We tell ourselves that we didn’t drive them away as we inexorably increased our range. As we returned to the warm lands and adapted our ways, hunting their animals that didn’t know our tricks so well, we used these others’ resources—to replicate and improve on our existing tools and techniques. Perhaps those mysterious singing people were already in decline when we found them. Perhaps the fact that they are no longer in our world doesn’t mean there’s blood on our hands. But our ancestors were strong and determined, and our brains were larger than those of the others, just a little. And we had been remade on the glacial anvil. How likely was it that our coming and their going was simple coincidence? By the time any of us could ask that question, it was too late. The others were already gone, and all we had were their stories and their bones.

By now you might be saying, “Hold on…,” because this doesn’t sound right to you, but again—and we promise for just this one more time—this isn’t your Earth. This is ours.

After the others had vanished, we were still around, refining our tools. Our development went down a slightly different path to yours because our brains and senses are slightly different. We evolved a sensitivity to certain stimuli, signals that don’t come into the world via regular channels. And early in the half million years during which we became us, chance mutation and evolutionary pressure bore strange progeny. In our distant past, we told ourselves that we had found doorways to other worlds. And why not? What good would a more precise knowledge have done? No point having a complete mastery of quantum entanglement when you still have to stick a spear in an aurochs for dinner.

However, in our more recent past, our scholars decided that we’d found unstable points in space–time. Here, the underlying fabric of the universe was being deformed by unknown forces, existing upon an axis that had no material analogue in our familiar universe. By then, we had already been exploiting these weak points for our technology. We started with regions where the regular laws of the world were suspended: we’d found water that flowed uphill, patches of localized weather, places where inconsistent objects suddenly appeared as though they’d fallen from the sky. All isolated, random, unreliable. Your species might have dismissed them as hearsay and madness. We, with our particular senses, did not. We knew something was there.