Of course, none of the narratives I am presenting in these interludes are necessarily true. We do not, after all, have a time machine to visit the many “Thens” I have described. So we interpret the timelines we encounter using what we find in their “Now.” We see the rusting war debris of the Silurian eurypterids or the bones of a lost Permian culture—still curled about their patinaed stone tools—and we extrapolate as best we can. We tell stories; all sentient life tells stories. In such a way, we make sense of profoundly alien things—other Earths filled with other thoughts. And sometimes what we find is close enough to us that we can learn its histories first-hand, visit its museums, view its curated artefacts. Or we might meet something advanced enough to communicate with us primates, crafting an interface we can look in the fabricated eye.

Looking back on some Earths, we’ve been forced to condense their tales so their story is easier to follow. We didn’t have time to tell you about the generation of global peace in the midst of the eurypterids’ terrible wars, or the death cults that riddled the early centuries of the immortal trilobites, or…a thousand million things. All we can do, in the end, is tell you stories. And all of them can be further reduced to a convenient cautionary tale that you—and we—may understand.

And we are closing in on our present, now. Not only is the guillotine shadow of the Permian epoch behind us, but the impact that terminated the Cretaceous period, too. These are events so cataclysmic that there is a reason they mark the end of those otherwise arbitrary geological periods. The life in the timeline I’m about to describe weathered those challenges exactly as ours did. The selfsame grab bag of species crept out to stare at that Cretaceous meteorite’s smoking crater. Birds, mammals, reptiles, insects, fish, plus an awkward gap next to the avians where the rest of their less flighty complement should be. In this timeline there are no dinosauroid survivors, no bold therapod hunters stalking the cold wastes of Bodmin Moor.

In the resulting Paleogene era, birds and mammals shove and elbow for terrestrial dominance. The birds will determinedly fatten up and make a bid for their dinosaur relatives’ lost crown. However, the mammals will prove to have better megafaunal elbows in the long run. Here in the Paleogene, across two continents you can already recognize as the Americas, this timeline branches away from the trunk perhaps forty million years ago. For the first time it’s the mammals’ turn at self-awareness, and they’re going to have a solid run at it.