I do not always agree with Zecharia Sitchin but he says that because monkeys don't have opposable thumbs and unable to be worked as slaves, aliens gene-spliced to make humans that were grown in female alien wombs.
The alien women said this was too heavy of a burden and the 'gods' of Sumeria made self-replicating humans.
Just like the black-goo of nanotechnology , self-replicating without natural enemies, humans became the virus of the Earth.
We grow, expand cities, inner city death, growth is only at the edges - just like viruses.
Mentioned in the Matrix with Agent Smith.