Marvel Universe Timeline
The following timeline describes the major events of Earth-616. Only major fictional historical events are listed here. Please note that few exact dates have been given for these events, so approximations are used instead. Note also that a sliding timescale applies to Earth-616, so that events involving its inhabitants can only be determined as having happened a certain number of years before whatever the current date is.
Billions of years ago
The Big Bang (approximately 13.7 Billion years ago, according to current data): Prior to the formation of the current universe, another universe existed which was destroyed when the Dweller in Darkness fractured the M'Kraan Crystal, hoping to feast upon the fear that the impending doom would bring to the universe's population. The Phoenix Force, however, reached out to touch every living mind and united them in peace, foiling the Dweller's plan. In spite of this, the destruction of the universe could not be stopped, but Eternity, the sentience of the universe, selected one being - the last living man in that universe, Galan of Taa - to survive and live on, merging with him at the point of universal collapse. Then, initiated by the Living Tribunal, this cosmic egg exploded in the Big Bang (see Oscillatory universe), forming the current universe, and from the cosmic crucible, Galan emerged as Galactus, a key cosmic being in the existence of the current universe. Other cosmic beings (including a reborn Eternity and Death) came into existence at or shortly after the beginning of the universe.
Several intelligent races evolved, but most died out, replaced by new ones as time passed by. Some, however, left behind a single, immortal survivor; these formed a loose brotherhood known as the Elders of the Universe.
One race achieved immortality and great power, but after an attempt to help two other civilizations that ended in their mutual destruction, decided not to interfere anymore, but only to record all events in the universe. They became known as The Watchers. Despite their oath, some have been involved in important events, such as the one appointed as Earth's Watcher, Uatu.
When life started to evolve on Earth (approx. 3 billion years ago) the Elder Gods were born from the planet's natural magical energies. However, most started devouring each other and devolved into demons. One who avoided this fate, Gaea the earth goddess, summoned the Demiurge, the planet's sentient biosphere, and mated with it, creating Atum, the sun god. Atum set about destroying the Elder Gods, but in doing so, he too became corrupted, and was transformed into the Demogorge, eater of gods. A small number of the Elder Gods, such as Chthon and Set, escaped by banishing themselves to other dimensions, and influenced mankind indirectly later on. Chthon left behind the Darkhold, a set of indestructible parchments (later bound into a book) whose black magic caused much evil through history. Only Gaea remained on Earth, where she would guide the evolution of life. Gaea, under many different guises, later become the mother goddess of Earth's second generation of gods. Demogorge then released the energies he had absorbed, returning to his form as Atum; these energies later led to the birth of the later generation of demons, such as Mephisto.
Millions of years ago
The mysterious cosmic beings known as Celestials began to experiment with the evolution of many intelligent beings across the universe, influencing races such as the Kree and the Skrulls.
About one million years ago, the Celestials arrived on Earth and experimented on the ancestors of humanity. This experimentation created two new races, the Eternals and Deviants. The Celestials also used the Earth as a womb or egg to birth a new Celestial. They gave some humans the ability to develop super powers (thus providing the potential for the existence of mutants and other superhumans later on) that would manifest themselves around the time that the Celestial was supposed to be born. They did this with the intent of these superhumans protecting the Earth/egg from damage in order to ensure the Celestials' birth. However, millions of years later the humans, with the help of Reed Richards of the Fantastic Four, discovered this plan and successfully ended it. Around this same time Set had created a new race, the Serpent-Men, to be his agents on Earth. They approached the Celestials, but were rejected for experimentation. The Serpent-Men then became jealous of the humans and vowed themselves enemies of the human race.
The Skrulls discovered the Kree and the vegetated race, the Cotati, who shared the same home world, and gave them a test to see which of them were worthy of sharing their technology: whichever race created the better achievement would get it. The Kree created a city in Earth's Moon (in a 'Blue Area' that contained an Earth-like atmosphere and gravity) for this purpose. The Cotati created an ecosystem. The Kree lost the contest, so they killed the Skrull envoys and stole their technology. They abandoned the city, which would later become a base for Uatu. A century later, the Skrulls discovered what the Kree had done, but by that time, they had become a powerful space empire themselves. The two races have hated each other since and have been fighting wars almost constantly.
