In the beginning the aliens came with a simple task – to seed a new world so that at some time in the future they could reap the information.
As advanced creatures, they understood that that information was the final destiny of everything - the seat of all power, thought and energy. While the universe would inevitably count down to nothing (due to the law of increasing entropy) they would live within an information bubble that would sustain their many races and thinking machines for a very, very long time (even they did not know for how long).
The aliens created DNA as a remarkable way of storing information, information that also had the ability to replicate itself. DNA based life became the preferred way to harness the energy from the many suns across many galaxies and turn it into information – increasingly effective with the higher level life that gained the ability to further develop their home planet.
The aliens were continually surprised that so few of the many sentient beings that had evolved from their initial seeding had not rumbled that there must have been an external factor involved. The DNA molecule with its fine double helix zipper and built in redundancy, carefully crafted to provide information transfer and with just the right amount of mutation when copying itself to provide for natures evolution requirements, could hardly have evolved on its own. Simple mathematics would have shown that the probability of such an instrument being created by natural forces required more time than available - even if you spread the task across all the planetary systems within several large galaxies.
(The aliens had long known that their own life forms came from another universe, tunneling into this one just after the big bang, which was itself an external event. They were too wise to worry too much about where sentient life first arose in some dimensionally distant universe.)
Now, on the earth, the human life forms using this complex DNA and lifelong information was stored in some nine billion people, (along with the other life forms and the plants that also created a fine repository of complex DNA based information).
Some seeded worlds produced creatures of significant abilities and sentience. Often these communities created stable long term viability within their own worlds by thinking through the way to survive against the, (always present), natural instincts that tended to destroy environments and create unstable progress. These highly successful life forms were invited to join the aliens in their quest for continuing survival. Most species did, after all, the aliens were not uncaring monsters.
During the (earth) year 2035, it became apparent to the aliens that this particular human farm had probably reached its zenith of production. It seemed likely that the humans would indeed devour many of their own kind and possibly many other life species in their headlong rush of the unsustainable use of resources and pursuit of global economics and materialism as the core factor for future success. Even though as a race they had clearly developed fine minds, they seemed unable to learn from historic events and plan a viable future.
So, the aliens came and harvested the information – turning all the human world’s life forms into processed mush, extracting along the way the fine information stored by many, many generations for over four Billions years. They then re-seeding the planet for another potential one billion years for an expected low level harvest before the sun engulfed it.