“But, Laoshi, didn’t they know what they were bringing back?”

“No, they didn’t, Pei. And that was the whole problem.” The teacher paused, tapping on the tablet in his hand with a small stylus. A picture appeared on the glasstic surface on the wall behind him. “Who can tell me what this is? Jia?” He gestured to the most hesitant of the students responding.

“The lander? The lander that came back?”

“Yes, Jia. That’s the lander that was sent to survey the moons of the planet Saturn.” The stylus moved and the word appeared on the glasstic screen. “That’s one of the other planets in the system with Terra, remember. This was the Odysseus.” Another word appeared. “It was a new type of craft that was meant to make it possible to explore more efficiently. It touched down on several moons, took readings and collected specimens, and then made its way back home. It was a great success. The scientists that sent it were very happy, because it worked and came back in good shape, so they were able to learn quite a lot.”

“So what happened, Laoshi?” Despite her reticence, Jia was getting interested in the story.

“Well, this was the first time an object that had landed on the surface of another planet was brought back to Terra unwrapped, so to speak. It wasn’t just the specimens that had been exposed to these other places, but the surface of the Odysseus itself. The scientists thought that the travel between the planets would have scoured it clean if any dust got on it. And that was mostly true. But on one of its stops, no one is sure which one, apparently a tiny pebble got lodged in between two pieces of the lander’s skin. And there were microbes on that pebble.”

“A microbe is a tiny little lifeform too small to see, right, Laoshi?”

“Yes, Tou. And in this case it was something that was perfectly harmless on the moon it came from, but in the atmosphere of Terra it behaved differently. You know how plants give off the oxygen they separate from the carbon dioxide that people and animals breathe out, right?” All the pupils nodded. That was a second-year lesson and they were now well into third-year. “Well, this microbe worked very much the same way. It took in carbon dioxide and separated it, and used the carbon in the process of growing and reproducing. But on the moon it came from there is very little carbon dioxide because there is very little oxygen.” All the students looked serious. Even a tiny child who had not yet started Lessons knew that oxygen was very important. “So when it was exposed to the air on Terra it started working hard. There was lots of carbon dioxide, so it got plenty of carbon to grow and reproduce. It was spread by the movement of the air—they called that ‘wind’. And there got to be more and more of these microbes on the planet before people even noticed them. You have already studied some about what happens when something alive is introduced into a new environment, right? What happens?”

“Either the new thing dies out, or it takes over.”

“It is not always quite that simple, Pei. But to make it simple, yes, it can take over. Humans did this by accident in colonizing new homes more than once, when they brought in some animal or plant that was not native. But it this case it happened to Terra, and it happened before they realized what was going on. This microbe did so well because of all the carbon dioxide on Terra that it spread like crazy. But it wasn’t quite like the Terran plants that took in carbon dioxide and gave off pure oxygen. This was adapted to give off carbon monoxide.”

The teacher paused. These students would learn so much more about this chapter in human history, but they did not need to hear it all at once. Years of Lessons remained to be assimilated. They would learn of the waves of mental deterioration that contributed to the death toll as the planet choked in its own air. They would read accounts of the struggles and the growth of the off-Terra colonies, as those who had left the planet in time fought to survive without the support they still needed. They would ultimately realize how this overlooked pebble had splintered humankind into divergent societies and even, eventually, into divergent species.

But they would not learn it all today.

“Carbon dioxide is a gas. A gas that is poisonous. And there was suddenly far too much of it in the air on Terra. Many people died, many left the planet to find other homes. Living on Terra now requires living in bubblecities and breathing through special filters. It’s a long story you’ll learn more of in other Lessons. Right now, class, you are due for Physical Training.”

The students rose and inclined their heads slightly to the teacher, in unison saying “Thank you, Laoshi.” Then they swam happily towards their next class.