Eternals vs. Inhumans...

I'm a long-time Marvelite...which is the current word to use to describe a fan of all things Marvel. The words used back in my day were "Marvel Zombie", which have a completely different meaning now.

As I ponder Thanos' eventual cinematic appearance and contemplate what FOX will do with their FF reboot, I am reminded that I still confuse these two groups.

So, in an effort to get it straight, I perused my handy-dandy resources...

and

 to submit the following...

The Eternals were originally human beings.

Over a million years ago, an alien species called The Celestials came to our galaxy and saw the genetic potential within Homo Sapiens. They collected a "handful of subjects" and performed experiments on them that altered their DNA in such a way that gave the subjects the ability "to mentally manipulate limited quantities of cosmic energy", which is the same cosmic energy Silver Surfer uses.

They didn't become immortal until later.

An Eternal scientist named Chronos (or Kronos) caused an explosion while experimenting with cosmic energy.

Now, Chronos (or Kronos) is the father of Mentor (or Alars)...who is the father of Thanos, as well as Eros (or Starfox)...meaning we may be seeing more than just Thanos in Avengers 2.
Because in a related story, Drax the Destroyer...now a member of Guardians of the Galaxy...was originally a human who died in a car crash and was resurrected as a being bent on destroying Thanos. But Drax had a daughter who was also in the car accident. She ended up being raised by Mentor (or Alars) and other Eternals on Titan and became Moondragon.
But that's another story...

The Inhumans were...wait for it...also originally human beings.

Only these "handful of subjects" were collected merely twenty-five thousand years ago by visiting Kree scientists who learned of the Celestials' experiments and decided to perform their own.

They created a "small tribe of highly genetically advanced human beings"  but unexpectedly abandoned their work and left the solar system.

So the newly created Inhumans wandered around until they found a place to call their own and built a city and founded a government based on "genocracy, rule by the genetically fittest".

It wasn't until much later that a geneticist named Randac isolated the chemical catalyst for genetic mutation he called Terrigen. He subjected himself to this Terrigen Mist and came out of it with powers rivaling that of the Eternals. Being genetically superior to those around him, Randac was named ruler and started the program that allowed all Inhuman citizens to be subjected to the Mist.

The Inhumans have closer ties to the FF than the Avengers, so it's possible we may see them in the Fantastic Four reboot. Of course, the Inhumans wound up on the Moon, which is where Jean Grey became Phoenix. But that's another story...