We often try to understand things by bringing an argument the kind of "if it wasn't good for us, we would had lost it" or "if that's what it is now with us, it must be good, it must be what evolution deemed best for us". I think this argument is sometimes misleading. Indeed evolution always helped finding treats that made the chances of survival higher... right? but survival to what point? There is no evolutionary benefit of living till you are 80 unless you can still procreate. From evolutionary perspective if you managed to have grandchildren, you succeeded, even if you didn't live long enough to see them.
I remember reading a fascinating book "Survival of the Sickest" by dr. Sharon Moalem. I had a chance to meet him informally when he was finishing this book and he is one of the most amazing and passionate people I've met. He is a geneticist, finishing his medical degree and generally awesome person. He was the first from whom I heard about the veggies who don't want to be eaten, the danger of soy and its methods of controlling animal population in it's region, as well as the most interesting of them - how many diseases helped us survive. His point was, that if something helped you survived a bit longer, till you could procreate, it was good from evolutionary perspective - even if it made you die at 35 of diabetes, heart failure, iron poisoning etc.
For example - his theory was that child-onset diabetes helped survive extreme cold temperatures during the small Ice Age (high sugar in blood slows freezing of blood, and prevents creating water crystals that could puncture the cell walls), genetic disorder of extreme high levels of iron supposedly helped survive bubonic plague and so on. I can't remember all, and I lent my book to someone.
So I've been thinking... how much of what we consider "good b/c evolution otherwise would get rid of it" isn't that good at all? And what we have to deal with b/c it is supposed to help us (or our descendants) another ice age (that would be thyroid problems for example)? How much "good" is only in particular consequences and simply is a side effect of our ancestors' temporary (thousands of years obviously) circumstances? We know now, thanks to epigenetics, that DNA can be influenced ("switched on and off") much more powerfully than we thought before, even if we do not understand all the processes involved. I wonder how can we know what was for sure evolutionarily beneficial to the species as a whole (easy one - switching to animal protein and fat helped brain development) and what is a genetic scum left in us as a result of particular circumstances that the whole species or only subgroup (like northern European who had to deal with the small Ice Age) had to deal with for longer period of time?
I know it's more of rambling than a clear question, but I find it fascinating to just ponder the complexity of our nature and evolution, and also the still quite shallow depths of our knowledge about ourselves.
Please let me know if that kind of writing is not allowed, so it can be deleted.