Also known as Homo Immortalis, the Eternals are biologically the apotheosis of human evolution. They possess incredible physical and supernatural power, easily on par or mightier than the New Gods dwelling on Earth.
The Eternals originally dwelt on Earth in the city of Titanos, some following Celestial edicts to continue their scientific research and others pursuing their own interests. Strict breeding programs suppressed reproduction rates.
A 'lesser' form of Eternal, called Homo Descendus, were developed as wildcard experiments. These 'Deviants' were not part of the control group and were allowed to roam freely. With a high percentage of superhuman traits the Deviants successfully established a highly advanced culture on Earth well before the rise of any human civilization. They occupied Lemuria and resided in the eco-sphere of the Savage Land.
When Oa and the Elder Gods raised the Demiurge to awareness, the Deviants recklessly launched attacks to stop them. This resulted in Oa culling committing genocide and wiping out anyone carrying dominant Deviant genes.
Around 750,000 BCE a civil war broke out between two dominant factions. Kronos and his brother Uranos came to blows. Uranos' faction lost and were exiled from Earth. These twenty-four Eternals conquered a small Kree outpost on the planet Uranus and claimed it for their own. They were preparing spaceship to return to Earth when the Kree sent a military vanguard and intercepted the Eternals. Four Eternals survived in hiding on Uranus, and seven survived the military encounter. These seven settled on the moon of Titan and developed it into a habitable colony.
The remaining Eternals dwelt on Earth until the events of Ragnarok. Kronos had been communing with Atlantean scholars into the nature of the Power Cosmic prior to the invasion force. He was able to help Atlantis fend off the Celestial war host and preserve Earth. The unexpected arrival of C'thulu overwhelmed him and he was consumed by the power he was unprepared to channel. Kronos was transmuted into a free-energy being and became part of the ideologuic manifestation of Time. Ripple effects of C'thulu's arrival awoke an ancient primordial evil, the Dragon of the Moon. Violence broke out on Titan and another civil war broke out.
After the events of Ragnarok, the Eternals established an outpost called Polaria and resumed diplomatic contact with their cousins on Titan. Eventually the largest body of Eternals relocated to that moon, using it as a platform for making contact with other alien races and dissuading any of them from tampering with Earth. Thanks to their advanced powers and technology they were able to maintain limited contact with the shattered realms on the branches of Yggdrasil, and often stood in for the New Gods in order to maintain the spiritual ties of humanity and the realms of magic. Guided by Zuras as their Leader, the Eternals have kept their presence largely hidden from humanity.
Around 10,000 BCE, there was a substantial population surge as Zuras permitted more breeding pairs following the restoration of the Demiurge. The most infamous Titan, Thanos, was born in this timeframe. After murdering a dozen of his fellow Titans, as well as his mother Sui-San, Thanos fled Titan. He would conquer great swathes of the galaxy and slaughter millions in order to appeal to Death.