"You know, none of this explains why there are Neanderthals out here in the North American Wild Animal Park."

"You surprise me, Son," said my father. "I thought you would have leaped to at least some conclusion. Instead you seem to be passively awaiting our explanation."

I hated it when Father patronized me. He knew that, so he did it whenever he wanted to goad me into thinking. It always worked. I hated that, too.

"You brought me here because of the way I reacted to Elizio's death," I said. "And because you're famous scientists yourselves, you got to pull strings and get me a special tour. Not everybody sees this, right?"

"Actually, anybody can, but few want to," said Father.

"And the biohazard stuff-that suggests some kind of disease agent. What you said about the evolved-from-Neanderthals scenario being close to correct suggests ... there's some disease loose in the wild here that causes regular people to turn into cavemen?"

Father smiled wanly at Mother. "Smart boy," he said.

I looked at Mother. She was crying.

"Just tell me," I demanded. "No more guessing games."

Father sighed, put his arm around Mother, and began to talk It didn't take long to explain.

"The greatest breakthrough in the medical treatment of disease was the germ theory, but it took an astonishingly long time for doctors to realize that almost all human ailments were caused by infectious agents. A few were genetic-such as cystic fibrosis and sickle-cell anemia but those all seemed to be recessive genes that conferred a benefit when you had one of them, and killed you only if you had two. All the others--heart disease, dementia, schizophrenia, strokes, nontraumatic cerebral palsy, multiple sclerosis, most cancers, even some crimes-all were actually diseases. What disguised them from researchers for so long was the fact that these diseases were passed along in the womb, across the placenta, mostly by disease agents composed of proteins smaller than DNA. Some were passed along in the ovum. So we had no way to compare a clean, healthy organism with an infected one until we finished mapping the human genetic code and realized that these diseases weren't there. When we finally tracked them down as loose proteins in the cells, we--"

"We?" I asked.

"I speak of our forebears, of course," said Father. "Our predecessors."

"You aren't in medical research."

"Our colleagues in science," said Father. "We've come a long way to have you quibble about my choice of pronouns. And anthropology is the science of which medicine is merely a subset."

I had a snappy retort about how nobody ever asks if there's an anthropologist in the house, but I kept it to myself, mostly because I didn't want to win points here, I wanted to hear the story.

"How do you inoculate an organism against in utero infection?" asked Mother rhetorically. "How do you cleanse an ovum that has already been infected?"

"What we developed," Father began, then interrupted himself. "What was developed."

"What emerged from the development process," said Mother helpfully.

"Was," said Father, "an elegant little counterinfection. Learning from the way these protein bits worked, the researchers came up with a protein complex that hijacked the cell's DNA just the way these infectious agents did. Only, instead of destroying the host cell, our little counterinfection caused the human DNA to check aggressively inside the cell for proteins that didn't belong there. There are already mechanisms that do bits and parts of that, but this one worked damn near perfectly. Nothing was in that cell that didn't belong there. It even detected and threw out the wrong-handed proteins that caused spongiform encephalopathies."

"Now you're showing off, my love," said Mother.

"It was perfect," said Father. "And best of all, self-replicating yet nondestructive. Once you introduced it into a mother, it was in every egg in her body after a matter of days. Any child she bore would have this protection within it."

"It was perfect," said Mother. "The early tests showed that it not only prevented diseases, it cured all but the most advanced cases. It was the ultimate panacea."

"But they hadn't tested it for very long," said Father.

"There was enormous pressure," said Mother. "Not from outside, from inside the research community. When you have a cure for everything, how can you withhold it from the human race for ten years of longitudinal studies, while people die or have their lives wrecked by diseases that could be prevented with a simple inoculation?"

"It had side effects," I said, guessing the end.

"Technically, no," said Father. "It did exactly what it was supposed to do. It eradicated diseases with smaller-than-bacteria agents. Period. Nothing else. The only reason that they didn't immediately spread the counterinfection throughout the world to save as many lives as possible was because of the one foreseeable hitch. Can you think of it? It's obvious, really."

I thought. I wish I could say I came up with it quickly, but my parents were nothing if not patient. And I did come up with it after a few false tries, which I can't remember now. The correct answer: "Aging is a disease. You get this counterinfection, you don't die."

"We were concerned about a population explosion," said Mother. "Even if people completely stopped having children, we weren't sure that the existing ecosphere could sustain a population in which all the existing children grew up to be adults while none of the adults died off to make room for them. Imagine all the children entering the workforce, while the older generation, newly vigorous and extremely unlikely to die, refused to retire. It was a nightmare. So, by the mercy of God, the counterinfection was restricted to a large longitudinal study centered on Manhattan, a smallish college town in Kansas."

"There was a quarantine of sorts," said Father. "The participants accepted the rules-no physical contact with anyone outside the city during the two years of the stud: In exchange, nobody dies of any kind of disease. They jumped at it."

"The counterinfection got loose!" I said.

"No. Everybody kept to the rules. This was science, not the movies," said Father. "But in the Manhattan Project, as we inevitably called it, for the first time the test included infants, newborns, children born after the study began, children conceived after the study began. We were so interested in the result with the aging population that it had never crossed our minds that ... well, it did cure aging. The people who have it would never die of old age. The trouble was, the children were born--"

"As Neanderthals," I said, making the obvious guess.

"And over time," said Father, "as cells were replaced, the adult bodies also tried to reshape themselves. It was fatal for them. You can't take an existing body and make it into something else like that. You had a few years of perfect health, and then your bones destroyed themselves in the frantic effort to grow into new shapes. The little ones, the ones who were changed in the womb, only they survived."

"And that's who I'm seeing out there," I said.

Father nodded. "It took fifteen years to find a way to sterilize them all without our counterinfection undoing the sterilization. By then there were so many, of them that to keep them all in their natural habitat required a vast reserve. It really wasn't all that hard to get the citizens of this area to evacuate. Nobody wanted to be anywhere near Manhattan, Kansas. So once again, Homo neanderthalensis has a plot of ground here on Earth. Homo neanderthalensis, the most intelligent toolmaking species ever to evolve naturally."

"But how could the counteragent cause us to revert to an earlier stage of evolution?" I asked.

"You weren't listening," said Father.