Bex was listening politely, but she looked distant. She has grown up with such earnest lectures, Joan thought. It must all mean little or nothing to her, echoes of a world vanished before she was even born.

Alyce subsided, the old frustration showing in her face. And meanwhile the plane continued to limp through the smoky sky.

To break the slight tension — she hadn’t meant to lecture this girl, only to distract her — Joan changed the subject. “Alyce studies creatures that are alive today. But I study creatures from the past.”

Bex seemed interested, and in response to her questions Joan told her how she had followed the example of her own mother, and about her work, mostly out in the desert heartlands of Kenya. “People don’t leave many fossils, Bex. It took me years before I learned to pick them out, tiny specks against the soil. It’s a tough place to work, dry as a bone, a place where all the bushes have thorns on them to keep you from stealing their water. After that you return to the lab and spend the next few years analyzing the fragments, trying to learn more of how this million-year-dead hom lived, how she died, who she was.”

“Hom?”

“Sorry. Hominid. Fieldwork slang. A hominid is any creature closer to Homo sap than the chimps — the pithecines, Homo erectus, the Neandertals.”

“All from bits of bone.”

“All from the bone, yes. You know, even after a couple of centuries’ work, we have dug up no more that two thousand individuals from our prehistory: two thousand people, that’s all, from all the billions who went before us into the dark. And from that handful of bones we have had to try to infer the whole tangled history of mankind and all the precursor species, all the way back to what happened to our line after the dinosaur-killer comet.” And yet, she thought wistfully, lacking a time machine, the patient labor of archaeology was all there was, the only window into the past.

Bex was starting to look distant again.

Joan remembered a trip she had taken to Hell Creek, Montana, when she was about this girl’s age, thirteen or fourteen. Her mother had been working there because it was a famous dinosaur-extinction boundary site. You could see traces of the huge event that had ended the dinosaur era, there in the rocks, in a layer of gray clay no thicker than her hand; it was the Cretaceous-Tertiary boundary clay, laid down in the first years after the impact. It was full of ash, the fallout of a huge disaster.

And underneath the clay, one day, her mother had found a tooth.

“Joan, this isn’t just a tooth. I think it’s a Purgatorius tooth.”

“Say what?”

Her mother was big, bluff, her face coated with sweat and dust. “Purgatorius. A dinosaur-era mammal. Found it right under the boundary clay.”

“You can tell all that from a tooth?”

“Sure. I mean, look at this thing. It’s a precise piece of dental engineering, already the result of a hundred and fifty million years of evolution. It’s all connected, you see. If you’re a mammal you need specialized teeth so you can shear your food more rapidly, because you have to fuel a faster metabolism. But if your mother produces milk, you don’t need to be born with your final set of teeth; the specialist tools can grow in place later. Didn’t you ever wonder why you had milk teeth? Joan, a lot of people are going to care a great deal about this. You know why? Because it’s a primate. This little scrap could be all that’s left of the most remote ancestor of you and me — and everybody alive — and the chimps and gorillas and lemurs and—”

And so on. The usual lecture, from the great Professor Useb. Joan, at age thirteen, had been a lot more interested in spectacular dinosaur skulls than ratty little teeth like this. But still, something about it had stuck in her mind. And, in the end, such moments had shaped her life.

“That’s the point of the conference, you see, Bex,” Alyce was saying. “It’s a synthesis. We want to pull together the best understanding we have of how we got here, we humans. We want to tell the story of humankind. Because now we have to decide how we are going to deal with the future. Our theme is the globalization of empathy.”

That was true. The real purpose of the conference, known only to Joan, Alyce, and a few close colleagues, was to found a new movement, establish a new way of thinking, a new approach that might actually stave off the human-induced extinction event.

Bex shrugged. “You think anybody’s going to listen to a bunch of scientists? No offense. But nobody has so far.”

Joan forced a smile. “No offense taken. We’re going to try anyway. Somebody has to.”

“And there’s no point in all that stuff anymore, is there? Your archaeology.”

Joan frowned. “What do you mean?”

Bex clapped her hands over her mouth. “I shouldn’t say anything. My mother will be furious.” Her Martian eyes were bright.

Alyce had withdrawn into herself again; she gazed out of the window at the billowing debris of forest fires a thousand kilometers away.

Suppose I threw you down the strata, back into time, Joan’s mother had said to her. After just a hundred thousand years you’d lose that nice high forehead of yours. Your upright-walker legs would be gone after three or four million years. You’d grow your tail back after twenty-five million years. After thirty-five million, you’d lose the last of your ape features, like your teeth; after that you’d be a monkey, child. And then you’d keep on shrinking. Forty million years deep you’d look something like a lemur. And eventually -

Eventually, she would be a little ratty thing, hiding from dinosaurs.

Sometimes she had been allowed to sleep in the open, in the cool air of the badlands. The Montana sky was huge and crammed with stars. The Milky Way, a side-on view of a giant spiral galaxy, was a highway across the night. She would lie on her back, gazing up, imagining the rocky Earth had vanished, its cargo of fossils and all, and that she was adrift in space. She wondered if that little Purgatorius critter would have seen the same sky. Had the stars swum about the sky, across sixty-five million years? Did the Galaxy itself turn, like some huge pinwheel in the night?

But tonight, she thought, the smoke from the volcano would hide any stars.

At the edge of the clearing, Purga crept out of a dense patch of ferns. It was night, but there was plenty of light — not from the Moon, but from the comet whose spectacular tail spread across the cloudless sky, washing out all but the brightest stars.

This scrap of forest lay in a broad, shallow lowland between new volcanic mountains to the west — the mountains that would become the Rockies — and the Appalachian plains to the east. Tonight the damp air was clear; but often mists and fogs blew in from the south, born over the great inland sea that still pushed deep into the heart of North America. The forest was dominated by plants that could extract moisture from the air: Lichen coated the gnarled bark of the araucaria trees, and even the low magnolia shrubs dripped with moss. It was as if the forest had been coated with a layer of thick green paint.

But everywhere the leaves were soured, the moss and ground cover ferns browned. The rains, poisoned by gases from the great volcanic convulsion to the west, had been hard on plants and animals alike. It wasn’t a healthy time.

Still, in the clearing, dinosaurs dreamed.

The thick night dew glistening from their yellow-black armor, ankylosaurs had gathered in a defensive circle, their young at the center. In the gentle Cretaceous air, these cold-blooded giants stood like parked tanks.