Planet Earth hasn't been a planet of peace and serenity over the last hundreds of millions of years. In fact, the planet appears to have been a battle ground over who was the greatest "God".

One of the original "god games" on this planet was who can build the biggest tastiest dinosaur. No doubt, some intergalactic fast food chain wanted the best McDinosaur burger around. When the winner was chosen, the losers got ticked off and destroyed all the Dinosaurs.

About 500,000 years ago, "The Annunaki" discovered that gold was a plentiful quantity on Earth. They started their mines in Africa and used their own people as labourers. Many of  the Annunaki spent thousands of years mining and they got tired of it. It was "Hell" to them.

To save their own people, they created a humaniod slave race called a "Lulu". 

The other gods in the galaxy got wind of this improved ape and challenged the Annunaki to a contest to make the best human being in the universe. You can see part of this contest in the Nazis's quest to create "A Master Race". 

The criteria for the winning entry was how well your human fought against other humans. Something along the lines of "The Highlander"(i.e. The winning specimen would chop the heads off of the other contenders.) 

As with the dinosaurs, many of the gods disagreed with the winning entry and/or the criteria for the winning entry. Wars between the gods and wars between their worshippers soon followed. Shadows of these wars are found in video games. 

History become a bloodbath between different tryants (god or man) where victory is oftened measured by "body counts"(i.e. dead bodies). 

Please note that different societies have their own secret histories like the Japanese and Native Americans. 

Earth with its multiple varieties of life forms is a exciting place for alien geneticists and/or advanced beings who want to help humanity.