The first soil beds were already bearing a crop, gen-enged potatoes, their leaves stained black by their adjusted Prox-friendly photosynthetic chemistry. The spindly roots were nutritious enough but they tasted odd to Yuri, faintly acidic maybe, and with a powdery texture. Potatoes, which after all had originated in the Andes, were a useful crop, robust enough to grow at altitude, or in the cold and damp. You could produce several harvests a year. And potatoes, it seemed, provided all nutrients essential for a human diet except vitamins A and D, and the seaweed helped with that. But the ColU was experimenting with other Earth crops, some gen-enged, that might be suited to the conditions. Strawberries, that required less light to flower than some plant species, and so were preadapted for Proxima’s feebler daylight. Wheat, flexible crops like soya beans, sweet potatoes, ready-to-eat salad crops like lettuce and spinach.

The ColU told Yuri, and anybody who would listen, that they really were pioneers in a new way of living, here on Per Ardua. On Earth, humans lived in a kind of sea of other organisms, including the bacteria that lived inside and outside their own bodies. Even in a dome on Mars you were living in a kind of closed sample of that wider sea, a droplet. Here, they were trying to recreate that sea of Earth life in an open environment, on an alien world. It scared Yuri to hear that nobody really knew how much of that bacterial sea you actually needed, in the long term, to survive.

And it pissed off everybody else to hear the ColU, and sometimes Mardina, speak of long-abandoned plans for further flights to bring animals out here, perhaps in iron wombs.

It was all marvellous – but somehow fantastically dull at the same time. It was only, after all, soil. The fact that they all seemed doomed to be dead and gone in a few decades, no matter how ingenious the ColU was, made it seem even more futile. Sometimes Yuri felt sorry for the ColU, which wanted to talk about its achievements and discoveries, but there was usually nobody who wanted to listen.

Now, for example, as they walked, the ColU essayed a conversation. ‘You are preparing to make astronomy observations, Yuri Eden.’

‘Transits, yes.’

‘Transits.’ With a whir, it lifted its camera eyes, entirely contained within its bubble-dome ‘head’, to the washed-out blue sky. ‘There is the Pearl, of course.’ The Pearl was the name they had given to Proxima e, the big super-Earth, the only planet visible in the sky of unending day. ‘Per Ardua is one of a family of six worlds. But aside from the Pearl, the only way we can see the other planets is by transits, when the inner worlds pass across the face of Proxima itself and cast a shadow . . . Six planets in all, and six of you left. I did wonder if you would think that was some kind of omen.’

Yuri looked at it curiously. ‘No. Anyhow, there’s only five of us now.’

‘Six if you include the ghost of Dexter Cole.’

The idea of the colony being haunted by the ghost of Dexter Cole, the first, lost, man to have been sent to Per Ardua, was a kind of black in-joke that had grown up among them. Yuri wasn’t surprised that the ColU had overheard, but he was surprised it referred to that kind of stuff. ‘Do you think that way? Omens and stuff? Ghost stories? You’re a machine. A creature of logic.’

‘We are all creatures of logic, at root. Of little switches turning on and off in our heads, metaphorically speaking. I do not think like a human, but I am endlessly curious about humans, and their ways of thought.’

‘Why? I mean, why did they program you to be curious?’

‘I need to understand you better, in order to serve you better. I am your doctor, your guide, your children’s teacher one day. It is my duty to be curious about you. Just as it is my duty to be curious about the life forms of this world.’

‘As we scrape them off to make room for potato fields.’

It laughed, a tinny, not unattractive, but quite unrealistic sound. ‘The native life is useful. And it is related to us.’ It said this gravely, as if making a grand announcement.

‘I don’t understand.’

‘It is what I have deduced myself,’ the ColU said with something like pride. ‘This was a significant achievement in itself. I do have a sophisticated genetic microlab on board, but when we began I didn’t even know what chemical basis any genetic material here might have. In the brief time we have been here I have managed to progress from that fundamental investigation to, by analogy, the discovery of the double helix . . . Yuri Eden, all Per Ardua life, like Earth life – that is, all I have sampled – belongs to a common family tree. And that family is related to the family of Earth life, as if they are two mighty trunks sharing the same root. But that commonality is deep, deep in time . . .’

Yuri, trudging in the hot light, said nothing. The ColU took that as an invitation to keep talking.

‘Life on both Per Ardua and Earth is based on fundamentally the same chemistry: carbon, hydrogen, oxygen, nitrogen. Perhaps that was inevitable, given the physical nature of worlds like these, rocky, watery worlds, rich in carbon. But the choices made in how life evolves are not inevitable. All life on Earth is based on two chemicals, two acids: DNA, which stores the information that defines a life form, and RNA, which interprets that information and uses it to assemble proteins, which are the building blocks of life.’

‘DNA as software, proteins as hardware.’

‘That is an antiquated reference. You are showing your age, Yuri Eden. Both DNA and RNA are based on a particular kind of sugar, called ribose. Life on Per Ardua has a similar basic architecture. The information store is not DNA – but it is a kind of acid, based on the same sugar choice as DNA, ribose. There were other plausible possibilities – dextrose, for instance.

‘Beyond that fundamental point, the two methodologies of life differ. Arduan genes do not use DNA; they use that ribose-based acid, which in turn encodes information using sequences of bases, but not the same sequences as DNA’s triple-base “letters”. Arduan life is based on proteins, which like your proteins are assembled from amino acids, but not from the twenty specific aminos used to construct your body, rather from an overlapping, non-identical set of twenty-four acids. Arduan life seems to rely on some genetic coding being stored in the proteins themselves – as if the genetic information is more distributed. This may help make the coding more flexible in the case of changing climatic conditions . . .

‘On the other hand, Yuri Eden, life on Mars is based on a variant of DNA much closer to Earth’s than the Arduan system, and a more similar protein set. You can see the implication. Earth, Mars, Per Ardua – all these families of life are related. Mars is a more recent branching from Earth. Or vice versa.’

‘Or it all branched off from what’s here, on a world of Proxima.’

‘Yes. This is panspermia, Yuri Eden. A lovely idea, of life being carried through space, presumably in drifting rocks, blasted up by impacts from the surface of planets. The worlds of a solar system, Earth and Mars, say, or Per Ardua and the Pearl, may readily share material. But it is much harder, more rare, for material to be transferred between star systems. Whatever came here from Earth, or travelled from Per Ardua to Earth – or came from a third source entirely – came long ago, deep at the root of all the life forms on all the worlds. I imagine a panspermia bubble spanning the nearby stars, Sol, Proxima, Alpha A and B, perhaps others further out, all sharing the same basic chemistry. Beyond that, maybe there are other bubbles, of other sorts of life chemistry – perhaps nothing like our own at all.’

‘And out of all that comes something as curious and busy as a builder.’

They were close to the forest fringe now. They came upon a garden of particularly large stromatolites, towering hemispheres each with a hardened carapace the colour of burned copper. They trudged on, parallel to the stromatolites and away from the track.