With the templates set in stone, it's time for life itself to appear. Although what I am about to talk about is not noted in any myths but is instead theories from us, it is widely believed to have been Mew due to it possessing DNAs of all Pokemon (or even humans). In the Myths, Mew is said to be the ancestor of all Pokemon and the first tribe to ever exist!
Be careful though, Mew is a tribe, but not a single Pokemon. Many people often make the mistake of assuming that all Mythical Pokemons as one just because we don't know and understand how they produce, not that they can't.
Mew is believed to have been shaped from the Lake Guardians as its base form baring a similar chubby cute look to them and its psychic powers, transforming into any type of Pokemon.
Now you might be asking, how does Mew, a single species, able to become the many species we are known to now? This is where the evolution of species comes in. The first thing we would think about from the word "evolution" was a Pokemon evolving to be stronger, but this kind of evolution might take millions, billions of years, if not millenniums! This type of evolution involves Pokemon earning traits to allow them to be better at adapting their environments for survival from breeding. Although what we call "Pokemon Evolutions" also has the same reason for evolving, this is an establishment of a species instead of just the Pokemon itself.
Oh yes! We are also some of the descendants of Mew! Many people in our world look very similar while many are very different, this is because the majority of us have evolved to look different to others while some look the same because this fits best for their occupation! Well, some are sort of different, but that might just be "regional variants"! Oh, wait... If we are descendants of Mew... And just what if, someone caught a Mew, doesn't it mean that this "someone" just caught their ancestor and is using them in battle? Oh boy, that's deep...
I do have to mention though, and this links to Arceus, the dragon type Legendary Pokemon from the Kalos Region, the Alola Region, the Galar Region and the Paldea Region might be highly related to the "Scales of Arceus" due to them relating to Balance, Light, Chemical and Reflective respectively, which are used to in this scale.
With millenniums passing, Mew soon evolved into the Pokemons (and us) we love and care about! Although some are extinct, that is...
