Ultima

by Stephen Baxter

  Titus insisted that they should stay close to the bank, fearing stronger currents in the middle of the channel—and, just possibly, more aggressive life-forms than they’d yet encountered. Even so, they swept on with what felt like ever-increasing speed.

  Without the physical effort of the march—the hardest work was the daily labor of hauling the craft up the bank for the night—and with Proxima sinking almost imperceptibly slowly in the sky behind them, the days passed in ever more of a blur to Stef. Even so it was a surprise when the ColU announced that they had already been traveling for sixty days.

  The character of the landscape around the riverbanks was changing once more. Much of the vegetation was waist high, and Stef was reminded of the prairies of middle America—or rather, of museum reconstructions she’d seen of such ecologies as they’d been before the climate Jolts. With the air cooler and Proxima lower still, the ground-blanketing “lilies” were no longer so successful, and plants that bore leaves tilted toward the star did better. There were even trees here, or tree-like structures, with big leaves competing for the life-giving light, some stubby and fern-like, some quite tall and rising above the “prairie flowers.” But in this more open country some terrestrial plants fared better too, and the travelers gratefully scooped up handfuls of wild potatoes, yams—even grapes from vines that grew laced over Arduan trees, a cooperation across the two spheres of life that the ColU said it found pleasing.

  The ColU never asked for stops. It seemed too aware of the pressure on them all to make good progress, and to push on with the journey. But sometimes, during their “night” stops, it would ask Chu, or perhaps Beth or Stef, to take it to sites of particular interest. Such as exposed rock formations—which were rare; this Arduan continent was worn as flat as the interior of Australia. And the ColU would ask for samples to be taken, for fossils to be sought.

  “You’ll remember, Beth Eden Jones, how frustrated I used to get! This planet was once so active, chunks of its surface forever churning up, that any fossils were destroyed, the very layers they had formed in disrupted—the whole fossil record was a mess. Now that the world is so much more quiescent, there’s at least a chance of finding some kind of decent record, at least of comparatively recent life-forms . . .”

  But all it ever found were what looked to Stef like matted banks of reeds, compressed into the sandstone and petrified. If there was no significant change, no extinction or evolution, she supposed, you were going to get a featureless fossil record, no matter how well preserved. Nothing but stems for—how long? Millions of years?

  What Stef did notice herself, and she discussed it with the ColU, was an utter lack of evidence of the works of humanity.

  “We know people were here, once, on this Per Ardua. Right? The potatoes and the grapevines wouldn’t be here otherwise. But where are their towns? Oh, the buildings would burn down and crumble away, but where are their foundations, and the waste dumps, and the outlines of farmers’ fields? Where are the remains of their roads and rail tracks? We were on the way to setting up farms and mines all the way to the terminator. But it’s all just as it was at the substellar—gone. How long would it take to erode all that to dust, ColU?”

  But, as always, the ColU refused to speculate about time. “We will know soon enough,” it said. “As soon as I see the dark-side sky. We will know how long then.”

  Once the ColU asked to be taken into a stromatolite garden, where those complicated mounds of bacterial layers, each about chest high, were growing together in a close crowd.

  “Of all the Arduan life-forms I have observed—save only the builders themselves—this is perhaps the most characteristic,” the ColU said happily from its pack on Chu’s back. “And the most enduring. Here, before all the rest, probably, and still growing strong, even now in the end days.”

  And Stef wondered, End days?

  “Yet,” the ColU said now, “there seems to be something subtly different about these specimens. Beth, Chu, do you have knives?”

  It had Beth slice open one of the stromatolites, through its carapace and thick trunk. Within was a greenish mush, vaguely stratified. Beth dug in with her hands, but yelped, “Ow!” and pulled back quickly. “Something bit me . . .”

  She called Chu and Stef over, and, more cautiously, they dismantled the slimy interior of the stromatolite, chunk by chunk.

  Then they found the ants’ nest. Black bodies, big, each maybe a thumb-joint long, came swarming out in protest at the intrusion of daylight.

