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Pokemon are a single, super-adaptable extraterrestrial species that has supplanted much of earth's ecosystems in a few hundred years, capable of widespread interbreeding and rapid metamorphosis when exposed to extreme conditions (such as continual battle) and all stemming from a single genetic progenitor (Mew, as created/brought to Earth by Arceus).
Humanoid Pokemon like Mr. Mime or the Machop line are the collective Pokemon DNA attempting to replicate and replace more extreme instances of humanity, failing in "cute" way.
Since the mass depopulation and infrastructure damage more recent world wars brought about by fully harnessing Pokemon in digital yokes (recall Lt. Surge) a society populated mostly by children, young adults, and the elderly has been slowly rebuilding itself from small, isolated towns and medical centers into one in which Pokemon are, instead of merely weapons of war, an omnipresent food/labor source that humanity lives symbiotically with.
Beyond Arceus, whose intentions in introducing Pokemon to Earth remain unknown, Deoxys is the sole example of a Pokemon born in conditions beyond our world and an example of Pokemon DNA adapting outside of Earth's ecosystems. Porygon is the sole example of a purely artificial Pokemon, born from the new technologies used to reduce Pokemon into a digital signature for easy transportation, storage, and enslavement. Mewtwo and Ditto were both attempts to replicate Mew using genetic engineering, both having failed in different, and spectacular, fashion.