by Greg Egan
‘So it doesn’t have to wait for the nuts to fall. Which means the teeth are there to help it stay off the ground?’
‘I suppose so,’ Grant conceded. ‘But there might have been any number of reasons in the past why that was a good idea. It doesn’t require co-evolution with the ants.’
Prabir turned to her. ‘If you’d come to this island knowing nothing about its history, nothing about the ordinary fauna of the region – if you’d dropped in out of the sky in a state of complete ignorance about this entire hemisphere – what would you think was going on here?’
‘That’s a stupid question.’
‘Humour me.’
‘Why? What point is there in ignoring the facts?’
Prabir shook his head earnestly. ‘I’m not asking you to do that. I just want you to look at this afresh. If you’d just arrived from the insular British Isles with an immaculate, theoretical training in evolutionary biology, but no contact for a thousand years with anyone east of Calais, what would you conclude about the plants and animals here?’
Grant folded her arms.
Prabir said, ‘I’m withdrawing my labour until you answer me. Forgetting all the history you know, what does this really look like to you?’
She replied irritably, ‘It looks to me as if the affected species originally shared territory with all the others, then became isolated on some remote island and co-evolved separately for a few million years – and now they’re being progressively reintroduced. OK? That’s what it looks like. But on what island is this meant to have happened?’ She spread her arms. ‘It didn’t happen here: you can vouch for that yourself. There’s no island in the whole archipelago sufficiently isolated, and sufficiently unexplored.’
‘Probably not.’
‘Certainly not.’
Prabir laughed. ‘OK. There’s no such island! All I’m saying is, when the account you’ve just given sounds so much simpler than a hundred separate genes in a hundred separate species marching back from the past in perfect lock-step – I have a lot of trouble seeing how it can’t be telling us something about the truth.’
Grant’s expression softened, her curiosity getting the better of her defensiveness. ‘Such as what?’
‘That, I don’t know.’
Prabir had rewritten the image-processing software to run directly on Grant’s camera. In the afternoon, she found the camouflaged fruit pigeons all around them.
Fluttering across the viewfinder between the pigeons were the butterflies. The wing patterns had changed dramatically – the dappled imitation of foliage and shadows they’d acquired was far less striking, far less symmetric, and far more variable from insect to insect than the old concentric bands of green and black – but when Grant finally captured one and Prabir saw the body, he knew they were the descendants of the insect he’d first seen pinned to a board in his father’s office at the university.
The tranquilliser darts were useless for insects, but Grant had a spray based on wasp venom that could temporarily paralyse the butterflies without killing them. Using a net to keep their victims from falling to the ants, they managed to collect half a dozen live specimens of both the pigeons and the butterflies.
Back on the boat, Grant killed and dissected one of the male pigeons, removing the testes and then working under a microscope to extract stem cells and various stages of maturing spermatocytes. She was hoping to catch the São Paulo protein in the act, though given the uniformity of the pigeons it seemed unlikely that it was still producing radical changes in their genome.
Prabir left her to it and stood out on the deck, staring back into the harmless shadows of the jungle, numb with relief as he realised how painlessly the day had passed. Between the distracting riddle of the altered species and the sheer physical effort of gathering samples, he’d had little time or reason to dwell on the significance of the place. And that was how it should be. He’d already mourned his parents, in the camp, in Toronto, and he’d recounted their triumphs a thousand times for Madhusree. There was nothing to be done here, nothing to be remembered, nothing to be learnt but the secret of the butterflies. He refused to imagine them trapped on the island. In the only sense that they survived at all, they’d left in the very same boat as he had.
And though Teranesia had proved to be no more dangerous than the other islands, he was still glad that he’d kept Madhusree away. She might resent his intervention for years to come. She might accuse him of standing between her and the memory of her parents. But the alien jungle would have meant even less to her than it did to him, and he’d spared her the pointless anguish of digging through the ruins of the kampung. His mother had told him, ‘Take her away! She mustn’t see!’ He’d completed what he’d begun when they’d set out on the boat. It had been a long trip, but now it was over.
Grant emerged from the cabin frowning, carrying her notepad.
‘More news from São Paulo,’ she said. ‘They’ve refined their model.’
‘And?’
She propped the notepad on the guard rail in front of Prabir; it was displaying a graphic of a couple of large molecules bound to strands of DNA. ‘I don’t know what to make of this. I was hoping they’d find evidence that some part of the protein resembled a transcription factor, and recognised disabled promoters—’
Prabir stopped her. ‘I used to know all these terms when I was a kid, but I’m pretty hazy now. Can you—?’
Grant nodded apologetically. ‘Promoters are sequences of DNA that sit next to the coding region of a gene, which is the part that actually describes the protein. Transcription factors bind to promoters to initiate the copying of the gene into RNA, which is then used to make the protein: to “express the gene”.
‘If a gene is accidentally duplicated, mutations that build up in the promoter of one copy might eventually stop that copy from being expressed. To identify a gene that’s become inactive like that, you’d need something that was capable of binding to a damaged promoter – something roughly the same shape as a transcription factor, but a little less fussy. And then to reactivate the gene, there’d be a number of possible strategies, either working base by base to repair point mutations in the promoter, or snipping the whole thing out and splicing in an intact version.’
