Marvel Comics this week fully introduced the lineup of its much-teased Avengers of 1,000,000 BC, a mysterious team of prehistoric heroes assembled by writer Jason Aaron and and artist Esad Ribic for September's Marvel Legacy #1. While Mike Deodato Jr.'s depiction of Ghost Rider atop a flaming mammoth was the undisputed fan favorite, the illustration of Odin, king of Asgard, raised the biggest question: How, and why, was the All-Father wielding the enchanted hammer Mjolnir more than a million years ago?

RELATED: Marvel Reveals Final 1,000,000 BC Avengers: Ghost Rider & Odin

Billed as telling "the startling origin of the Marvel Universe," the 50-page one-shot brings together heroes of the Early Pleistocene: Odin and the Spirit of Vengeance, joined by the extra-dimensional entity Agamotto, a manifestation of the Phoenix Force, a bearer of the Star Brand, and perhaps the very first warriors to bear the mantles of Black Panther and Iron Fist.

"A lot of you want me to know that there were no Homo Sapiens in 1 million BC," Aaron wrote Friday on Twitter. "And I gotcha. But this is the Marvel version of prehistory ... A version with Asgardian gods, cosmic firebirds, mystic kung fu cities, flaming mammoths and ... something else. Something very big. Buried deep in the earth. Something that's the key to understanding where we're going next. And where it all started."

But what about that hammer, which the writer confirmed is Mjolnir?

As Marvel comics (and movie) fans know, Mjolnir has long been the favored weapon of the mighty Thor, not his father. If the hammer was designed for god of thunder, then how can Odin be carrying it a million years in the past?

The basics of Mjolnir are long established: Made of the incredibly durable (but wholly fictional) metal Uru, the hammer was forged in the heart of a dying star. Several enchantments were placed upon the weapon, the most notable being that only someone worthy to wield its awesome powers is able to lift it -- a feat most famously accomplished by Thor.

RELATED: 15 Characters Who Have Wielded Thor's Hammer

However, as part of his massive and overarching Thor saga, Aaron revealed an entire new origin for the hammer last year in The Mighty Thor #12. In that issue, the Jane Foster version of Thor travels to Omnipotence City, where she learns "The Untold Origin of Mjolnir": Long, long ago, a great sentient storm -- the God Storm, the Mother Storm -- the size of a galaxy arrived from the far reaches of space. It was so strong that it pulverized moons and moved planets. Wrathful, it eventually turned its eye to Asgard, where Odin fought against it for days, using all of his immense power to keep the storm at bay. The battle only came to an end when Odin trapped the storm inside a piece of the unbreakable metal Uru, which the dwarves forged, using a burning star as their fire, into Mjolnir.

But even trapped within the weapon, the Mother Storm had a mind of its own. Odin deemed the hammer too dangerous for anyone to wield, and cast an enchantment upon it so that it would never be lifted.

It's possible, then, that Aaron has been planning the story of the Avengers of 1,000,000 BC (or at least parts of the story) for a while. "The Untold Origin of Mjolnir" established that the enchanted hammer predated Thor Odinson by thousands of years -- maybe even longer. So it's certainly not a reach for Mjolnir to have existed in 1,000,000 BC.

Although Odin was recently depicted as unable to lift Mjolnir, he has done so with ease in the past. However, in the teaser image for Marvel Legacy #1, it appears as if the All-Father is having difficulty wielding the weapon; it looks downright heavy. That could be a result of the Mother Storm raging within. In The Mighty Thor #12, it's said the hammer is so old now that the storm is likely dead. But a million years ago? Odin would likely engaged in a constant struggle to keep the Mother Storm, and Mjolnir, in control.

But what threat would be so dire that Odin would risk lifting the enchantment so he could arm himself with an untamed Mjolnir? Something very big, as Aaron teased. Big enough to require not only the king of Asgard, but Earth's Mightiest Heroes of 1,000,000 BC.

Marvel Legacy #1, by Jason Aaron and Esad Ribic, arrives in stores Wednesday, Sept. 27, from Marvel Comics.