The Eternals are an evolutionary offshoot of humanity; they are beings possessing greater powers and longer lifespans than the mainstream human race.
The Eternals were created over a million years ago by the enigmatic Celestials, who arrived on Earth to perform genetic experiments on the nascent human race. For purposes unknown, they hoped to test the adaptability of the human gene, accelerating the evolution of a handful of subjects and gave them the genetic potential to mentally manipulate limited quantities of cosmic energy. Thus was born the race of Eternals. In a similar way, the Celestials also performed experiments that led to the creation of the Deviants, and they also implanted latent genetic material in other specimens that would one day permit benevolent mutations in human beings.
At first the Eternals' genetic potential for extraordinarily long lifespans and virtual invulnerability lay dormant. A devastating civil war eventually broke out among the first known generation of Eternals. The two opposing factions, one led by Kronos (also “Chronos”) and one by his brother Uranos, clashed in a conflict of ideologies over their people's destiny. Kronos's side prevailed, and the warlike Uranos and his surviving band of followers were exiled into outer space. These exiles found a supply depot left by the alien Kree on the planet now known as Uranus, and used it to construct a spaceship in which they attempted to return to Earth and renew their war with their brethren. Four Eternals, however, stayed on Uranus and gave rise to a small colony of Uranian Eternals (one which was finally discovered in the 1950’s by Marvel Boy and his father). The rest of Uranos's followers were attacked by a Knee armada before reaching Earth. The survivors among Uranos's band built a civilization on Saturn's moon Titan, giving rise to its own colony of Eternals. One of Uranos's followers, Aflok, was taken by the Kree for vivisection. Learning that the being had derived from Earth, the Kree determined to perform genetic experiments on Earth's pre-humans themselves. Their experiments would result in the Inhumans. Kronos was working in his laboratory in the Earth Eternals' city of Titanos on an experiment with cosmic energy when he accidentally caused an explosion that demolished Titanos and subjected the Eternals to a cosmic particle bombardment. This irradiation activated the latent genes in the Eternals that gave them their long lifespans and virtual invulnerability. Kronos's atoms were scattered, but his astral form still survives on this plane of reality, quietly observing life in the universe and once encountered by Maelstrom who was seeking ways to become a cosmic entity.
After Kronos's physical death, his eldest sons, Zuras and Mentor (A'Lars), initiated the first creation of the Uni-Mind, a collective melding of the minds and bodies of the Eternals. While in the form of the Uni-Mind, the Eternals determined that Zuras should be Kronos's successor as their leader. To avoid disunity among the Eternals, Alars, who differed with Zuras as to what the Eternals' goals should be, left for outer space. Eventually, Alars arrived on Titan, where he met with Sui-San, the last survivor of a civil war among the Eternals there. Together Alars and Sui-San repopulated Titan. However, since Sui-San was not affected by Kronos' cosmic energy bombardment, their children were not as powerful or long-lived as the Eternals of Earth. Alars renamed himself Mentor, and the race of Titanian Eternals, although recently greatly reduced in number, still exists today.
These new Titanian Eternals made many achievements, not the least of which was ISAAC, an immense computer system that could monitor and maintain all of the life support functions of the inner world.
Under A'lars' leadership, Titan became a haven for peace. Then, A'lars (also known as Mentor) sired the last two of his numerous children: Starfox and Thanos. Thanos was born a mutant, and grew up with dark visions of conquest and destruction. Thanos left Titan, recruited an army of mercenaries, and returned to attack. Only 100 of Titan's thousands of inhabitants survived, and Sui-San perished in the slaughter. Eventually, the Titanians rebuilt the colony and restored it to a place of meditation and the pursuit of knowledge.