Prabir said, ‘OK, that all makes sense. Now what have the modellers found?’
Grant hit a button on the notepad and animated the graphic. ‘This bloody thing just crawls along causing havoc during DNA replication. What normally happens is: the double helix unwinds, the two strands separate, and DNA polymerase comes along and stitches together a new complementary strand for each of them, from free-floating bases. What the São Paulo protein does is slide along each single strand, cutting it up into individual bases, while splicing together a whole new strand of DNA to take its place. Then DNA polymerase comes along and duplicates that.’
Prabir took the notepad from her and slowed down the animation so he could follow the steps. ‘But what’s the relationship between the old sequence and the new sequence?’
‘Basically, the new one is the old one, plus noise. SPP changes shape as it binds to each base of the original strand – it assumes a different conformation depending on whether it’s cutting out adenine, guanine, cytosine, or thymine – and that in turn determines the base it adds to the new strand. But the correlation isn’t perfect; there are some random errors introduced.’
Prabir laughed, disbelieving. ‘So it’s just an elaborate, self-inflicted mutagen? These creatures might as well be bathing their gonads in radiation or pesticide?’
Grant replied dejectedly, ‘That’s what they’re claiming.’
He replayed the animation. ‘No. This is crazy. If you wanted to add a few extra random errors to your offspring’s DNA, would you take the easy way out and just alter your DNA polymerase slightly, so it made occasional mistakes – or would you invent a whole new system like this for making deliberately flawed single-strand copies?’
‘Well, exactly,’ Grant said
. ‘And even if you had a good reason to take that approach, the whole protein’s vastly over-engineered. There are commercial enzymes that do something similar, and they’re about one-hundredth the molecular weight.’
‘Maybe there’s a bug in their software. Or maybe there’s some logic to the changes, some pattern that they simply haven’t noticed.’
Grant shrugged morosely. ‘They’ve synthesised some of the protein for real now; they’re doing test-tube experiments as we speak, to try to confirm all this.’
She seemed to be taking it all too much to heart. Prabir said, ‘You know that the things we’ve seen here can’t be explained by random mutations. Maybe there’s still some way this can be compatible with your theory. But whatever’s going on, at least we’re closing in on it.’
‘That’s true.’ She smiled. ‘They have synthesised protein in São Paulo, and I have cultured fruit pigeon spermatocytes. By morning they’ll know what happens in a test tube, and we’ll know what happens in a living cell.’
When Prabir woke, this prediction had been fulfilled. Grant had been up since three o’clock trying to make sense of the results.
The experiments in São Paulo had confirmed the computer model: fed a few hundred different test strands of DNA, the protein had chopped them up and synthesised new strands of exactly the same length, copying the original sequence but introducing random errors. Another group, in Lausanne, had repeated the work and found the same thing.
Grant had detected RNA transcripts for the São Paulo protein in the pigeon spermatocytes, which implied that the protein itself was being made in the cells; she had no direct test for it. But when she compared sequence data for cells before and after meiosis, the error rate was about a thousand times less than for the two experiments in vitro.
She said, ‘There has to be a second protein, some kind of helper molecule that modifies the whole process.’
‘So they need to look harder at the sequence data?’ Prabir suggested. ‘The gene for it must be in there somewhere.’
‘They’re looking. SPP alone is a bit like a pantograph with a whole lot of superfluous hinges. So maybe this is something that binds to it and stabilises it – not enough to produce perfect copies, but enough to allow its internal state to reflect the last few dozen bases to which it was bound.’
Prabir opened his mouth to say, Turing machine, but he stopped himself. Most processes in molecular biology had analogies in computing, but it was rarely helpful to push them too far. ‘So it could recognise a sequence of something like a promoter, even though it only binds to one base at a time?’
‘Maybe,’ Grant agreed cautiously. ‘They’ve also got hold of samples from the Ambon fruit pigeon, and they’re going to see what pure, synthetic SPP does to an entire chromosome, in the presence of nothing but a supply of individual bases.’
As they waded ashore, Prabir looked down into the warm, clear water where he’d swum with Madhusree, then across the dazzling white beach where they’d played. He wasn’t just cheating her out of a role in the study of the butterflies, he was depriving her of the chance to demystify the island, to purge it of its horrors the way he was doing for himself.
But he could never have brought her back here. He could never have undone the one good thing.
Grant wanted to collect specimens of the butterfly’s other stages, so they spent the morning doing nothing but searching appropriately succulent leaves for the spiked larvae, and the branches of the same trees for pupae. The original versions would not have been hard to spot: both had been covered in bright-orange patches, warning colours to signal their toxicity. Grant found signs of leaf damage that looked promising, but there were no culprits nearby. If the larvae had switched strategy and opted for camouflage that was as efficient as the adults’, their movements would be far too subtle for the image-processing software to detect.
