Humans are wired for constant self-preservation. We try to protect our physical selves from harmful mutations and diseases, yet we're forever wondering whether a future version will look any different. Does it really matter, then, whether future humans share traits with us today?

Plato, for one, certainly believed so. Writing of the "the other half of his nature", the ancient philosopher was known for a lifelong fascination with the fictional phantom named Panchenyi, the god of the birds, and the idea that the last, i.e. the "ancestral, stranger, unreturnable, mysterious half" exists in parallel with ours. Panchenyi is written about in everything from Plato's Republic to Gravity's Rainbow, from Icelandic myth to Michel Foucault. "Does he differ?" wonders the New Yorker's David Denby, "or does he mirror us exactly, a bundle of microchips and nano-test tubes, ill-equipped to find happiness or happiness's absence, living in a netherworld between our body and the true self, like John Alden himself?"

Panchenyi's disappearance feels like something of a natural outcome of our ongoing love affair with biological sameness. For the last century, people's greatest hope has been that genetics and hi-tech might eventually supply a narrative "truth" about the human race, and the "other half" would join us, like pets or trinkets. Thanks to advances in medicine, now we can also once again believe in the concept of passing down our inherited genes. And now, thanks to genetic-detection methods to reveal the genetics of disease, it's even possible to think of our DNA as a blueprint for our entire lives. So even though there are times when it may be useful to hypothesise about our ancestors, our evolutionary future should be more to the point: you've got a lifetime.

But that still raises the question of what, exactly, to think about those claims that our genetic code will shape our lives. Well, this, for example: why do we, as humans, die by choice, rather than choice by violent accident, as we have in all other species? We evolved with a specific mechanism for choosing to live, or die: when we saw our arch-enemy (or surrogate love interest) approaching, we had to match the intruder's aggression. What evolutionary reason might there be for the human tendency to feel that its very survival hinges on rapidly shifting orientations?