  The ColU seemed thrilled. “How wonderful! More ecological integration, more cooperation. Perhaps the terrestrial insects feed off detritus trapped in the layers of the growing stromatolite. And the structure as a whole must benefit from the internal mixing-up of the insects. Two life-forms originating on worlds light-years apart, evolving ways to live and work together, for the benefit of all.”

  “I expect there’s a moral lesson in all this,” Beth said drily.

  Again Stef was left with more questions than answers. Yes, she could understand the evolution of a cooperative community like this. But how long had that evolution taken? Time—the great mystery of this new Per Ardua.

  They cleaned off their knives, packed the mushy organic material back into the wounds they’d created, and—with one last silent apology from Stef to the mutilated stromatolite—they returned to their riverside camp.

  • • •

  Ninety days out from the substellar, their faithful river at last flowed into a broader body of water, a lake perhaps, maybe even a sea. It was wide, stretching beyond their horizon, and choked with green life.

  They decided not to try to cross it in the boat. So they camped on shore for a couple of days while reassembling their boat into a cart, and began the process of hauling their way around the lake, hoping to find a way to continue southeast, following their great circle. The haulers grumbled, and Titus chided them for their lack of fitness after so many days on the river.

  After a couple of days they came to what appeared to be a broad isthmus, a neck of land separating their own lake and a neighbor that looked even more extensive. The isthmus led to what appeared to be higher ground to the southeast, densely carpeted with forest.

  With relief that they were able to resume their southwestern course, they continued across this natural bridge. Titus strode boldly at their head, hauling on his harness with the vigor of a man half his age, Stef thought. He was magnificent in this setting, with the light of the slowly descending Proxima glimmering on the water around him, and casting an ever-lengthening shadow ahead of him: he was the last of the Romans, pursuing one last impossible mission. Not that she was about to tell him so.

  They reached the bank of forest at the far side of the isthmus. Compared to the substellar forest, this was sparse, patchy—but, in the long shadows, quite gloomy. Titus and Clodia spent a day scouting out a likely route, and settled on another water course, heading south from the isthmus and cutting a track through the forest.

  On they marched. As Proxima lost ever more height in the sky, so the nature of the vegetation around them changed again. The trees grew taller now, with big flaring leaves that strained to the northwest toward the lowering star, and at their feet the gathering shadows were broken by a greenish glow, reflections from the huge sprawling triple leaves. In some of these pools of illumination they found termite mounds, familiar from Earth, feeding off the reflected light of another star: another cute example of the cooperation of interstellar life in this strange environment.

  They reached yet another milestone: a hundred and twenty days since leaving the substellar camp. When Stef looked back she saw that the disc of Proxima, dimmed and bloated by refraction in the thickening air, now touched the horizon. And when she looked ahead she could see splashes of light, islands in the sky. She remembered this from her last jaunt across the terminator, with Yuri Eden and Liu Tao, long ago. She was seeing the light of Pro
xima catching the peaks of mountains, while their bases were in permanent night, the shadow of the planet.

  That was when Mardina announced she was pregnant.

  65

  They built a camp in a valley of twilight.

  They had walked into the shadow of the world, Stef realized. The sky, laden with thick cloud, was pitch-black. The only light they had, save for their own torches, came from the mountain that loomed over this valley, worn by time but still so tall that its summit and higher flanks were splashed by sunlight, and some of that daylight reflected into the valley below. Stef suspected they had stalled in this last scrap of light, before penetrating the interminable dark ahead, for reasons of instinct as much as logic; they couldn’t bear to leave the unending Proxima day behind.

  Titus Valerius, as always, took charge. First he had them build a camp on a rocky outcrop rising from the generally muddy ground—and it would always be muddy here at the terminator, the ColU had warned, when it wasn’t snowing or icebound. It always rained at the terminator. As warm air from the day side spilled over into the chill of endless night, it dumped its moisture, and the ground everywhere would be waterlogged. But at least on this rock they could build a fire, and sleep out of the damp, and keep any rain off with their tent canopy supported by a frame of stem-tree trunks.