They stopped and ate lunch in the middle of the forest, in a rare spot where the ground was rocky enough to keep the shrubs at bay. Prabir still didn’t feel safe sitting down until he’d sprayed a cordon of insect repellent on the ground; the ants were never content to stay inside the orchids, waiting for easy prey. He wasn’t sure why they didn’t swarm up the trees and take nestlings; maybe they were lacking some crucial adaptation for the task, or maybe it just wasn’t worth the energy.
Grant said, ‘So your whole family was here for three years, from 2010? Was your sister born on the island?’
He laughed. ‘We weren’t quite that isolated. We went to Ambon on the ferry four times a year. And we flew down to Darwin for the birth.’
‘Still, it must have been a rough place to raise a young child.’ Grant added hastily, ‘I’m not criticising your parents. I’m just impressed that they could cope.’
Prabir shrugged. ‘I suppose I took that for granted. I mean, the people in small villages on the other islands had better access to transport, clinics they could get to, and so on. But we had a satellite link, which made it easy to forget the distance. I even had lessons from a school in Calcutta; they’d set up a net service for kids in remote villages, but I could join in just as easily.’
‘So at least you had some friends your own age through the net.’
‘Yeah.’ Prabir shifted his position on the rock, suddenly uncomfortable. ‘What about you? What were your school days like?’
‘Mine? Very ordinary.’ Grant fell silent for a while, then she took out her camera and began scanning the branches around them.
She said, ‘The butterflies spend a lot of their time quite high in the canopy. Maybe they lay their eggs there.’ She lowered the camera and asked casually, ‘What are you like at climbing trees?’
‘Seriously out of practice.’
‘It’s like riding a bicycle. You never forget.’
Prabir gave her a stony look. ‘You’re the field biologist, remember? I’m the desk zombie. And I don’t care how ancient you are: you’re twice as fit as I am.’
‘You put that so gallantly.’
Prabir said flatly, ‘I’m not doing it! The deal we made in Ambon—’
Grant nodded effusively. ‘OK, OK! I only asked because I’m not used to judging the strength of the branches of these species. I thought you might be more confident, since you must have climbed them as a kid. I’ll go back to the boat and get a rope—’
‘A rope? You’re not serious?’
‘I had a bad experience in Ecuador,’ she admitted. ‘I broke a lot of bones. So I’m ultra-careful now.’
Prabir’s resentment faded. There was a principle at stake, but he didn’t want to be petty and sadistic. ‘I’ll do it, but you have to pay me. Ten dollars a tree.’
Grant considered this. ‘Make it twenty. I’ll feel better.’
‘With a conscience like that, who needs labour laws?’
Grant selected a nutmeg tree. Prabir took off his boots and rolled up his trouser legs. He hesitated, unsure how to begin. The lowest branch of this tree was just above his head; he must have been able to scale a sheer trunk once, gripping the bark with his arms and legs – he’d even climbed coconut palms – but he felt certain he’d make a fool of himself if he tried that now.
He grabbed the branch and raised himself up, then hooked his feet around it and hung sloth-like for a while before figuring out how to right himself. It was a clumsy start, but once he was standing squarely on the branch, with a firm grip on the next one up, he was elated. The scent of the bark, the feel of it against his soles, was utterly familiar; even the view straight across into the other trees was far closer to anything he remembered than the view from the ground. He glanced down at Grant, not wanting to lose perspective, not wanting to be drawn back too strongly.
She shaded her eyes and looked up at him. ‘Be careful!’
He took a few steps along the branch, feeling it flex, trying to recalibrate his old instincts for his adult weight. He called down, ‘I promise you, I have no intention of breaking my neck for a caterpillar.’
He sc
oured the clusters of leaves hanging around him for signs of larval feeding, but there was nothing. He climbed higher. Fruit pigeons fled as he approached, a rush of air and a blur of motion. There were foul-smelling beetles on the trunk, but they scurried away from the repellent. There’d been pythons in the trees once, but even the lowest branches wouldn’t have taken the weight of anything remotely like the one he’d met in the mangrove swamp; as long as he didn’t panic and fall to his death, he probably had nothing to fear from its tree-dwelling cousins. Assuming they hadn’t acquired venom.
Twenty metres up, Prabir found something hanging from a slender branch. At first glance he’d mistaken it for a nutmeg fruit, but then a hint of unexpected structure had made him look again. When he was close enough to examine it properly, he found a butterfly, wings folded, suspended from the branch. It had to be a pupa, but it looked more like a tiny sleeping bat than an insect about to emerge from metamorphosis – and it still looked more like a nutmeg fruit than anything else. He touched it warily; it even felt like a nutmeg fruit.
He took out his notepad and recorded some vision, to document the attachment method before he broke the pupa free. The silk girdle around the bulk of the insect was virtually undetectable, the colour matched so well; the short length anchoring it to the branch looked exactly like a stalk. He sent the images down to Grant, and spoke to her through the notepad; it was easier than shouting.
‘What do you make of that? Pretty good camouflage, at the risk of being eaten by mistake.’
‘Maybe they smell bad to the fruit pigeons,’ Grant suggested.
‘Why not just – oh, forget it.’ Whatever anything did, why not do it differently? It was frozen history, not rational design. He broke the pupa free, and dropped it in his backpack. ‘I’ll go up one more level, just to see if there are any larvae.’