Partly, too, it was because the phenomenal Neanderthal sense of smell and their ever-vigilant Companion implants made it safe to be out even in the dark; Ponter’s hometown of Saldak, located at the same place in his world as Sudbury was in this world, didn’t illuminate its streets at night.

And partly it was because the Neanderthals used clean solar power for most of their energy needs, rendering their skies far less polluted than the ones here.

Mary had made it to her current age of thirty-eight before seeing the aurora, and she didn’t anticipate any reason to come back to Northern Ontario, so tonight, she knew, might well be the last time she’d ever see the undulating northern lights.

She drank in the view.

Some things were the same on both versions of Earth, Ponter had said: the gross details of the geography, most of the animal and plant species (although the Neanderthals, never having indulged in over killing, still had mammoths and moas in their world), the broad strokes of the climate. But Mary was a scientist: she understood all about chaos theory, about how the beating of a butterfly’s wing was enough to affect weather systems half a world away. Surely just because there was a clear sky here on this Earth didn’t mean the same was true on Ponter’s world.

But if the weather did happen to coincide, perhaps Ponter was also looking up at the night sky now.

And perhaps he was thinking of Mary.

Ponter would, of course, be seeing precisely the same constellations, even if he gave them different names—nothing terrestrial could possibly have disturbed the distant stars. But would the auroras be the same? Did butterflies or people have any effect on the choreography of the northern lights? Perhaps she and Ponter were looking at the exact same spectacle—a curtain of illumination waving back and forth, the seven bright stars of the Big Dipper (or, as he would call it, the Head of the Mammoth) stretching out above.