In prehistoric times, superior beings, that we would call aliens, came to Earth to mine for a substance they depleted on their own planet. Upon arriving, they saw that the species that were on Earth were not intelligent enough to help them mine.
These superior beings decided to mix their DNA with the most intelligent species DNA to create workers. In doing so, they created the human species. Once all of the unknown substance was mined, the aliens went back to their planet, leaving us to live on this planet and fend for ourselves.
This theory explains why we are so much smarter than any other species on this planet. It also explains why there are so many diseases. We didn't develop at the same pace as other animals and the influx of DNA in our systems was too much to handle.