An argument in favor of "enhancement" that I see pop up a lot when reading transhumanist literature is the fact that we already improve upon our bodies/health with vaccines. The major problem with this argument is that it assumes vaccines are inherently good. This is most certainly not the case.
Jonas Salk, inventor of the polio vaccine, came out 16 years after the vaccine had been administered to say that the vaccine itself was the main cause of polio cases during the time of its use. It is also interesting to note that Salk was involved with eugenics, and very interested in the "evolution" of the human species.
There is a host of information linking vaccinations to ill health, a comprehensive list of which exists: here
The point I really want to make in this post is that many transhumanist arguments are lacking in depth. The simple fact is that vaccines are not entirely beneficial (understatement), but the argument that we "enhance" ourselves with vaccines assumes the fact that they are. We have indeed been augmenting our bodies for some time now. What is necessary before going further with this process is to truly question the steps that have already been taken. Exactly how is the human race evolving? What methods are being employed to do so, and are they mutually beneficial to all? Who, or what is the driving force behind this evolution? Take time answering these questions, they are important.