The Eternals are basically old mythology mixed with sci-fi or cosmic angels. They are not aliens, but actual early hominids (homo-sapiens) that were experimented on by the Celestials.
If you notice by their names, its quite obvious:
- Ikaris's name (Richard Madden) is based on Icarus, the son of Daedalus, who flew too close to the sun that his wings burned and fell to the earth.
- Sersei's name (Gemma Chan) is based on Circe, a Greek enchantress, who has the power to transform anything into something else.
- Thena's name (Angelina Jolie) is based on Athena, the Greek goddess of Wisdom and Battle Strategies.
- Makkari's name (Lauren Ridloff) is based on Mercury, the Roman god of speed and messengers. That's why she's super fast in the trailer.
- Phastos's name (Brian Tyree Henry) is based on Hephaestus, the Greek god of the forge, blacksmiths, etc. He's the inventor of the group.
- The little girl Sprite (Lia McHugh) is based on a sprite, an ancient fairy-like human. She's forever young.
- Gilgamesh (Ma Dong-seuk/Don Lee) is based on Gilgamesh, the ancient Sumerian King.
- Kingo's name (Kumail Nanjiani) is based on Kingu, a Babylonian god
- Druig's name (Barry Keoghan) is based on a druid, a high ranking Celtic priest or teacher
- Ajak's name (Salma Hayek) is based on Ajax, a Greek hero of the Trojan War.
This movie is supposed to take place across thousands of years, so in the Marvel universe, they influenced or had a hand in a lot of human mythology/culture. That's why you see the Ishtar Gate and other Babylonian structures in the trailers. If you read the Neil Gaiman run, they lost their memories as Eternals and have been living on earth as humans for a long time. This is going to be the most interesting and insightful Marvel movie to date if they execute the plot well.