Evolution seems more like building a tree where mostly all you can do is ascend the tree and add finer detail, leaving the trunk and branches (our evolutionary history) in place. It seems unlikely that, say, vertebrates are in the future going to "undo" the major evolutionary developments of the past and lose their skeleton, body symmetry, number of limbs, lungs, alimentary canal, nervous system, brain, etc. We see things like these developing in the evolutionary tree and mostly staying in place once created. Sure some fins turned to limbs, some gills to ears, but once things like that happened they seem to stay in place.
I wonder what evolution would look like if we could see it sped up from the origin of life to billions of years into the future? A building up of complexity to begin with, but those major branches of the evolutionary tree remaining pretty stable it would seem. Continual ongoing change, but of smaller and smaller scope, perhaps - building on what came before.
Try looking at whale skeletons over time. What isn't beneficial gets undone.
https://en.wikipedia.org/wiki/Recurrent_laryngeal_nerve#Evid...
Similarly, beneficial and complex traits, like eyes, can "regress" if nothing selects against that trait. Plenty of species have lost their sight, making them less generally fit for many environments, because in a certain place and time those species could reproduce even without perfect vision, or just as the result of genetic drift.
I'm guessing there may be at least a couple of reasons for this:
1) Large scale structures evolved over long periods of time, involving layer upon layer of genetic change. This isn't going to be undone quickly or by any localized change, and those rare cases where a genetic change/defect does impact some fundamental aspect of the body plan (e.g. a frog with six legs) are very unlikely to be successful.
2) It seems possible that evolution acts to preserve large scale structure that has proved itself over time, and changes to which tend to be detrimental. In the same way that sexual reproduction seems like an evolution hack to evolve faster, then perhaps animals have also evolved genetic hacks to preserve/stabilize large scale structures that are critical to survival.