Complex sentence: It seems that orders with aquatic nymphs or larvae become evolutionarily conservative once they have adapted to water, which could partly explain why mayflies are more primitive than dragonflies if mayflies made it to the water first, even if dragonflies should have an older origin.
Split into simpler sentences: It seems that orders with aquatic nymphs or larvae become evolutionarily conservative once they had adapted to water. If mayflies made it to the water first, this could partly explain why they are more primitive than dragonflies, even if dragonflies have an older origin.