Marvel Universe: In the Beginning was Genetic Manipulation

Long, long ago, before recorded history, the Celestials came to Earth. The mysterious, metal-clad, six-hundred-meter-tall cosmic beings experimented with our  genes for mysterious reasons, creating a super-powered offshoot of the human race known as the Eternals. Then, for reasons just as mysterious, they left.

The Celestials

The Eternals built a secret city called Titanos, and were led by the brothers Kronos and Uranos. Uranos wanted the Eternals to conquer the humans, whilst Kronos thought it would be better to just stay hidden in their city and do science. There was a civil war, and Kronos won, because science.

The Eternals by Daniel Acuña

Uranos and his followers were exiled from Earth, and went to live on a planet that they named after their leader - Uranus. Fortunately, many years later, we humans gave it the exact same name, otherwise things might have gotten quite confusing. On Uranus, the Eternals encountered a dormant outpost built by the aliens known as the Kree. Finding a Kree Outpost causes it's robotic Sentry to activate and attempt to kill you in order to preserve the secrecy of the outpost. The exiled Eternals managed to survive and destroy the Sentry. But destroying a Kree Sentry is a crime punishable by death. (This is why nobody likes the Kree.) A Kree force travelled to Uranus to carry out the sentence, but most of the exiles survived by tricking the Kree into thinking they were dead. Whilst in the Solar System, the Kree also decided it was a good idea to experiment on the genes of the human race. Apparently this is a thing aliens like to do to us. Another super-powered offshoot of the human race was created, this time called the Inhumans.

The Inhumans by Jae Lee

Meanwhile, Kronos' son Zuras became the new leader of the Eternals. Titanos had been pretty much destroyed in the war, so they built a new secret city on top of mount Olympus, as well as other smaller cities around the world. These Eternals still live on Earth today, and are often confused with the Greek gods. I have no idea why.

Kronos' other son, A'Lars, decided to leave Earth in order to prevent the possibility of a second civil war, and he and his followers settled on Saturn's moon Titan. These days they are called the Titans, and A'Lars is known as Mentor of Titan.

Iron Man meets the Titan Eternals

 At some point after, during, or possibly even before this, mutants began to emerge. Many people think that the first mutant was Apocalypse, who was born during the time of Egypt's Old Kingdom. But the oldest living mutant is actually, Selene who is a lot older than him, which makes me  think that there must have been others too, just less immortal, and therefore, dead. Apocalypse found a crashed Celestial transport ship, and was contacted by the Celestial known as Exon the Searcher, who revealed that in addition to creating the Eternals, the Celestials had actually created the mutant X-Gene and placed it in to the genetic makeup of the human race. The Celestials then endowed Apocalypse with great powers and tasked him to lead the world into a new mutant age, which set him on a millennia-long genocidal war. Hmm.

Now why do we care about any of this? Well firstly, Mentor's son is very important. He is Thanos, the Mad Titan, one of the Avengers' most powerful enemies. Yes, this means Thanos is actually a distant relative of you and me.

Secondly, it was Mentor who, knowing the damage his son could do, took a mortally wounded human named Douglas Arthur, and turned him into the superhero known as Drax the Destroyer. (Yes, this is a rather different origin than the one Drax is given in the Guardians movie, but I don't think anyone cares because the movie was awesome.)

Thirdly, the Inhumans are important these days due to the fact that Marvel has been trying to turn them into a semi-replacement for the X-Men. They are supposedly doing this to purposely mess up Fox, who own the movie rights (and possibly a lot more) to the X-Men. But I think the real reason is a lot simpler: Marvel want to make an Inhumans movie, because they are one of the few things they still own the rights to. They have been putting a lot of effort into turning the Inhumans from an elite kingdom of separatists into everyday superheroes you would expect to bump into whilst going shopping. Unfortunately, no one cares.

There is significant evidence that Marvel is going to retcon Quicksilver and the Scarlet Witch into being Inhumans instead of mutants. As if their origin wasn't already unnecessarily complicated and completely bonkers. This is going dangerously close to messing-with-Fox territory, and is probably why Fox suddenly shoehorned Quicksilver into Days of Future Past.  But again, I think the true reason for this is simpler: it's an attempt to get people to care about the Inhumans. And to be fair, it may actually work.