Barlow: Right now we're looking out the window at these glorious trees. The only way we have trees that can stand up against the wind and gravity, and move their leaves up toward the light of the sun, is because previous generations of tree cells have died but haven't been immediately recycled. That's what wood is: dead cells that now provide support for the very thin layers of living cells between the wood and the bark.

But the litany of examples that come out of the biological sciences really starts with evolutionary biology. The basic underlying premise of Darwin's theory of evolution by natural selection is the understanding that species have gone extinct in the past. Around the beginning of the 1800s, paleontologists were out discovering fossils and they realized that enough of the world had been explored to be able to state definitively that a certain animal whose bones they'd found—be it a mastodon or a mammoth—no longer existed on the earth. Whether people believed in God or didn't believe in God, there was huge resistance to the idea that nature could have produced something so “imperfect” that it would go extinct.

Once that was understood, Charles Darwin could open his eyes to what he was seeing in the world and visualize the whole complexification of life for the first time in history. It was now possible to see how wave upon wave of species had come into existence and gone out of existence, and that through that process, there were tendrils of complexification. Eyes developed. Fins developed. Wings developed. Neurons developed into congregations that we call brains. All of that depended on the death not just of individuals but of whole species. So fundamentally, if we love ourselves and our consciousness enough to want to keep going forever, then from a biological standpoint, we must embrace death. Death is the reason we're sitting here talking about the prospect of immortality. We're only here because death is creative.

Another example from biology is fetal development in animals. First, egg and sperm come together as zygote and reproduction starts happening, with cells doubling and doubling and doubling. We start off as a sphere. And then, through differential death—that is, some cells go on and reproduce for a certain number of cell divisions, some die earlier, and so forth—we take on shape. The great evolutionary gift of multicellularity is the celebration of form and shape, and that's because of the death of individual cells within the development of creatures.

Neurons begin dying in human brains from infancy, and scientists have always thought that that was just the aging process starting early. But in the last five years they've discovered that the death of neurons actually allows more connections to be made between those neurons that remain. And as we know from complexity theory, and from our own lives, it's the connections that allow creative intelligence to emerge. We've also discovered that even in a healthy adult, there is a winnowing away and a replenishment of cells. It's called programmed cell death. Part of it is due to oxidation, and the people working on life extension are trying to reduce that oxidation. But even with minor injuries, if the body stopped discerning when to recycle the elements from certain cells and start anew, we would be wasting away even faster than we already are. Cells that forget that there is a time in their life that they need to die and stop reproducing—whether they've had their DNA damaged by toxic chemicals or radiation or some other means—are called cancer.

So, from the smallest levels within our bodies to the largest levels out there in the universe, we have a whole nested reality in which death is not just natural, it's creative. It's what allows everything to be. Were it not for death, there would be no such thing as food. Everything we eat was once alive. When you're eating salad, or anything that's uncooked, those cells are still alive right at the moment you're eating. You're killing them as they go into you. Even if immortality comes about in some way, we still can't eliminate death from the whole cycle of life.

WIE: Ray Kurzweil thinks that we're no longer going to need food because our digestive systems will be replaced by little nanobots in our bloodstreams that will administer all the right nutrients at the precise time we need them. Therefore, eating food will become just an aesthetic activity.

Barlow: Once you go the route of thinking about immortality, so much sort of gets swept up along with it. If we got to the point where we were no longer growing food, would we no longer value leaves falling in the autumn and becoming compost? How much of our aesthetic appreciation of the world would change if we didn't have the poignancy of death—of looking at death in the autumn, and the beauty of the leaves, and also having a resonance within us that, especially as we get older, we're in the autumn of our lives too? Would we just completely lose our connection with the natural world? I don't know.

Or take another example. If we were immortal, eventually we'd get to the point where, at least on this planet, we would have to outlaw having any more children. And the warm, fuzzy part of us wants to always go on having babies. But also from the standpoint of cultural development along an evolutionary trajectory, there's something to be said for a world in which you can always have children. There's something to be said for what happens to human beings when their formative years occur at later and later stages of cultural development. I met a woman recently who told me that in her work with kids, she is seeing that this generation of children is remarkably different from previous generations because the world they're growing up in is changing so fast. I often think, “What would I be like today had I grown up with the understanding that my ancestors include the stars?” So much of who we are has to do not with what we read or learn later in life but with how our imaginations form as children.

So I, for one, would not want to live on a planet without death and without children, where we're all just grown-ups together forever. For some reason, people who are into immortality find the prospect of immortality more alluring than the prospect of a mortal life, and I don't go there. I mean, if someone were to give me the choice to actually download my brain and live forever, or have some sort of nutritional supports so I would live forever, I would absolutely say No. I can't imagine a worse hell.

WIE: What if the technologies for immortal life become so widespread and so built into the fabric of everything that it becomes natural to live forever, or at least for a very long time? And all you have to do is take a pill to stop the dying process, to stop all kinds of diseases and degeneration. At some point, people who take the position you're taking are going to have to say, “I choose death.” So I'm curious: At what point would it be? It obviously wouldn't be now, because we all have that choice every day. You could choose death over life today if you wanted to, and you are choosing life.

Barlow: Right.

WIE: Well, how do you know it's going to be any different in fifty years? Maybe in fifty years, you'll still wake up and say, “I'm going to choose life.” And you will. And then in another fifty years, life will still be rich and interesting, and once again you'll want to choose life. Given the option, can you be sure that at some point you would want to choose death over life?

Barlow: The thing is, I don't view it in those terms. The prospect that unless I chose to die, I would live healthily forever, or for a thousand years, is appalling to me. Where would we get our motivation for not putting things off till tomorrow? Or next century? Where would we get our sense of real poignancy in a moment of joy with a spouse? The moments that we feel most alive are when we recognize that our experience is passing, that at some point it will be gone. But if we're always at a point where our experience will never go away—like if I were going to be married for a thousand years—then I just think life would be diminished. It wouldn't be as rich. We would have to develop a whole new psychology, and it ain't the one I've got right now.