I thought for a moment. "Homo neanderthalensis isn't an earlier stage," I said. "There was no more evolution after that."

"Only a disease," said Father.

It seemed too incredible to me, as an eleven-year-old who prided himself on understanding the world. "Human intelligence is an infection?"

"Passed from mother to child through the ovum," said Mother. "By a disease agent that alters the DNA in order to replicate itself. We should have realized it from the fact that in utero development recapitulates evolution, but there is no stage in which the fetus passes through a habiline form. We did not evolve past it. The DNA is hijacked and we are born prematurely, grossly deformed by the disease. Neotenic, erectstanding, language-mad, lacking in sense of smell, too feeble to survive on our own even as adults, in need of clothing and shelter and community to a degree that the Neanderthals never were. But ... smart."

"So now," said Father, "do you understand why medical science has to rely on inoculation to fight off cancer, so that a small percentage-far smaller than ever before in human history, but not zero-a small percentage dies? Elizio died because the only alternative we've found is for this race of perfectly healthy, immortal, dim-wilted beings to inherit the Earth."

I stood there for a long time in silence, watching the Neanderthals, trying to see how their behavior was different from ours. In the years since then, I have come to realize that there was no important difference. Being smarter hasn't made us act any differently from the Neanderthals. We make better tools. We have a longer, more thorough collective memory in the form of libraries. We can talk much more fluently about the things we do. But we still do basically the same things. We are Neanderthals, at heart.

But I did not understand this at the time. I was, after all, only eleven. I had a much more practical -- and heartless -- question.

"Why do we keep this park at all?" I asked. "I mean, they're going to live forever. And all the time they're alive, they pose a danger of this counterinfection getting loose outside the fence. Why haven't they all been killed and their bodies nuked or something so that the counteragent is eliminated?"

Mother looked appalled at my ruthlessness, but Father only patted her arm and said, "Of course he thought of that, my love."

"But so young, to be so-"

"Practical?" prompted Father. "There was a long debate over exactly this issue, and it resurfaced from time to time, though not for decades now: The ones who argued for keeping the park talked about the necessity of studying our ancestors, and some people talked about the rights of these citizens who, after all, can't help their medical condition and have committed no crime, but it was all a smoke screen. The real reason we didn't destroy them all, as you suggested, was because we didn't have the heart."

"They were our children," said Mother, crying again.

"At first," said Father. "And later, when they weren't children anymore, we still couldn't kill them. Because they had become our ancient parents."

Now, though, I have come to think that while they were both right, the answer is even deeper. We didn't kill them, and we continue not to kill them, despite the reality of all those dangers, because they are not "they" at all. There, but for the fact that we happen to be the tiniest bit ill, go we.

I had troubling dreams for months afterward. I had mood swings, alternating between aggression and despair. At times, my parents wished they had just answered my questions about Elizio by taking me to the priest and getting me on the roster of altar boys.

But they were not wrong to take me there, any more than they had been wrong not to tell me up till then. I needed to know before my education was complete. Those who do not know, who continue through adulthood oblivious, in a sense remain children, forever naive. Within the fence of the North American Wild Animal Park is the Garden of Eden, and the people there eat freely of the Tree of Life. Here, outside, in this world of thorns, we dwell in the Valley of the Shadow of Death, madly eating of the fruit of the Tree of Knowledge, as much of it as we can get before we die.

You cannot straddle the boundary. If you bring children into the world on this side of the fence, you must take them to eat the fruit of the tree not too young, not before they're able to bear it. But don't wait too long, either. Let them see, before you die, that death is truly the gift of a merciful God.