  Then, once they were established, Titus gathered them in the glow of the fire. In the deepening cold they were already wearing extra layers of clothing, stuffed with padding; they all looked fat and clumsy.

  “We’ve done well so far,” Titus said. As he spoke, he ladled out a stew of potatoes and cabbage. “Mostly thanks to the rivers. A hundred and twenty days to these shadow lands, faster than I anticipated. But we’ve still got the same distance to cross again, and in the dark and the cold all the way, as I understand it. Yes?” He looked around at them somberly. “Some of you know this world; I was never here before. Sitting here I find myself uncertain about whether this mission will even be possible, the six of us dragging a cart through the dark for thousands of miles. Well, we must do the best we can. Just as we planned, we will now consider our situation, and prepare for the adventure ahead.”

  Stef smiled at his choice of words. Adventure, not ordeal. The man was a natural leader. Looking around the group, she saw that he held everyone’s attention—everyone save Mardina, perhaps, who seemed unable to eat the cabbage, and was folded over on herself, her knees drawn up to her chest.

  “There are six of us, plus the ColU,” Titus said now. “Four of you, all save myself and Clodia, will make one last effort to gather supplies. Clearly nothing will grow on the ice of the dark side, I understand that, so you must gather what you can from the nearside vegetation that grows in the sunlit areas a little way back, or even on the illuminated peak above us. By the time we leave, our cart must be full, our packs bulging. Perhaps we can find a way to reduce more of the food, to boil it, compress it. If the challenge is too much, we can do this more gradually, setting up a series of caches, pushing deeper into the cold.”

  Stef put in, “At least we’ll have no trouble with warmth, thanks to the Romans’ kernel oven. There will be no trees growing on the farside ice, no fuel for fires.”

  “True,” Titus said.

  Beth said, “So while we’re foraging and boiling potatoes, you and Clodia—”

  “We will be scouting,” Titus said with a grin. “We’ll go exploring into the dark, a little way at least. Looking for a route forward. And looking for a way to shorten this trip.”

  Beth frowned. “How would that be possible?”

  “I’ve no idea. But then, I’ve never been here before.” He glanced at the ColU, which sat on a folded-up blanket. “And, in a sense, neither have any of you, since—if I understand your hints correctly—somehow a great span of time separates this world from the one you knew before. Who knows what might have happened in all that time? Perhaps Per Ardua had its own Romans who left behind a road, straight and true as an arrow, leading us straight to the antistellar.”

  Stef smiled. “I suppose it’s worth a look.”

  Now Titus turned to Clodia. “And I will have you at my side, child, because you will be a valuable companion on such a mission. I’ve seen enough on this journey already to know that.”

  “Thanks,” Clodia said flatly.

  Titus looked at Mardina. “The alternative is for you to stay here and assist Mardina. You may imagine how much I know about pregnancies. Perhaps it would help Mardina to have another young woman at her side.”

  Mardina looked back at him bleakly. “Forget it. My mother’s here. And Stef.”

  “And me,” the ColU said. “Remember my programming. I was designed to fulfill the medical needs of a growing colony. Indeed I administered the birth of Beth Eden Jones herself, many years ago. While I am no longer capable of practical intervention, I can—”

  “You can shut up,” Mardina snapped at it. “That’s what you can do. I don’t need anything. Not yet.”

  Titus glared at her and at Clodia. “At any rate, your rivalry over the boy, Chu Yuen, is over, at least for now. Yes? When the baby comes, you can work out for yourselves how you want to organize your lives, and your loves.”

  Stef smiled at him. “Titus Valerius! I’m shocked. I thought you upright Romans were monogamous.”

  “Different moralities apply on the battlefield.”

  “I wasn’t aware we were on a battlefield.”