Tens of thousands of years ago
The Eternals underwent a series of civil wars. Some of them went on to colonize the planet Uranus and Saturn's moon, Titan. As a result, they missed being irradiated by an accidental explosion triggered by an experiment which turned all of the Eternals on Earth fully immortal. Their enemies, the Deviants, also developed advanced technology and created new races (such as the Subterraneans) and huge monsters that would lay dormant until modern times.
The Kree discovered the Eternals and decided to try to create their own superhuman race out of Earth's humans. The result of this experiment would become the race known as the Inhumans.
The first powerful human civilization developed in the continent of Atlantis. One of its first notable kings was King Kull, who helped to exterminate the Serpent Men. Later, Atlantis developed a highly technologically advanced civilization. They engaged in a war with the Deviants, who ruled the continent of Lemuria. The Atlanteans used weapons that drew upon their continent's geothermal energies, debilitating its geological foundations. 
Just prior to the sinking of the two continents, the Darkhold was used to create vampires, the first of which was Varnae. Around the same time, Set's worshipers created the Serpent Crown, a magical artifact of great power that allowed Set to influence its users.
When the Celestials returned to Earth, the Deviants tried to attack them, which resulted in the Celestials retaliating with an explosion so powerful it wrought havoc worldwide, sank Lemuria, and caused the unstable Atlantis to sink as well. Some Atlanteans managed to survive; some apparently adapted themselves to life underwater via unknown means; this process turned their skin blue and made them able to only breathe water, not air. These Atlanteans became the first Homo mermanus or Homo mermani.
As a result of the cataclysm caused by the Celestials, technology was lost to the human race for millennia, and they reverted to barbarism. During this period, the Hyborian Age, Conan, a descendant of Kull, became an unintentional champion of humanity, and later became king of the land of Aquilonia. One of the sorcerers of this era, Kulan Gath, eventually manifested in the present, along his enemy, Red Sonja. Most of the civilizations of the time eventually became ancestors to actual historical civilizations, like Egypt. (Note: Marvel no longer has the rights to use Kull, Conan or Sonja, but they haven't been officially erased from the continuity; they are just no longer cited by name. Gath, being a character created by Marvel, can still be used.)
Thousands of years ago
The second generation of gods appeared before mankind and were worshiped by pagan civilizations. Most of the legends involving them are true in the Marvel Universe, though not exactly in the form recorded in mythology. Similarly, the second generation of demons began to prey on humanity. Some pretended to be legendary infernal beings such as Satan.
The man who would become Kang the Conqueror traveled back in time to ancient Egypt. He took control of Egypt and ruled as "Pharaoh Rama-Tut" until deposed by the time traveling Fantastic Four. The West Coast Avengers and Doctor Strange were also present via time travel, and they influenced the course of the Fantastic Four's battle with Rama-Tut in their fellow heroes' favor. 
The immortal Mutant; En Sabah Nur, who would later become known as Apocalypse, was also present; he had been a slave in Rama-Tut's court, but was freed when his powers manifested themselves. At the time, he was unsuccessfully attempting to woo a member of the royal family named Nephra; when she rejected him, he in turn rejected humanity.
7th century BC - Ancient Greece dawned. This civilization brought forth great minds, like Socrates and Plato. King Philip II united the Greek states, and his son, Alexander the Great, extended the great culture to other states.
At least a thousand years ago, the Celestials returned to Earth and forced the gods to stop intervening in the development of human civilization.
In 625 BC., the city of Rome was founded and expanded to form the Roman Empire. In 476 A.D., the last Roman Emperor, Romulus Augustus, was defeated by the German Goth Odoacer. This is the start of the Dark Ages in Europe.