNF-404 (or at least the people who made the Heroic Shield) may be his ancient ancestors. He does also theorize that a meteorite may have carried "organic components", which is technically possible, but I'm not sure how that would occur in a way that carries said components from one non-destroyed planet to another and leading to creatures evolving in eerily similar ways. Honestly, I'll chalk that one up to an early theory of his, a red herring on Nintendo's part, and thus not something to think about too much. Especially sense, in the Buddy Display, he considers his ability to somewhat recognize Earth dogs as an "ancient memory... imprinted on my genome". While I'm not sure of the science behind such a claim, we also have to take this from a story telling perspective - which is that Nintendo is all but screaming "there is a connection here". Not just in relation to the aliens and the planets, but also in relation to the dogs.

Speaking of the dogs, Olimar's notes on the final boss of the game presents yet more evidence to a connection - this time focusing more on the connection between Earth dogs and space dogs.

[ALT ID 1: Piklopedia, Olimar's notes, Ancient Sirehound (segment): These results would indicate that 99.9 percent of the canine creature's DNA matches Oatchi's, and that the only difference between Oatchi and Moss may be the natural presence or absence of a leaf tail. END ID 1]

[ALT IDs 2 and 3, as one naturally leads into the other: Piklopedia, Olimar's notes, Ancient Sirehound (segment): If these two theories prove to be true, it would suggest the possibility that this ancient creature is a common ancestor to two canine species from vastly remote star systems. And that, perhaps, there are even more hidden secrets out there that could hint at a fascinating yet inconceivable truth the universe has yet to reveal... END IDs 2 AND 3]

I'm not sure what else to say here; the evidence speaks for itself. The Ancient Sirehound and Moss - both from PNF-404 - closely match the genetics of Oatchi, who is from an entirely different part of the universe. For the Sirehound to be genetically related to Oatchi, then members of the Sirehound's species would have had to leave PNF-404 and migrate to other planets. Their care takers - humans - would have had to go with them.

I also find it worth pointing out that the Sirehound's arena is filled with various human items, such as shoes and toys. It's honestly a little haunting to find them so deep in a dark cave, unused by humans for who knows how long. The ruined toys in particular really get to me. We don't know how old the Ancient Sirehound in particular is, but the idea that humans once hid in this cave from something or that their dog brought the items here for comfort is really depressing.

[ALT IDs: various hard-to-see images of the human items in question, including: pillows, pipes. gardening tools, a fork, adult sized shoes, baby sized shoes, various children's toys, and other objects I couldn't quite make out. All of them are either partly submerged in water or at the very edge of water, leaving them covered in mud. END IDs]

I think by now it's safe to say that, yes, humans left Earth with their dogs at some point in time. Yet there are still MORE pieces of evidence to suggest this idea, this time coming from Shepherd's family history. Because we really needed to be beat over the head with evidence, I suppose. All are presented in numerical order as shown in-game.

[ALT ID 1: Rescue Journal, Shepherd Family History, entry 1: Thanks to the technology that has come from space exploration, our lives have grown much easier and far safer, and on-planet rescue requests have decreased dramatically. But as migration into space continues to grow, our services continue to be needed. So I'm happy to announce we are extending our rescue services to the frontiers of space! Signed, Paniya Shepherd, 82nd Captain. END ID 1]

[ALT ID 2: Rescue Journal, Shepherd Family History, entry 2: We've done it at last! We have flown to space! Out in this great, limitless expanse, not a sound can be heard. And yet, I can still clearly hear the pounding of my own heartbeat. Space--how it overflows with possibilities! There can be no doubt. This is one giant leap forward for our kind. Signed, Ed Shepherd, 65th Captain. END ID 2]

[ALT ID 3: Rescue Journal, Shepherd Family History, entry 6: The Shepherd family has lived with dogs from the very beginning. We understand that if we love and respect these creatures as we do our own--if we pay attention and observe their behavior closely--then we can break down the walls between our species and truly understand one another. Signed, Kain Shepherd, 16th Captain. END ID 3]

