Our world is called Wel, and before it was peopled two brothers traversed the void - an infinite black desert - and came upon it. The bottom was lifeless dark water. One brother was Strong, the other Clever. Strong Brother knelt to draw water from the well, and was pushed / fell / threw himself in sacrificed/, and shattered and died at the bottom.
His body rotted and Beasts appeared. Maggots made his flesh to dirt, and left his bones the mountains. His hair became the forests, and the buzzing of flies set in motion the air and made the weather.
Clever Brother looked down, and sweat from his brown fell onto the body of his brother, and grew into the First Men, of which there were 60. Strong were the First Men, the World-Shapers (for the had in them the strength of a god), and they strove with and compelled the beasts to remake the world to their design. And pitiless: When each type of Beast had done its work to prepare the way for the next, they were destroyed. The Firsts cared for nothing but their labours, and lived alone in metal hovels scattered in the wilderness. They were shorter than us, and went unadorned, and their work was nature and had no straight lines.
Clever Brother spat in contempt / to judge the depth / to do penitence by unslaked thirst /, and his spit grew in the world and became the Second Men, the Law-Makers, of which there were 3600. They had language and taste, and would not endure the coarseness and wildness of the First. When they decided the world was well enough wrought, they subjugated the Firsts with Law, waged bloody war against the Beasts which remained, and set the First who would yield against the few who would not, and imprisoned them. The Seconds were very tall and slight, and of great dignity. They laid out charters and set up cities, but were not enough to people them; Their works were geometric monuments, beautiful but empty.
Clever Brother then wept, and his tears became the Third Men, of which there were 216000. They had passion, though not the puissance of the First, or the artifice of the Second. They inhabited the cities and inherited the charters, but saw the wideness of the world and chaffed against the limits that had been imposed. Some wished to cultivate, and they cultivated; Others wished to husband, and they caused vast herds to be raised up to husband; All wished to multiply, and they multiplied. The Seconds were irate to see the narrow strictures of their Law defied, but the Thirds would not yield. Their stature was between their two predecessors, and their appearance and manner better mirrored the nature of Wel. Their works were chaotic and ambitious, frequently failing or developing to unforeseen ends, and lacking overarching design.
The Seconds tightened their grip and, and the Thirds rebelled, and the imprisoned First were broken free and some of their world-altering Beasts were raised up. Then everywhere war raged. Columns of fire lanced from the sky, and great adamantium arrows pierced the bedrock, and seas were boiled, and mountains fell. Everywhere tumult and struggle bent every effort of every man. Clever Brother was nicked by a sling's slight bullet, and a gout of blood fell to the world. It became the Fourth Men.
The Fourth Men were cultivated by every faction. They were not meant to be free shapers of the world, but slaves of war, and they were multitudinous chimera of man and animal. Some had the bodies and the claws of lions, and tore warriors apart to be consumed. Others were like alligators, and could lie in silent ambuscade in still waters. Others had no quality except numbers, and emerged full-formed from pits in the soil to enter raging battle for any master.
Ash then covered the face of the world, and Clever Brother went away to lie sleeping in the darkness, his back to Wel. In the Winter Night the Free peoples eventually prevailed, and the Law-Makers and their servants and slaves died in battle, or as fugitives, but some fled to the far corners and deep recesses of the world. Famine and darkness and rogue Beasts and masterless Fourths lead to an interval of barbarism.
The long night ended, and those only who walked upon the face of the world were called the Fifth Men. They are not the product of Clever Brother, but a mixture of all which came before. They are us.
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The events behind the myth are the two colony ships which arrive to people an empty world. One is deliberately crash landed to provide material for terraforming, and vast bioengineered creatures are raised to effect this under the control of sixty scientists. You can infer the rest from this.