Regarding the claim that without an advantage, left-handedness would vanish due to natural selection and human evolution:
Presently, the human race is deteriorating at a frightening rate. We are no longer evolving, thanks to medical “innovations” and inventions that save lives everyday. We are DE-volving.
Take the common vestigial organ, the appendix. Appendices are vestigial organs that are not only useless, but in some rare cases, deadly. Appendicitis strikes an average 15 people out of 20,000 in the United States each year. Not a very high number, but, still, one would think the number of people with appendices is decreasing because of natural selection. This is not the case, however, because each year about the same amount of appendices are removed, allowing the individual with the faulty appendix (and faulty genes) to live and reproduce, passing those genes down to his/her kids. Children without appendices have been born in the world today, and five million years ago (if the organ was vestigial then, which it was not), the appendix-free population would immediately flourish while the percentage of people with appendices would plummet until the appendix was abolished forever. Now, in this age of mapping the human genome and other great medical advances, the appendix will never be abolished completely.
It is the same with near- and farsightedness. Glasses and contacts repair these problems that would have horribly inhibited the daily function of our ancestors. Not being able to hunt, these individuals inflicted with problems such as this would be forgotten and would die off, natural selection at work once more.
So you see, as long as we keep fixing these genetic defects* that manifest themselves in our bodies as physical problems, the human race will continue deteriorating.
*This obviously does not include genetic defects that are incurable or untreatable, or are completely devastating such as cystic fibrosis.