[ALT ID 4: Rescue Journal, Shepherd Family History, entry 7: "Beyond the sky itself there lies a beautiful blue planet." Those words have been passed down from captain to captain, calling to each of us. I can't imagine what sort of place this blue planet might be, but it's said to be home to even more dogs and folks that we have here. One day, I hope to fly beyond the sky...and into space. Signed, Maris Shepherd, 7th Captain. END ID 4]

[ALT ID 5: Rescue Journal, Shepherd Family History, entry 8: Many years have passed since we first began to make this land our new home. Still, we need more time to complete our environmental adaptations. In the meantime, we must survive so that, one day, we can pass down this new history to our children, and pass down the ways we've learned to rescue and protect everyone. One day we will live here in peace. Signed, Mark Shepherd, 2nd Captain. END ID 5]

[ALT ID 6: Rescue Journal, Shepherd Family History, entry 9: The surface! We made it at last! Finally, we can begin making a fresh start here on this planet. Of course, it will take many long months and years before what we've built here will be anything like what we once had. But we won't be alone. Our beloved dogs will be by our side as we walk this long, difficult road. Signed, Aral Shepherd, 1st Captain. END ID 6]

There is so much here, all confirming that ancient Giyans (and Captain Shephard's direct ancestors) 1) migrated to this new planet in search of a "fresh start" and 2) proceeded to migrate to other parts of space many generations latter. It also hints that they know of a "blue planet" - a common nickname for Earth - which is home to more people and dogs. Most certainly, it is the planet they came from - making Giyans human, or at least an evolution of humanity, and perhaps all other alien races in Pikmin 4 (from Hocotations to Koppaites to Ohrians to Conohanins to anyone else from the twenty one named planets (excluding PNF-404) in the series) are also descendent from Giyans. It would explain why they all look so similar in game - though we can safely assume that Hocotations are at least descendent from Giyans based on Olimar's notes. Further more, the leafling cure requires cells from a non-leafified individual who is genetically similar to the individual in question. We're never told how Yonny gets these cells (and, given we're talking about Yonny, I'm too afraid to even ask) but the fact that the medicine works for anyone regardless of what their home planet is suggests that they are, in fact, more or less the same species.

How different the various aliens look from Humans is easily chalked up to evolution. We know minor changes in height could, theoretically, happen within just one or two generations spent on a planet with different gravity than Earth's. It would make sense that this would be exacerbated over many many more generations, especially if the gravity is much stronger. Which, to quote the Pikipedia, "According to Brittany, the gravity on Koppai is roughly ten times stronger than on PNF-404." Given their matching heights, it's probably safe to assume similar applies to the other planets, and thus over the generations the people shrunk. Again, the exact scientific validity of this is questionable, but given the semi-fantastical nature of Pikmin I believe we can suspend our disbelief enough to let this story work. We already have to do that for how the Pikmin as a species function, after all. As for oxygen becoming toxic in the 8th history entry Mark Shephard does state that "we need more time to complete our environmental adaptations". Be these natural or artificially induced (as he is only of the second generation), it is again safe to suspend our disbelief to say that humans just adapted to these new worlds. Besides, Koppaites at the very least retained (or possibly gained back?) the ability to tolerate at least low amounts of oxygen.

The real problems come in the time frame and the loss of history. It's never stated one way or the other if people are aware of their ultimate connections back to Giya, so I won't discuss that, but it is very obvious they have forgotten their connection to Earth/PNF-404. Furthermore, it's made clear that the Giyans lost the ability to travel through space by the 7th generation and only regained it by the 65th. If they had access to space flight, they would certainly have archives of their heritage - and they do indeed seem to have some idea of what it is, given how Earth has become a legend. Yet, the Shephard family history as far back as the migration to Giya was preserved - why not anything else? The inconsistencies and questions make this part of the tale hard for me to ignore, yet we have no answers. This is pure speculation, but if I had to provide an answer I think a solar flare hit Giya and destroyed their technology, making them start over. Perhaps these early Giyain-Humans were so reliant on technology at this point that there were no books or oral traditions to preserve this heritage lost when the tech was destroyed? Though, somehow, the Shephard family would have to be one of if not the exception to this. Again, the previous is pure speculation, but it's the best we can do for the time being.

There's also the question of when and how Earth dogs changed to look like the Sirehound, which would of had to happen before the migration to space given that Oatchi has the same body plan. And then why did Moss's species become so much smaller?