Eternals
The Eternals settled on a high mountain in Greece, and their appearances to early humans generated many of the myths and legends of gods. A schism in their ranks caused a civil war, with the result that a group led by Uranus left Earth and settled on Titan, a moon of the planet Saturn. There they found a Kree Sentry, which alerted the Kree to the newcomers' presnce.
The Eternals remaining on Earth, however, followed a different evolutionary path. Chronos, brother of Uranus, began an experiment intended to give his people long life and health. The experiment exploded, discorporating Chronos, but it had the intended result, and the Eternals became immortal and nearly impossible to kill. The Eternals were also able to develop a group consciousness called the Uni-mind, in which the whole race's pooled thoughts conversed and explored.
Alars, one of Chronos's sons, was overcome by the Uni-mind, but rather than war with his brother Zuras, he also left Earth and settled on the moon Titan, where the other Eternals had lived. Unfortunately, his cousins had attracted the attention of the Kree, and their civilization was destoryed when they used Titan as a base in their war with the Skrulls. Alars found a survivor named Sui-san and married her, founding a new civilization of immortal Eternals on Titan. (Note: According to the Watchman of Earth X, Sui-san was actually a disguised Skrull, though Alars (Mentor) did not discover this until she was killed. Many of the Titanian Eternals would therefore be part Skrull, though with the exception of Thanos, Eternal genes dominate the phylogeny.) This Eternal civiization was also devastated by war, this time at the hands of Mentor's scorned first-born Thanos. However, many of the immortal Eternals survived and rebuilt their homes.
Eventually, the Eternals of Earth went to war with the Deviants, and a long conflict ensued. The Eternals stayed out of mankind's way for many years afterwards. However, Ikaris, who took over as leader when his father Zuras disappeared, dispersed the Eternals to live as "normal" humans across the world. By and large, the Eternals married humans repeatedly over the centuries, but while their children had some altered genomes, since Eternals are still fundamentally human, it rarely resulted in anything but a normal child.
The Eternals have almost never come into contact with the X-Men. Years ago, Ikaris (along with Pixie of the First Line) did help Angel find his teammates when he was lost in the Deviant tunnels used by the Mole Man, but Angel was transformed to stone by Pixie's dust, and so has no knowledge of the event. More recently, Titanian Eternal Starfox was a member of the Avengers for a time, as was Terran Eternal Sersi.
At the turn of the millennium, Apocalypse set in motion events that would bring the Deviants to war with humanity, which he felt would cull the weak from the strong. At the same time, Zuras returned, and disapproved of the way Ikaris had led his people in his absence. Zuras left Olympus, and Ikaris, upon discovering what happened to the Deviants, gathered some of the far-flung Eternals living "normal" lives into a team that stopped Apocalypse's machinations. Calling themselves the "New Breed, " they took new codenames and appeared as a new super-team. However, they have not been seen again since that initial appearance.