The Celestials featured in Marvel's Celestials for quite some time, with Guardians of the Galaxy suggesting these towering space-gods are somehow associated with the Infinity Stones and its sequel introducing Ego the Living Planet. Although Marvel has rightly earned a reputation for playing the long game, in the case of the Celestials they've taken things in a completely different direction - one that feels a little more comic-book-accurate at first glance, eschewing the idea Celestials are composed of "living light" and instead making them the beings who wove worlds and galaxies into existence.
There is, however, one crucial different between the Celestials seen in Eternals and those seen in the main comic book continuity. In the comics, the powerful beings called Celestials travel the universe seeking out inhabited worlds, and they experiment upon the beings they find there. They typically create two evolutionary offshoots, Deviants and Eternals, who become locked in combat for millennia. The outcome of this conflict will ultimately determine the planet's fate when the Celestials return to judge it. In the MCU, however, the Celestials seek out worlds with the potential for life in order to implant an embryo within it. The Eternals are sent to guard the precious life-energy that the future Celestial lying at the planet's core will feed on. The Celestials are still creators, but they are not associated with anything like the kind of sophisticated genetic engineering seen in the comics.
This Eternals retcon is actually vitally important when the MCU gets round to introducing mutants. Although mutants are typically described as the next step in human evolution, in reality in the comics the potential for mutation—the crucial "X-gene"—was artificially implanted within humanity by the Celestials. Even Earth X, the miniseries that inspired the MCU's version of the Celestials, found a way to make this idea fit with its cosmology. Because this is the case, many comic book readers had approached Eternals expecting the film to set up the introduction of mutants and the X-Men, assuming it would show the Celestials experimenting on humans millennia ago. But Marvel Studios took a different approach, and now the comic book origin of the mutant race cannot happen—at least not in the same way.
Ironically, this may be for the best. The Celestial origin of mutant-kind in the comics has always sat uncomfortably with the franchise's central themes. While the X-Men are symbolic of diversity, tolerance, and equality, the idea that mutants are a result of something being done to humanity—rather than a natural progression of humanity's evolution—can feel like a clashing concept. Indeed, most comic books quietly ignore this Celestial background, with the writers seemingly aware of the thematic inconsistency. Marvel Studios is wise to avoid the problem outright, and hopefully their version of mutants will simply be the natural next step for humanity, if/when the X-Men's MCU origin actually takes place.
Eternals' portrayal of the Celestials means the biggest question facing the MCU after Disney acquired the bulk of Fox's film and TV empire is still unanswered. Marvel Studios has regained the film rights to the X-Men, and it's only a matter of time before mutants become integrated into the MCU. How that actually happens still remains to be seen. Eternals has avoided one potential origin story, meaning viewers are none the wiser for what approach Marvel will take in the end.
