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 fulfil 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 organise 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 south-east, 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, which 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 Earthlike photosynthesis replaced by the Per Arduan kind – the version tuned to Proxima light, which exploits the dense infra-red 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 upwards, is a product of a deep integration of many forms of life. Genesis through symbiosis, indeed.’

‘And now, here on Per Ardua, we’re seeing the same thing over again. How long would it take? How much time has elapsed here since humans arrived? How far into the future have we been projected, ColU? More than millions of years, more than hundreds of millions …’

The ColU simulated a sigh. ‘I apologise for my reticence. You have asked these questions many times before. I can make only rough guesses based on the data I have so far, the evidence from the geology here, the biology – even from the evolution of the star itself. I will be able to make much more accurate estimates of the date when I see the dark-side sky, and I can gather astronomical data. But of course there is an upper bound.’

Stef frowned. ‘An upper bound? How can there be an upper bound on the future – what upper bound?’

‘The End Time,’ the ColU said simply.

That was when Mardina and Chu burst into the camp, scuffed and dusty and breathing hard.

Mardina said, ‘You keep saying you want to see the sky, ColU.’

‘Yes—’

‘Well, your luck is in. You can see it from the slope, not much of a climb from here. Chu, get him into his pack.’

‘See what?’ Stef demanded. ‘The stars?’

Mardina gave her only a quizzical look. ‘Sort of. See for yourself. Come on! And where’s my mother?’

The four of them, Stef, Chu, Mardina and Beth, stood on a hillside, looking out over the night lands of Per Ardua, over an ocean of dark. Only the faintest reflected glow from the summit above reached them here.

And above them, in a terminator sky marred for once only by scattered cloud …

Not stars, no, Stef saw. Not just stars. It was a band of light, an oval, an ellipse – no, surely it was a disc tipped away from her, all but edge on. The overall impression was of a reddish colour, but bright white sparks were scattered over the pink, like shards of glass on a velvet cushion. There was a brighter blob at the centre, and lanes of light sweeping around that core. As eyes adapted to the low light she saw finer detail, what looked like turbulent clouds in those outer lanes, and here and there a brighter spark, almost dazzling. When she looked away from this tremendous celestial sculpture, she could see stars – ordinary stars, isolated sparks scattered thin, though many of them seemed reddish too. But the sky was dominated by the great ellipse.

And, oddly, the thing she noticed next was Mardina’s hand slipping into Chu’s, and squeezing tight.