Approximately 65.5 million years ago, Earth was locked in an epic battle. Dinosaurs, the native population, were struggling with all they possessed against hostile invaders. This unknown race had hopped over (from, we now guess, one of Neptune’s moons) and landed their mother ships on Earth, setting out to conquer one continent at a time.
The dinosaurs had quickly decided that they could not stand up to the enemy in direct combat, and had moved to guerilla tactics, a word that would not be invented for millennia. Using bows, swords, and crude flint-lock rifles, they ambushed the alien race wherever they could, and also stepped on them.
Unfortunately for both parties, a third force was entering the fray, having latched, groupie-like, onto a passing asteroid. These chitinous, malevolent entities crashed their temporary home into the planet, wiping out the majority of both sides. The dinosaurs, well-used to secrecy, went into hiding, while the few Neptunians who remained were quickly wiped out with freeze rays and cold-fusion bombs.
After waiting out the resulting nuclear winter, the crab people tunneled their huge ships into the crust, using the lava to power their cappuccino machines, and observed the evolution of the planet. They attempted to institute control over primitive life, but found to their sorrow that, unless directly supervised, these creatures were not intelligent enough to avoid low-hanging tree branches. So a large number of them packed up and went to go investigate Venus.
To this day we are not certain what happened on Venus, but currently the entire planet is locked in a giant poisonous cloud and there is no visible life. So we can assume that the crab people hijacked the Venusian right wing and started a successful fascist revolution. Or they gave the Venusians reality-TV technology.
When the Venus expedition returned, they found to their delight that intelligent life had arisen. They immediately began manipulating mankind–but were rudely surprised by the dinosaurs.
The dinosaurs, somewhat more intelligent and mostly now possessed of opposable thumbs, had gone before the crab people, walking among men and teaching them many secrets, including an deep-seated horror of all crawly things which has lasted ever since.
The crab people, their ships now embedded in the planet, were unwilling to start the same losing battle as the Neptunians had sixty million years before, and so they quietly and sinisterly sent their agents among the human society, disguised as hobos, madmen, or journalists.
Even now they watch, and wait, seeking for that moment when they can place a crustacean on the world throne, and seize mankind in their greedy, sharp claws.
Or so I hear.