Ponter sighed, then did so. “Then if the unthinkable were to happen, and I were to lose my woman-mate again, I might not be so devastated, since she might not really be totally gone.”

Selgan lifted his eyebrow and both shoulders slightly. “Exactly.”

Ponter rose to his feet. “Thank you for your time, Scholar Selgan. Healthy day.”

“I’m not sure we’re finished yet,” said Selgan. “Where are you going?”

“To do something I should have done long ago,” said Ponter, marching out of the circular room.

Louise Benoît came into Jock Krieger’s office at the Synergy Group. Jock didn’t have any geologists on staff, but Louise was a physicist, and she’d spent all that time working down at the bottom of the Creighton Mine, so he’d assigned this task to her.

“Okay,” she said. “I’ve worked it out, I think.” She spread two large charts on the worktable in Jock’s office. Jock got up from behind his desk and joined Louise at the table.

“This one,” she said, pointing a red-painted fingernail at the chart on the left, “is a standard paleomagnetic chronology made by our people.”

Jock nodded.

“And this one”—she indicated the other chart, which was filled with strange symbols—“is the comparable chart we got from the Neanderthals.”

Even though Mary Vaughan had found no evidence that the Neanderthal magnetic field really had reversed, Jock had used his clout to make the swapping of paleomagnetic information a top priority. If the Neanderthals were wrong about the magnetic field collapsing rapidly, well, then Jock would know he was worrying for nothing. But he wanted to be sure.

“Okay,” said Louise. “As you can see, we’ve mapped a lot more geomagnetic reversals than they have—over 300 in the last 175 million years. That’s because there’s a more complete record in sea-floor rocks than there is in meteorite finds.”

“Score one for our side,” said Jock, dryly.

“So,” continued Louise, “what I’ve done is pair up the reversals that do match—that is, the ones that both they and we have evidence for. As you can see, although their record has many holes in it, there’s a one-to-one correspondence almost all the way to the present.”

Jock looked at the sheets, Louise’s finger guiding his eyes. “Okay.”

“Well,” said Louise, “that makes perfect sense, of course. You know my theory: that there was only one long-term universe until consciousness dawned forty thousand years ago.”

Jock nodded. Although quantum-mechanical events caused countless brief splittings of the universe, and probably had since the beginning of time, those splittings made no macroscopic difference, and so the resulting universes had always collapsed back together after a nanosecond or two.

But the acts of conscious beings caused splits that could not be healed, and so, when the Great Leap Forward took place forty thousand years ago—when consciousness emerged—the first ever permanent split occurred. In one universe, Homo sapiens acquired that initial consciousness; in the other, Homo neanderthalensis did—and they had been diverging ever since.

“But wait a minute,” said Jock, peering at the Neanderthal chart. “If that one there is the last recorded magnetic reversal that we know about—”

“It is,” said Louise. “They’ve got it listed as about ten million months ago, which is 780,000 years ago.”

“Okay,” said Jock. “But if that’s the most recent one on our chart, what’s this one here?” He pointed to what was apparently another, more-recent reversal indicated on the Neanderthal chart. “Is that the one they said began twenty-five years ago?”

“No,” said Louise. She had too much of the academic in her for Jock’s taste. She was clearly leading him to make his own discovery, but she obviously already knew the answer herself. He wished she’d just tell him.

“Then when was that one?”

“Half a million months ago,” said Louise.

Jock made no effort to hide his irritation. “Which is?”

Louise’s full lips spread into a grin. “Forty thousand years ago.”

“Forty thou—! But that’s when…”

“Exactly,” said Louise, pleased with her pupil. “That’s when the Great Leap Forward occurred, when consciousnessemerged, when the universe split apart for good.”

“But…but how is it that they know about a magnetic reversal then and we don’t?”

“Remember what I said the first time we were talking about this? After the magnetic field dies away, it’s a fifty-fifty chance as to what polarity the new field will come up with. Half the time, it’ll be normal, and—”

“And half the time it will be reversed! So this event must have happened after the universes split—and since the universes were no longer in lockstep, it happened that the polarity came up reversed in the Neanderthal world—”

Louise nodded. “Leaving a record in meteorites.”

“But in our world, it came up with the same polarity it had had before the collapse—leaving no record.”

“Oui.”

“Fascinating,” said Jock. “But wait—wait! They had a reversal forty thousand years ago, right? But Mary says that when she took a compass reading in the Neanderthal world, it now has the same polarity as our world does, so…”

Louise nodded encouragingly; he was on the right track.

“…so,” continued Jock, “there was a recent, rapid field collapse in the Neanderthal world, and this time, when the field came up again, just six years ago, it had flipped its polarity once more, back to matching what it is on this Earth.”

“Exactly.”

“All right then,” said Jock. “Well, that’s what I wanted to know.”

“But there’s more to it than that,” said Louise. “Much more.”

“Spit it out, girl!”

“Okay, okay. It’s like this. Earth—the one and only Earth that existed at that time—experienced a magnetic-field collapse forty thousand years ago. While the magnetic field was down, consciousness emerged—and I can’t think that that’s a coincidence.”

“You mean the collapsing of the magnetic field had something to do with why we developed art?”

“And culture. And language. And symbolic logic. And religion. Yes.”

“But how?”

“I don’t know,” said Louise. “But remember, anatomically modern Homo sapiens have existed since one hundred thousand years ago, but they didn’t get consciousness until forty thousand years ago. We had the same physical brains for sixty thousand years without ever making art or exhibiting any of the other signs of true sentience. Then—click!—something happened, and we were conscious.”

“Yes,” said Jock.

“You know some birds use magnetite in their brains to tell direction?”

Jock nodded.

“Well, we—Homo sapiens—have magnetite in our brains, too. No one knows why, since we obviously aren’t using it as a built-in compass. But when the magnetic field collapsed forty thousand years ago, I think something happened to the magnetite that caused the—the ‘booting up,’ shall we say, of consciousness.”

“So what’s going to happen when the magnetic field collapses again?”

“Well, on the Neanderthal world, nothing happened during their most recent collapse,” said Louise. “But…”

“But?”

“But they don’t use fossil fuels. They don’t have billions of cars. They don’t use chlorofluorocarbons for air-conditioning.”

“Yes? So?”

“So their atmosphere—and their ozone layer—is completely intact. Ours isn’t.”

“What’s that got to do with magnetic reversals?”

“Earth has two methods for shielding its surface from solar and interstellar radiation: the atmosphere, and the magnetic field. If one goes down, the other covers for it…”

Jock’s eyes went wide. “But one of ours is already down.”

“Exactly. Our ozone layer is depleted; our atmosphere is chemically altered. When the magnetic field collapses again—and it looks like it’s starting to do that right now—there’s not going to be any backup shielding in place.”