"Uh-huh."

"And," she continued, "you can read the fossil record back to almost the beginning of life, but as to how life actually arose, again, no one is really sure. We speak nebulously about self-replicating macromolecules supposedly arising spontaneously through some random series of events."

"What are you talking about?" I said, baffled by where all this was going.

"I’m talking about time travel, Dr. Thackeray. I’m talking about why time travel is inevitable."

She’d lost me completely. "Inevitable?"

"It had to come into existence. The future must be able, with hindsight, to rewrite the past." She leaned forward slightly in her chair. "Someday we’ll be able to create life in the laboratory. But we will only be able to do it by reverse-engineering existing life. Something as complex as the universe, as complex as life, has to be reverse-engineered. It has to be built from a known model."

"Not the first time, obviously."

"Yes," she said, "especially the first time. That’s the whole point. Without time travel, life is impossible."

"You mean someone from the future went back into the past and created life?"

"Yes."

"And he knew how to do it because he had the lifeforms from his time as models to study?"

"Yes."

I shook my head. "That doesn’t make sense."

"Yes, it does. For years, physicists have bandied about something called the strong anthropic principle. It says the universe must — must — be constructed in such a way so as to give rise to intelligent life. The purpose of the anthropic principle was to explain the existence of our unlikely universe, which has a number of remarkable coincidences about it, all of which were required for us to be possible."

"For instance?"

Ching-Mei waved her hand. "Oh, just as one of a great many examples, if the strong nuclear force were even five percent weaker than it is in this universe, protons and neutrons couldn’t bind together and the stars wouldn’t shine. On the other hand, if the strong force were just a little more powerful than it is in this universe, then it could overcome the electrical repulsion between protons, allowing them to bind directly together. That would make the kind of slow hydrogen burning that stars do impossible; instead, hydrogen clouds would explode long before they could coalesce into stars."

"I think I’m getting a headache."

She smiled ever so slightly. "That goes with the territory."

"You’re saying someone from the future went back in time four billion years and created the first life on Earth."

"That’s right."

"But I thought the Huang Effect could only go back — what did the diary say? — a hundred and four million years."

"The Huang Effect was a first-generation time machine, created for a very specific purpose. It might not be the only or the best solution to the problem of time travel."

"Hmm. Okay. But it’s not just the creation of life you’re talking about."

"No."

"You’re also saying that someone from the future — the very far future, I’d guess — went right back to the beginning, back some fifteen billion years, and created matter."

"That’s right."

"Created it, with exactly the properties needed to give rise to us, having learned how to do so by studying the matter from his or her own time."

"Yes."

I felt slightly dazed. "That’s mind-boggling. It’s like — like…"

"It’s like we’re our own God," said Dr. Huang. "We created ourselves in our own image."

"Then what about the Sternberger?"

"You’ve read the diary. You know what that other version of you does in the end."

"Yes, but—"

"Don’t you see?" she said. "The Sternberger mission was only one of many instances in which time travel was used to set things right. The flow of events requires periodic adjustment. That’s chaos theory for you: you can’t accurately predict the development of any complex system. Therefore, you can’t just create life and leave it to evolve on its own. Every once in a while you have to give it a push in the direction you want it to go."

"So — so you’re saying that someone determined that the timeline had to be altered in order to give rise to us?"

"That’s right," she said.

"But the time-traveling Brandy wrote that he could hunt dinosaurs, or do anything else, with impunity — that any changes he made wouldn’t matter."

"I’m sure he believed that — he had to, of course, or he never would have done the things that needed doing. It was crucial that he believe that lie. But he was wrong. There was a mathematical string between the Sternberger in the past and the launch point in the present. The changes he made did indeed work their way up that string, altering the timeline as they did so, rewriting the last sixty-five million years of Earth’s history, making our world possible. By the time the string had been hauled all the way back to 2013, the conditions that had given rise to the Sternberger had been eliminated, and our version of the timeline existed instead."

I sagged against the padded back of the steno chair. "Wow."

"Wow, indeed."

"And the other you who invented the time machine?"

She looked down. "I’m clever, but not that clever. I think it was more likely that its birth was induced."

"Induced?"

"Made to happen. The technique must have somehow been given to me from the future, perhaps by little clues or experiments that went a seemingly serendipitous way."

"But why you? Why now?"

"Well, here near the beginning of the twenty-first century we’re probably at the very earliest point in human history at which a time machine could be built, the very earliest that the technology existed to put the parts together, even if we couldn’t really understand the theory behind those parts. In fact, it was necessary that we not fully understand it, that the time-traveling Brandy believe that he’d spin off a new timeline, which he would then abandon, rather than actually change the one and only real timeline."

"So you don’t know how to make a time machine anymore."

"No. But there was one. It did exist. The Sternberger did go back into the past, did change the course of prehistory in such a way as to make our present existence possible."

"But then what happened to that other Brandy? That other you?"

"They existed long enough to make a midstream correction, to steer the timeline in the way it was meant to go."

"Meant to go? Meant to go by — by the powers that be?"

She nodded. "By what we will become. By God. Call it what you will."

My head was swimming. "I still don’t get it."

"Don’t you? The trip by the Sternberger was necessary to adjust things, but it also means that there’s no way another time-travel mission from this present to that part of the past could ever be made to happen again. Once the correction had been made, once the temporal surgery had been performed, the — the incision, shall we call it? — the incision would be sutured up, to prevent any further tampering, lest the correction be undone." She sounded wistful. "I can’t ever build another time machine, and you can’t ever travel in time again. The universe would conspire to prevent it."

"Conspire? How?" And then it hit me. "Oh my God. Oh, Ching-Mei, I’m sorry. I’m so terribly, terribly sorry."

She looked up, a tightly controlled expression on her face. "So am I." She shook her head slowly, and we both pretended not to notice the single teardrop that fell onto the desk. "At least Dr. Almi was killed quickly in that earthquake." We sat in silence for a long, long moment. "I wish," she said very softly, "that that had been what had happened to me."