Evolution or ideal design?

To my understanding, scientifically, most of this can be explained by natural selection. As species evolve from one generation to the next, nature dictates which random mutations will be passed on to future generations depending on whether a newly evolved trait enabled the individuals in the population to survive better with it than without. For example, say all humans in a population didn't have opposable thumbs (opposable thumbs allow the digits to grasp and handle objects). You give birth to 4 offspring, one of which suddenly has this ability. That offspring will be better suited to survive than their siblings, and hence will have a better chance of making offspring's of their own.

On a similar note, people often misconstrue the phrase 'survival of the fittest' associated with natural selection, as meaning that survival is the reward for the strongest, the most vigorous, or the most dominant in a population. However, fitness does not necessarily mean strength so much as the capacity to adapt successfully. This might mean developing adaptations for more efficiently obtaining food, or escaping predators, or enduring climate change - in other words, for thriving in a given set of circumstances.

Another misconception is that evolution always progresses to better creatures. But that's not necessary true because if a species becomes too narrowly adapted to a given environment, they may ultimately lose the genetic variation necessary to survive sudden changes. Evolution, in such cases, will lead to extinction.

It seems weird that we can't assume certain things are better. I mean wouldn't a simple nonvertibra be more adaptable and quick to heal unlike humans which don't regenerate much of anything? Most creatures have bilateral symmetry. Most have some type of vision. A lot of abilities tend to keep popping up?

And a lot of features are lost too. The skeletons of humans, for instance, retain evidence of a tail-like structure called the coccyx that is probably a relic from previous mammalian ancestors. It has little apparent function in modern humans, same with wisdom teeth, and perhaps the appendix (personally I don't think so, but most academics would disagree). Relic features such as these are called vestigial structures. In some plant-eating mammals, the appendix is a functioning organ that helps to digest plant material. In humans, however, the organ lacks this purpose and is considerably reduced in size, serving only as a minor source of certain white blood cells that guard against infection. So the take-home message here is that if a feature doesn't improve your overall fitness, nature will eventually cull it out of existence.