Soon Beth was asleep; she had a soft, gentle snore.

Yuri had time to inspect the route they were following. After all, it was the last time he expected ever to come this way. The ColU was following its own tracks along the bank of a broad, braided river bed. Like most of the channels down which the builders guided the flow of their lake this bed had been here already, but was dry as bones before the lake came. Now the bed was littered with the detritus of the passage of the waters of the lake: snapped stems, a few broken builder traps, dead aquatic creatures from fish analogues to crab analogues and jellyfish analogues, and others they had yet to identify. There was even some terrestrial-origin seaweed, the gen-enged laver brought to this world by the starship Ad Astra.

After years of observation, even the ColU had no real idea how the builders managed these hydrological transfers so effectively. The lake stayed in stable locations for months or years at a time – it had turned out that the site where the shuttle had landed had been the longest stay so far, and in fact the intervals between moves were generally getting shorter. It was clear the builders used existing water courses, although they would sometimes dig out or extend canal-like connecting passageways, and their characteristic middens were used to guide the flow of the water precisely where they wanted it to go.

And, wherever the lake finally pooled, there were always local streams and springs to feed it. The mystery of that was that as the land’s wider uplift continued – and the ColU constantly reminded them that some dramatic geological event was apparently unfolding to the north of here – the pattern of the region’s springs changed all the time, as underground aquifers were shifted or broken, the water tables realigned. The builders always seemed to know in advance where the useful springs would be, and how to re-establish the lake. The builders didn’t have maps, but they evidently knew about geography; they must be able to visualise the landscape in some way.

It was as Yuri mused on this that the ColU’s theorising broke into his day.

The ColU jolted to a sudden stop.

Beth muttered and stirred. Yuri stroked her head, and she calmed again. He looked around. There was nothing special here, no obvious reason to have stopped.

The ColU backed up a little way, then rolled forward with a grinding of ageing gears. Again Beth stirred, before settling.

Yuri whispered urgently, ‘Hey! What’s wrong with you?’

The ColU’s voice was a matching whisper. ‘Yuri Eden?’

‘Why have you stopped? Get going before this one wakes up, or Mardina will slaughter the lot of us.’

‘I am sorry. I had not realised I had stopped.’ It rolled on with a sight lurch.

‘So what was all that about?’

‘Yuri Eden, call it an existential crisis.’

Yuri groaned inwardly. Not again.

He knew he’d have to tell Mardina about this episode, whatever it was; she was concerned about anything erratic in the ColU’s behaviour. The ColU had made it clear from the beginning that to have been forced to help transport the colonists across the planet, if they’d attempted to escape from the landing sites that had been planned for them by the starship crew, would have violated its deepest layers of programming. So when the lake had first shifted, in its algorithmic soul the ColU faced a conflict between mandates to keep its human charges alive, and to stay close to the original landing site. The preservation of life had won out. But Mardina, who knew a lot more about ISF AIs than Yuri did, fretted that some deep internal damage might have been done. All of which was over Yuri’s head, let alone the head of his seven-year-old daughter, his little muda-muda.

Now, reluctantly, he asked, ‘What existential crisis?’

‘I have come to a conclusion which baffles and alarms me. I have just received, from my internal laboratory facilities, the results of the analysis of a novel organism which enabled me to complete a genetic mapping – you’re aware that among my long-term projects has been the construction of a tree of life, for the Arduan native flora and fauna—’

‘You know, I wish I just had a truck.’

‘Yuri Eden?’

‘Like the rovers on Mars. A truck I could just drive. The number of conversations like this that I’ve had with you over the years—’

‘I can’t help it,’ the ColU said, sounding almost miserable. ‘I can’t constrain my curiosity. Nor should I. Until my understanding of this world is complete enough—’

‘Just tell me.’

It paused, as if gathering its thoughts. ‘Yuri Eden, I have told you that life on this world is similar in its fundamentals to life on Earth, but not identical. I believe the two biospheres may be linked by a panspermia process that operated at a very early date. The earliest days of life on Per Ardua might have been like the early days of Earth, a world of simple bacteria, drawing their energy from chemical reactions in the rocks. But all the time much more energy, a hundred times as much, was available, washing down from the sky—’

‘Proxima light.’

‘Yes. The next step was the development of kinds of photosynthesis, creatures that could draw energy directly from that light. The new kind colonised the surface, while the older ones survived, sinking deeper into the planet. And there they still reside in great reefs, in caverns, in porous rock and aquifers, dreaming unknowable dreams. Just as on Earth, life on Per Ardua is actually dominated by the bugs in the deep layers, mass for mass. But on the surface, as photosynthesis evolved, ultimately oxygen was released as a byproduct.’

‘Like the green algae on Earth.’

‘Yes, Yuri Eden, this step, oxygen production, was evidently difficult to achieve; on Earth it occurred only once, and in fact came from the coupling of two older photosynthetic processes. I have yet to fully understand the equivalent process on Per Ardua – it is necessarily different because the energy content of the light here is heavy in the infrared – but it is evidently just as complex, just as unlikely to have happened.’

‘Yet it did happen.’

‘It did, and I have been able to date the event from traces in the Arduan genetic record: some two billion, seven hundred million years ago.’ It paused. When Yuri didn’t react it went on, ‘The next great step in the emergence of Arduan life, again mirrored on Earth, was the development of a new kind of cell: a much more complex organism, a cell with a nucleus, a cell with different kinds of mechanisms within a containing membrane. Of course the energy available from burning up all the oxygen concentrating in the air helped with that. Such complex cells are the basis of all multicellular life, including you, including the builders. This was an information revolution, not a chemical one; these complicated creatures needed about a thousand times as much genetic information to define them as their simpler predecessors.’

‘Another unlikely step.’

‘Yes. But again it occurred on both worlds. And on Per Ardua this came about some two billion years ago.’ Another pause. ‘Yuri, I am not sure you are grasping the significance of—’

‘Just tell me the story,’ Yuri said. He stroked his daughter’s hair, growing sleepy himself.

‘Multicellular life emerged some time later – evidently another difficult step to take. Seaweeds first on Earth, like the lavers we imported to Per Ardua . . .’

The new camp was coming into view, the lake settling into the contours of its latest shoreline. Yuri saw builders busily working all around the lake’s edge, and smoke rising from Mardina’s camp fire.

The ColU was still talking about ancient life. ‘Of all the great revolutions of life this is the easiest to identify on Earth because it left such clear traces in the fossil record. On Per Ardua, of course, there is no fossil record to speak of. And yet—’