Whether and how we descended from animals is debated by scientists. Whether we will descend back into animals is being debated by Parliament. Prime Minister Gordon Brown is pushing a series of bills that would, inter alia, allow the mixture of human with animal cells for scientific research.
There is much to be said on both sides of the debate over embryonic research (verily: both sides talk a lot). But cutting through it all, I believe, is the question of whether one accepts the primacy of currently living adults over all humanity, past and present. Do we accept that we, the living, should alone determine whether and how nascent generations shall live? That is, are we alone human?
C.S. Lewis addresses this question brilliantly in The Abolition of Man. Francis Fukuyama revisits the topic in The End of History and the Last Man, which offers a rationale for Nietzche's "last man" (from Thus Spake Zarathustra). All three see humanity collapsing inward on itself and losing all control over itself precisely when it fully masters its environment.
To these add: when humanity can freely use its own offspring purely for the enrichment of its own material well-being, it has ceased to be humanity in any full sense of the word. Instead of the strong laying down their lives to protect the weak, we will be the strong laying down the weak to protect ourselves. Rather than giving life to future generations, we will be taking it from them. And not only taking their lives, but taking their humanity by admixing them with animals for our own benefit - and perhaps soon for our own amusement.
Thus passes the manhood of Albion.