“These are your follow-up extinctions,” Jao said to Jorv, “The ones you say sometimes follow the main events thirteen to fifteen million years later. They subdivide the twenty-six-million-year intervals, and they’re growing outward at different rates, so we can’t fit all of them into the picture. Yet.”
“Very thin,” Smeh said.
“So we start with what we do know. Here’s the dinosaurs.”
One of the arms of the original cross thickened and darkened to burnt orange. The tenuous dotted line following it began to blink for attention.
“Okay, anything there?” Jao asked.
Ame and Jorv looked at each other. “The Miocene crisis,” Ame said immediately. “A mass wipeout of shellfish, plankton, some land animals. But it’s only twelve million years later.”
“Close enough,” Jao said. “Now, let’s skip back one hundred and fifty-five million years to the other really major crisis. The one that wiped out half of all animal families on Earth. Have we got a secondary extinction associated with it?”
“Yes,” Ame said, catching her breath. “A bigger one this time. A very large extinction of marine organisms that could fall within a ten- to fifteen-million-year period. We couldn’t fit it into our data before without stretching it.”
“All in knowing how,” Jao said with a grin, while Smeth smoldered.
Jao thickened the axis and shaded it over to include the following arm. “Very interesting,” he said. “The two big ones are at right angles to each other. Forming two arms of the older cross. And it’s the arms sprouting in their wake that turn out to be the real killers.”
“As if whatever it is was getting stronger,” Ame suggested.
“Yah.”
“Jao—”
“Not yet. Let’s fill in as many blanks as we can first.”
Over the next several minutes they assigned eleven extinction episodes to the rotating spokes. Nine of them fit the pattern of the eight major spokes, and two fell within the secondary following position.
“How do you explain the missing pieces?” Smeth asked.
“How should I know?” Jao rumbled. “Insufficient data. Fluctuations in the strength of the spokes. Maybe factors that we haven’t figured yet. The sun catches up with a spiral arm every hundred million years or so and stays inside for ten million years. It bobs up and down through the plane in a thirty-three-million-year cycle, if it behaves like the other stars at that radius. Maybe the dust intensifies the killer effect on some passes. Maybe it does the opposite and acts as a shield. Why don’t you try to combine all the cycles and see what you can work out? The important thing is that everything we do have fits the pattern.”
His belligerence died. Like everybody else in the observatory he was staring at the one big fact that hung before them in the rotating holo image.
“The ninth extinction and the first extinction are doubled up on the same spoke,” Ame said in a half whisper.
“It came back again for a second swipe,” Jao said.
“The second visitation was the last extinction before human beings evolved.”
“The first swipe was the big double event,” Jorv said. “First the trilobites and all that plankton—ninety percent of sea life. Then half of all animal life on Earth.”
“And if there was a … a similar follow-up,” Ame said, “it would have come at just about the same time that Original Man disappeared from the universe.”
Smeth’s harsh voice grated through the ensuing silence.
“It couldn’t be. Man is an intelligent being, not a—a dinosaur! He would have found some way to protect himself. Or flee. After all, it isn’t as if the entire Milky Way was sterilized the way the Father World’s galaxy was. Some life survived each of these—extinctions and went on to evolve.”
“When the dinosaurs disappeared,” Jorv said, his young voice getting away from him, “no species of land animal weighing more than twenty pounds survived. Man’s ancestors were very small and primitive. It was the highly evolved species that went. The second time around, that was man.”
Jao stared thoughtfully at the rotating orange arms of his holo model. “Original Man had only spread a few hundred light-years. At most, a few thousand. You can’t travel faster than the speed of light. You can’t outrun something that extends to the galactic rim and sweeps the galaxy laterally. They could only have caught up with the previous killer arm.”
He retreated into gloomy contemplation. Nobody else seemed very lively, either. Bram was just about to say something, when Jun Davd did it for him.
“Let’s not jump to conclusions,” Jun Davd said. “This is all highly speculative. Jao doesn’t have a theory, just a hypothesis. We need more data. We’ll set up a long-term computer model and keep feeding our observations into it.”
Yah, I’ll get on that today,” Jao said.
Jan Davd went on, “Original Man’s sector of the galaxy is about thirty thousand light-years from the center, and on our outward spiral, we’ll sweep great areas of the disk over a real-time period of tens of millennia. We’ll be able to make observations that were not possible for Original Man, no matter how much further advanced than us he was.”
Ame and Jorv looked puzzled, and Jun Davd added gently, “The time wasn’t there for him, you see.”
As it sank in, the tension began to go out of the room. A few tentative smiles made their appearance. Ame brought her chin up and said to Bram, “You’ve brought us this far, Bramtsu. Nothing can frighten us now. This galaxy is humankind’s heritage, and we’re here to claim it.”
Smeth edged forward, trying to reestablish contact with Ame. “That’s right,” he said. “The whole thing may be nothing more than a statistical fluke, anyway.”
“There’s just one thing.” Jao’s voice brought their heads around again. “If the eight-arm model is correct, our sector of the galaxy is due for another brush soon.”
His hand swept the board, and the rotating orange lines snapped out of existence. The universe came terrifyingly back into sight, a raw torrent of light that slammed them across the eyes and turned the human figures into stark silhouettes. Yggdrasil’s plunge toward periastron had carried it past the black hole’s equator, and the accretion disk had risen out of the floor.
CHAPTER 5
“There’s a star in there somewhere,” Jun Davd said through his suit radio. “Its light may be blocked, but it’s shining its heart out in the ten-micron infrared range.”
Bram clung to a cleat with one gloved hand to keep the trunk’s slow rotation from shedding him into space and turned up the magnification of his helmet visor. “A body-temperature star,” he said. “That fits the picture, all right.”
He peered past the leafy horizon at a void that was frosty with stars again. After more than two decades of braking, the starbow had separated and strewn its baubles across the sky. The drive was off, and Yggdrasil was towing the probe now, not the other way around.
But despite the magnificence of the sprinkled stars, it was the sight in the center of his image compensator that occupied Bram’s full attention.
There was a scratch across the sky—a perfectly straight line, as if a cosmic thumbnail had scraped away the black. Next to the long scratch was a collection of bright squiggles, like cursive writing in an unknown language.
“The straight line’s about ninety million miles long,” Jun Davd said. Within the crystal bowl of the new space suit, his dark profile was intent on the distant object. “That’s about the same length as the radius of the infrared emission shell the instruments can detect. Does that suggest anything to you?”