Galactus

"Of all the creatures in the vastness of the Universe, there is none like me. I was present at the birth of the Universe, and I shall be there at its end. Though I ravage worlds to live, I bear no malice toward any living thing. I simply do what I must to survive. And why must Galactus survive? For, no matter how many worlds I devour ... How many civilizations I destroy ... It is my destiny to one day give back to the Universe – Infinitely more than I have ever taken from it. So speaks Galactus!"

- Galactus

As Galan
Galactus is the sole survivor of a universe that existed prior to the current universe. Originally Galactus was a humanoid named Galan, who was born on the planet Taa, a paradise-like world whose civilization is said to have been the most advanced of any of the known universe of that time. However, this universe was in its final stages of collapse. Originally, like all universes in the Multiverse, this universe had been a "Cosmic Egg"; a primal sphere of disorganized, dense, compact primordial matter. The sphere underwent a "Big Bang," an explosion that hurled the matter outwards, where much of it eventually condensed into stars and planets. This universe expanded in size for billions of years, and then contracted, undergoing a "crunch" over the following billions of years. All of the matter of that universe was plunging towards a central point, where it was collapsing into a new "Cosmic Egg." Taa's civilization was one of the last still in existence. Lethal radiation caused by the "Big Crunch" this universe was experiencing was wiping out all life across the universe. Galan, a space explorer, was dispatched to travel through the cosmos to find a means of saving Taa, but he found none. The radiation eventually killed off all but a tiny fraction of the population of Taa. Knowing their deaths were inevitable, Galan proposed to the remaining survivors that they die gloriously by piloting one of their starships directly into the heart of the "Cosmic Egg." As the starship containing Galan and his fellow survivors approached the focal point of the Big Crunch, the heat and radiation killed all the passengers except Galan, who strangely found himself filled with new energy.

Rebirth as Galactus
At the moment Galan's universe met its end, the Phoenix Force amassed the positive emotions of all living beings in the cosmos to preserve them from eternal damnation, enabling the Sentience of the Universe — the previous universe's equivalent to Eternity — to meet with Galan. Within the "Cosmic Egg", the Sentience of the Universe revealed itself to Galan and informed him that though they both would die in the final moments of the universe and they would both survive through a joint heir born into the next universe. The Sentience of the Universe let the mortal Galan gestate within it, and thus Galactus, the devourer of worlds, was conceived. Eventually, the "Cosmic Egg" underwent another "Big Bang," thus creating the current universe. Eternity and Death, the ethereal embodiments of the new universe, were created in this "Big Bang," and the newborn Galactus was simultaneously hurled outward in his recreated starship. The nascent Galactus drifted in his starship for eons as life began to come into existence in the universe, including the races that would give rise to the Elders of the Universe. Eventually the nascent Galactus's starship was sighted by Ecce of the Watchers, an ancient species who possess vast psionic and energy manipulation powers who had undertaken to observe the events of the universe. Ecce drew the starship down to his planet to study it. There he learned that the being within it had power beyond comprehension and would eventually be forced to consume entire worlds to satiate his hunger for life-sustaining energy.