[Lacynth40]

Also, let's not muck about with real world science here. About 5000 years ago is the start of any written human language. As far as Mayans at 23000 BCE... The earliest human artifacts in South America have been dated to about 14000 BCE. And the Mayans weren't the first human civilization there. So off by well over half. But, Shadowrun isn't even the same world. It just seems pretty close. Seriously, IRL, New York was not hit by a massive earthquake in 2005, VITAS didn't wipe out a quarter of the world's population in 2010, and the Awakening did not happen in 2011 (Much to my chagrin...). The Earthdawn stuff didn't happen on old Mother Earth here, so we can assume that the archaeology findings have been a little different in the Shadowrun world. It's just close enough to to feel familiar, but it isn't the same world.

[CitizenJoe]

Divergence starts in the 1980's or 90's, but back tracking from there should hold true (or be retroactively explainable as false assumptions due to the new discovery of magic).

It could be that the cycle started when sufficient sapient life was spread enough to cover the globe.  Maybe the first worlders were following ley lines, activating them as they migrated.  Eventually, the ley line pattern activated and started the magic cycles.

[Mirikon]

According to tales told by Icewing and Vasdenjas in the Earthdawn books Horrors and Dragons, respectively, long before the first dragon flew, there was a time of darkness. It was the Age of the Dark One. The Dark One had no Name, as Name-Givers know them. It was simply a being of darkness and corruption. It created the horoi, and delighted in watching them slaughter eachother, making more to replace those that died.

Then, there was one horoi not like the others. This was Nightslayer, the First Named creature in the world. It flew away from the endless battle, and landed upon a lonely outcropping of rock, where it saw the sun. It cried nine tears out of joy, and those tears became Dragon, Elf, Human, Obsidiman, T'skrang, Dwarf, Windling, Troll and Ork. The dragon roared, declaring its name, Dayheart, and then Nightslayer named the rest. Thus, Dragons are unique amongst Name-givers, for they are Name-takers as well. Every Dragon names itself.

The Dark One saw what Nightslayer had created, and made to destroy them. But Nightslayer protected the first Name-givers, and would not let the Dark One harm them. The Dark One cursed Nightslayer, saying, "Ungrateful horoi, you know not what you do. But you will pay for your insolence. I, Verjigorm, will hunt your children for the rest of time. I will slay every last one of them, and my minions will feed on their pain and terror. But I will not give the mercy of death to your favorite--the Dragon, the one you created in your image. The Dragon will know eternal pain. As you betrayed me, the children of the Dragon's line will betray you. I will corrupt them, twist their souls and make them my own. Then I will return to reign over all the world."

Then Verjigorm turned and left with its minions, but not before throwing a great ball of fire at Nightslayer. Nightslayer gathered the nine under his wings, and sheltered them from the blast. The ball struck like a thousand thunderclaps. The earth shook and the sky rumbled, and clouds filled the sky. And when it was done, the First Named was dead, and its children, the Name-givers were left to await the return of the Dark One called Verjigorm.

[CitizenJoe]

That's where I got the Dragons are Horror Spawn. 

Of course, you can't take that story literally, but taken as a metaphor, it can explain many things.  The ongoing battle may have been going on in the Horoi metaplane and the lump of rock Nightslayer found was Earth's metaplane.  The tears represent DNA infusions which changed the local manasphere.  It is conceivable that Nightslayer also infected the local elemental metaplanes to invest creatures with those powers.  So the Obsidiman, T'skrang, and Windling might be invested with earth, water and wind respectively.  Fire and Wood creatures just didn't interact with other people enough to be statistically significant.  Meanwhile, investing humans with this DNA caused divergent expressions, resulting in the elf, dwarf, troll and ork races.

If you want to go real tinfoil hat, you can also assume that Death (the Passion) is Nightslayer and that fireball resulted in Death's Sea, trapping her there.  From there (probably her own metaplane) she protects the world from the Dark One's forces.  You can also assume that the Passions were allied Horoi that followed Nightslayer and the faith of Nightslayer's children is what empowers them and makes them 'good'.  Later on, they get corrupted into what we now call the Seven Deadly Sins.