When archaeologists discovered the Undersea Ruins, they were left extremely puzzled by the fact that the reliefs and sculptures contained virtually no evolved pokemon. Trainers were depicted in art with evolving pokemon like Marill and Remoraid, and a vast plaza in the city center surrounded by stands was difficult to interpret as anything other than a pokemon arena, yet neither artwork nor biological signatures left any evidence that the people who built the ruins had ever encountered evolved pokemon at all.
Faced with a puzzling mystery which had vexed the scientific establishment for years, a group of experimental archaeologists won permission to attempt a very careful battle in the wreckage of the stadium, with a pokemon list carefully vetted to not damage structures which had stood for 100 years. Both found their pokemon capable of evolving, but they visibly struggled to maintain their shape. One of the participants found the battlefield frustrating, regretted his choice of pokemon until he was reminded this was not an actual match, and wondered why trainers would ever use evolving pokemon here when so many strong basic pokemon could be found in the vicinity. The other, an ex-Rocket with a Dark Slowbro, exploited effects caused by evolution again and again in order to win the match.
It was argued not long after the experiment that some of the art depicting Slowpoke with Shellder in the ruins was actually intended to depict Slowbro, and some Grimer were likewise reinterpreted as Muk. For while evolution was not unknown in the Undersea Ruins, it rarely lasted long enough for artists to depict it accurately, and pokemon, no matter how divergent their evolutions, were almost always regarded by the people who built the Undersea Ruins as fundamentally being their original forms.