  “Tell that to the ice. Why, I remember once on campaign—”

  “Not now, Father,” Clodia said, and she turned her back.

  • • •

  After a night’s sleep Titus and Clodia bundled themselves up in layers of clothing, packed bags, and slipped away, off to the southeast, deeper into the dark.

  The rest got on with collecting foodstuffs and fuel for the fire. Chu, Mardina and Beth explored the diffusely lit valley, and made longer treks back into the lands of daylight. Chu and Mardina also made some climbs up the flank of their mountain, into the island of life and light up there. Beth found the steeper climb all but impossible herself, and she was unhappy about leaving it to her pregnant daughter. But the ColU pointed out the pregnancy was barely begun, its own tests showed that Mardina was as healthy as could be expected, and there was really no reason to hold her back.

  Stef assuaged her own guilt by doing what she could at the camp: refurbishing the cart, preparing the food they gathered, fixing meals.

  And she worked with the ColU at its studies, biological, geological, astronomical.

  The species of vegetation the youngsters brought down from the illuminated summit turned out to be complex. Some of it was familiar, descendants of either Arduan life or terrestrial. But some was stranger, what appeared to be essentially terrestrial root crops but with leaves with a peculiarly Arduan tinge to the green. The ColU grew excited at this, and insisted that Stef dice up samples to be fed into its own small internal laboratory for analysis.

  “Do you remember our own trek to the far side with Yuri Eden and Liu Tao, long ago? We passed these terminator islands of light that I longed to explore. I could see even then that such islands really were isolated from each other, especially as we pressed deeper into the dark, just like islands in an ocean. And just as on Earth, islands are natural laboratories for evolution . . .”

  It took it a full day of analysis before it was prepared to announce its conclusions.

  The remnant ColU unit had only a tiny display screen, meant for showing internal diagnostics of the AI store itself. Stef squinted to see with tired, rheumy eyes. “That’s a genetic analysis,” she said at last. “But there’s a mixture there. Of terrestrial DNA, and the Arduan equivalent . . .”

  “All from the one plant,” the ColU said. “An unprepossessing tuber that you might trip over in the dark. I’m not even sure if it would be edible, for humans—”

  “Just tell
me what you found, damn it!”

  “Integration. A product of a deep integration of the two biospheres. Colonel, this plant is like a terrestrial vegetable, but with Earth-like photosynthesis replaced by the Per Arduan kind—the version tuned to Proxima light, which exploits the dense infrared energy that Proxima gives off. Do you see? In the very long run, it is as if there have been two origins of life on this world, Stef Kalinski. The first origin was when Arduan life emerged—and we know even that was related to the emergence of life on Earth; there was a deep biochemical linkage enabled by panspermia. And the second origin was when humans arrived at this world—Yuri Eden and Mardina Jones and all the rest—and brought with them a suite of life-forms from Earth.”

  “Ah,” Stef said. “The ISF thought they were exploring the stars. In fact they were seeding life.”

  “Ever since Lex McGregor walked here and made his speeches, the dual biosphere has been evolving. At first there must have been extinctions on both sides, as forms unable to adapt to the new conditions went to the wall. After that, over a hundred thousand years, a million years, there must have been speciation as new forms emerged and adapted to the new conditions. New kinds of potato, adapted to the thinner Proxima light. And in ten or a hundred million years, there would be time for integrated ecologies to emerge, as the surviving life-forms evolved together.”

  “Like the ants in the stromatolite. Like bees and flowers, back on Earth. But this is more, deeper, this mutated metamorphosis. A symbiogenesis,” Stef breathed.

  “Exactly. The deepest symbiosis possible, the most intimate life cooperation of all. It is just as the mitochondria in your own body’s cells, Stef, were once independent organisms. They became integrated into your cells to serve as sources of energy, yet they retained their own genetic heredity, a kind of memory of their free-swimming days. Terrestrial life, from amoebas and complex cells upward, is a product of a deep integration of many forms of life. Genesis through symbiosis, indeed.”