Meltan: Missing Link between Mew/Ditto and ancient Steel-Types?

To keep it succinct:

-It's long been speculated that Mew "seeded" the Earth and space with the earliest form of Pokemon DNA (probably) creating Ditto.
-Meltan (when caught) turns into Ditto. It doesn't *evolve* into Ditto, however.
-Oak said it was not only mythical, but ancient.

The biggest plot hole for me in the "Mew Seeded Earth" theory was that if Ditto evolved into all these different types, where was the so-called Missing Link between Ditto (a puddle of goop) and a more "structurally sound" pokemon type. What would Ditto have looked like in the interim, before Magnemite but still after millions of years of evolution?

I think we got our answer.

https://imgur.com/a/NgyfuXg