This is an area where I have fully ambivalent feelings. Many technical assists to human biology - eyeglasses, for example - were viewed with suspicion when they were first introduced, but are regarded as unremarkable now. Life extension is becoming almost alarming. Jonathan's daughter has a life expectancy over 100, and serious scientists believe that lifespans over two centuries are possible quite soon. Genetic tinkering is already well beyond any general philosophical consensus about what is "good for us," and what is a legitimate intervention. If we get good enough at remaking ourselves that human striving becomes nearly irrelevant, or important only at the margins, are we still humans?
Not only do I not know where to draw the line - I don't know by what method we should draw the line. This writer attempts to set up a framework within Natural Law.