Evolution

Some kind of animal howled, out in the night.

He threw a little more wood on the fire, and buried himself deeper in his mound of greenery.

Sidewise had been right. Mars was missing.

The replicators, Ian Maughan’s robot probes, had survived. The program had been designed as a precursor to human colonization of the planet. The replicating robots would have been instructed to build homes for human astronauts, to make them cars and computers, to assemble air and water, even grow food for them.

But the humans never came. Even their commands ceased to be received.

That wasn’t troubling, for the replicating robots. Why should it be? Until they were told otherwise, their only purpose was to replicate. Nothing else mattered, not even the strange silence from the blue world in the sky.

And replicate they did.

Many modifications were tried, incorporated, abandoned. It did not take long for a radically better design to converge.

The replicators began to incorporate the factory components within their bodies. The new kind looked like tractors, pilotless, trundling over the impassive red dust. Each weighed about a ton. It took each one a year to make a copy of itself — a much shorter reproduction time than before, because they could go where the resources were.

After a year, one of the new replicator types would become two. Which after another year had made two more copies, a total of four. And in another year there were eight. And so on.

The growth was exponential. The outcome was predictable.

Within a century the factory-robots were everywhere on Mars, from pole to equator, from the peak of Mons Olympus to the depths of the Hellas crater. Some of them came into conflict over resources: There were slow, logical, mechanical wars. Others began to dig, to exploit the deeper materials of Mars. If you mined, there was still plenty of resources to go around — for a while, anyhow.

The mines got deeper and deeper. In places the crust collapsed. But still they kept digging. Mars was a cold, hard world, rocky for much of its interior. That helped the mining. But as they dug deeper and encountered new conditions, the replicators had to learn quickly, adapt. They were capable of that, of course.

Still, the penetration of the mantle presented certain technical challenges. The dismantling of the core was tricky too.

Mars weighed one hundred billion billion times as much as any one of the tractor-replicators. But that number was small in the face of the doubling-every-generation rule. Because of the continuing conflicts, the pace of growth was a little slower than optimal. Even so, in just a few hundred generations, Mars had gone, all but a trace of its substance converted to the glistening bulks of replicators.

With the whole planet transformed to copies of themselves — using solar sails, fusion drives, even crude antimatter engines — the swarm of replicators had moved out through the solar system, seeking raw material.

The next day, roaming into the country around the town, Snowy saw birds, squirrels, mice, rabbits, rats. Once he thought he saw a goat; it fled at his approach.

Not much else. There didn’t even seem to be many birds around. The place was silent, as if all the living things had been collected up and removed.

Some of the rats were huge, though. And then there were the rat-wolves he thought he had glimpsed. Whatever they were, they fled at his approach.

Rodents had always been in competition with primates, Sidewise said. Even at the peak of their technical civilization, people had had to be content with keeping rodents out of sight, and out of the food. Now, with people out of the picture, the rats were evidently flourishing.

It was easy to hunt, though. Snowy set a few snares, in a spirit of experimentation. The snares worked. The hares and voles seemed peculiarly tame. Another bad sign if you thought about it, because it meant they hadn’t seen humans for a while.

At the end of the second day, Ahmed had them sit in the ruins of the church, in a rough circle on corroded stone blocks.

Snowy was aware of subtle changes in the group. Moon was looking down, avoiding everybody’s eyes. Bonner, Ahmed, and Sidewise were watching each other, and Snowy, with calculation.

Ahmed held up an empty ration packet. "We can’t stay here. We have to plan."

Bonner shook his head. "The most important thing is finding other people."

"We’re going to have to face it," Sidewise said. "There are no other people — nobody who can help us, anyhow. We haven’t seen anybody. We’ve seen no sign that anybody has been in this area recently."

"No contrails," Ahmed said, pointing to the sky. "Nothing on the radio, on any frequency. No satellites. Something went wrong—"

Moon laughed hollowly. "You can say that again."

"We can’t know how events unfolded. Before the end it must have become — chaotic. We were never recalled. Eventually, I suppose, we were forgotten. Until we were revived by chance."

Snowy forced himself to ask the question. "How long, Side?"