WARNING: The following contains major spoilers for Eternals, now in theaters.
In Eternals, the Celestial Arishem created the superpowered group as a way to correct his mistakes of the past and aid in the growth of new Celestials. To prevent the Eternals from getting out of his control, Arishem prevents them from evolving. They never change during the thousands of years spent through Earth's history and are later revealed to be synthetic creations. Yet despite this, their abilities somehow manage to change throughout the Marvel Studios' film.
The Celestials would be perceived as god-like beings to many in the universe, holding the ability to create planets and stars to continue the cycle of creation. Yet Arishem reveals that in order for the new Celestial to be born, they need to emerge from a planet with a high population of intelligent life, with Earth being one such candidate.
To guarantee a planet's population growth, Arishem created the Deviants: creatures designed with the sole purpose of eliminating predators and allowing intelligent life to thrive. However, the Deviants themselves began to evolve to the point where Arishem could no longer control them as they began to retaliate. And so, once again, Arishem created another solution in the form of the Eternals. The Eternals were set to wipe out the Deviants and complete their original task of growing life. Only this time, his creations were to have their mind wiped with every new planet and not given the ability to evolve -- or so Arishem says.
Each Eternal has their own unique powers, such as Sersi's power of matter transmutation. Although she can turn any material into another, the movie makes a point of establishing that Sersi's powers don't work on sentient beings. For example, she couldn't turn a human into metal. Yet around halfway through Eternals, Sersi manages to turn a Deviant into a tree, much to her surprise. With this newfound ability, she uses her powers to turn the emerging Earth Celestial into marble, once again affecting a living being.
Sersi's apparent power change puts Arishem's story into question. He told the Eternals that they are synthetic beings, much like androids. That much seems true since none of them age or look any different after thousands of years on Earth. Yet if they were truly unable to evolve, then Sersi's powers couldn't have possibly changed.
There are a few possible reasons to explain this, with the simplest being that Sersi had the power all along and never realized, which would be strange and a bit convenient. The moment she turns a Deviant into a tree is when she's wresting with one underwater. It's possible that this life or death situation could've triggered the ability to finally work. However, it seems unlikely that she has never been in such a situation before with a Deviant after the thousands of years fighting them.
It's also possible that when Sersi adopted the orb from the deceased Ajak's chest, her abilities were amplified. This orb allowed Sersi to communicate with Arishem and was used to create a uni-mind during the end battle. Therefore, it's entirely possible that orb powered-up Sersi, even if it wasn't specified.
And finally, it could be that the Eternals are able to evolve after all. They're capable of developing their own personalities and decisions, actively going against their creator's wishes, so it's not a stretch to say their bodies could also develop slowly over time. Not to mention their powers may be something separate from their physical bodies as well, such as magic that isn't bound by their body development. Either way, there's a chance that the Celestial was lying all along, which could raise many questions about their intentions going forward.
Eternals is currently in theaters.