I'm still wondering, as it was not revealed in the anime version, why the parasites were sent to Earth? Was there any purpose to it?
This may not be the real purpose (and as Gao Weiwei commented, there will be more hints), but Hirokawa (the leader of the cabal and mayor of the neighbor town) states during a meeting (in which Reiko Tamura was present at the time, but did not object) that Parasytes were sent to earth as a counter to the environmental damage humans were doing.
Whether this was the real purpose, or whether Horikawa was mistaken, or whether he was just pushing some hidden agenda, is unknown.
But the fact that Reiko (the
person one that did the most research on Parasyte's nature) did not object may back that hypothesis.