They essentially abide by the Prime Directive from "Star Trek"; that is: The Eternals live by code of noninterference, only becoming involved with human development if they are either being attacked by giant mutant monsters called Deviants (from which the Eternals will defend humans), or if humanity requires a little push to aid in the development of its civilization (in one scene, the Eternal Phastos, played by Brian Tyree Henry, invents the plow, kicking off an agrarian era in Mesopotamia in the 6th century BCE).

The Eternals serve an impossibly ancient and incredibly vast space creature called Arishem, who has a millennia-long-term plan for the them, only known by the team's leader Ajak (Salma Hayek).

The Earth — the planet itself — serves as an egg to an unborn Celestial, and the Earth will be destroyed when it hatches. .

In this broad earth of ours,.

The passage from "Song of the Universal," as it's used in "Eternals" is not only literal — there is literally a seed perfection nestled in the broad Earth — but a direct clue as to what is happening with the Earth.

The Whitman poem — recited by a character named Whitman, hmm — was not the only hint about the Celestials that Zhao dropped in either.

In the Babylon sequences of "Eternals," the character of Makkari (Lauren Ridloff) makes mention of The Emerald Tablet.

The Earth carried it in her belly, and the Wind nourished it in her belly,

It ascends from the earth to the heaven

"The Earth carried it in her belly?" "It ascends from the earth to the heaven and becomes ruler?" These are all pretty open allusions to a powerful space deity nesting in the heart of the Earth

Whether or not ancient humanity knew about Arishem's plan to use the Earth as an egg for another Celestial is unclear; perhaps the author of The Emerald Tablet somehow knew about the Celestial gestating at the planet's core