I'll be reviewing this book formally elsewhere, but it's worth also drawing attention to it here. Agar is an unlikely recruit to the bioconservative ranks, having previously written a book that offers a cautious defence of human enhancement technologies. On this occasion, however, he's produced a lucid critique of what he calls "radical enhancement". As I'll be pointing out in my review, he may not have changed position, exactly, as he's defined radical enhancement fairly narrowly, as involving "improving significant human attributes and abilities to levels that greatly exceed what is currently possible for human beings."
We've also published a lengthy review of this book, by Jamie Bronstein, over at The Journal of Evolution and Technology.
The arguments that seem dearest to Agar's heart, and will, I think, merit most discussion, involve the claim that a posthuman life would be impoverished by human standards (not by the standards of the posthumans themselves, who may be perfectly happy with their lives, or by some kind of objective standard that is inescapably binding on all rational beings). This raises issues about whether such human standards exist and, if so, whether they are the ones we should apply.
Agar wants us to adopt a "species-relativism" about values, which does not necessarily mean that we should be speciesist. Our species-relative values may turn out to involve concern for the interests of non-human animals. However, they may also involve a desire to preserve various kinds of relationships, approaches to life, social institutions, and so on, that would (arguably) have no appeal to posthumans, with their indefinitely long lives and/or vastly augmented, ever-developing intellects. To preserve these things and lead lives that we ourselves see as valuable, we will need to stay human.
I don't necessarily agree with all, or any, of this approach, and will saying more about why in my review, but Humanity's End is certainly an important contribution to current debates about enhancement, emerging technologies, and transhumanist philosophies.