(Longtime readers and Twitter followers might recall that this is a post i've been promising - threatening? - to write for at least a few years. There's not a lot of new material here but it might be new to folks with a less-than-encyclopedic recall for late Bronze age continuity.)
Evolution is the process of gradual biological change in large populations over multiple generations through incremental changes enabled by natural selection. Evolution is not, as is often supposed, random - but it is blind, without any set goal other than the perpetual survival of species. There is no "purpose" to evolution, no ultimate teleological endpoint to which the improvement of species tends - in fact, the very idea of "improvement" is a qualitative judgment wholly alien to the process of selection.
Except that this isn't how evolution works in the Marvel Universe.
Oh, don't get me wrong: natural selection works just fine, and the large majority of natural history on Earth 616 occurred almost identically to our Earth. (There are some differences, many of which can be explained by recourse to the mystical manipulation of the Elder Gods on the primordial Earth, but that's mostly a separate story involving Set and Gaia and the destruction of the dinosaurs.) But for the most part, natural history on 616 proceeded very similar to our own, until sometime around the point where - in our own fossil record - the ancestors of Homo sapiens first split off to form the primate branch that would culminate in our own species. Somewhere along the line of Nakalipithecus or Ouranopithecus, Earth was visited by the first host of the Celestials.
The Celestials tampered with these distant human ancestors and planted the seeds of later development. The Celestials had identified man's ancestors as the most advanced creatures on the planet, with the most potential for the kinds of improvement which would lead to the eventual development of civilization. So they tampered with proto-man, and inserted into his genetic code the source of a three-way divergence in human evolution, a split which culminated in the creation of three distinct branches of Homo sapiens - the godlike Eternals, the savage and genetically unstable Deviants, and the mainline of "normal" humanity. In addition to these three main branches, the Celestials also planted the seeds of a fourth, later development - the creation of Homo superior, mutants.
It is important to remember that in our world evolution is not and cannot be teleological. This is just one reason why eugenics in the "real world" has always been a bad idea. Putting aside every other consideration, it's just terrible science, an idea invented to justify racial distinctions that have no basis in physiology, and which if pursued to its logical extreme would tend towards achieving the opposite of the desired goal - that is, the weakening and attenuation of the organism through the gradual pruning of hybrid diversity. But in the Marvel Universe, human evolution is and has always been teleological: humans on Earth 616 have a roadmap in their genes implanted by 500-foot tall space gods for mysterious purposes. For whatever reason, the Celestials decided that humanity needed to exist and needed to exist for very specific reasons. (It's worth noting that Earth isn't the only planet they altered in this manner - the Skrulls and Kree are also the products of ancient tampering.)
The reason why the Marvel Universe has such a strange origin story at its center is actually quite simple. In the mid-1970s Jack Kirby returned to Marvel from DC and proceeded to create a number of new series for the company, many of them in the same vein as his increasingly weird and highly eclectic output for DC. One of these series was The Eternals, an epic sci-fi story in the mold of The New Gods, predicated on an ancient war between the Eternals and Deviants reaching back to the very dawn of human life on Earth. The series, like much of Kirby's output, was influenced by then-current cultural trends, specifically, the popularity of Erich von Däniken's Chariot of the Gods and the "ancient alien" theory. The series was not considered a success in its time, and a series of creative compromises between Kirby and Marvel editorial ensured that the series' 20-issue run ground to a stop in the throes rapidly diminishing returns.
Perhaps the most significant element in the series failure was Kirby's inability and / or disinclination to properly place the series within the context of the larger Marvel Universe. In the series' early run, there's very little indication - other than throwaway mention of SHIELD and the Thing (of the type which could easily have been inserted at the behest of editorial) - that the series actually takes place in the Marvel Universe. There's little in it that explicitly contradicts continuity, but the very premise of the series was such that, if the series was to be considered "canon," it would change the complexion of the entire line.
Perhaps it might seem like something of an obscure point, but consider the fact that once The Eternals was officially part of continuity, everything in the Marvel Universe had an origin. Every human character - from Spider-Man and the X-Men all the way down to the Punisher - was part of a massive genetic experiment on the part of ancient space gods that literally spanned the whole of human history. The question of whether or not The Eternals could be considered canon was the subject of heated debate during the early months of the series' run. The series' editors were initially hesitant to confirm or deny. Kirby himself was, to all appearances, extremely nonplussed by this reaction. While it is certainly true that he was one of the architects of the system that eventually became known as the "Marvel Universe," he had never shown himself to be spectacularly invested in the propagation of the Universe concept for its own purposes. He was perfectly happy to have a shared universe as long as it allowed him to draw Thor fighting Galactus, but he wasn't invested in the concept in the way that those fans who later became the second generation of Marvel creators were.
The idea of the Universe being a higher goal and purpose in and of itself separate from the considerations of individual creators and their series was probably very unsettling for him. His 70s run on Captain America was very much set in the context of the Marvel Universe, but that was a long-running series with an established history and supporting cast. He didn't invent the Falcon, for instance, but he was perhaps the best writer the Falcon ever had. But the idea of making every idea fit into this singular context was alien to his catholic creative tendencies. Certainly, there could have been no expectation that his 2001 adaptation would ever be folded into the Marvel Universe - and yet, it eventually was, to the extent that Marvel still makes occasional use of the rectangular Monolith from Kubrick's film (something that, according to Tom Brevoort, they've never even bothered to run past MGM). Even Devil Dinosaur was eventually made to fit into the Marvel Universe.
If there was any lingering doubt in the late 1970s that The Eternals was destined to remain a part of the Marvel Universe regardless of its creator's wishes, this doubt was annihilated by Roy Thomas. Thomas devoted a full year and a half of his run on Thor to folding the The Eternals into Marvel history, a series of stories that culminated in Thor #300, wherein the united pantheons of Earth confronted the Fourth Host of the Celestials by animating the Destroyer armor in an attempt to save humanity from the judgment of Arishem. (There's some other interesting stuff in there as well - Thomas wasn't just invested in incorporating Kirby's work into continuity during this run, but also the Niebelungenlied and its various permutations, along with establishing pantheons of god to correspond to all major Earth mythological systems. It is thanks to Thomas, for instance, that the Marvel Universe has an underutilized version of Vishnu who gets together and has lunch with Odin and Zeus. Thomas' influence on Grant Morrison has yet to be widely acknowledged.) From that moment forward, the Eternals, the Celestials, and all their baggage, have been an integral - if oft-ignored - cornerstone of Marvel's cosmology.