Ben seemed exhilarated to be walking, stretching his legs, thumbs hooked in the straps of a backpack. “Australia is a place for creatures who walk,” he said. “That’s what we humans are adapted for. Look at your body some time. Every detail of it, from your long legs to your upright spine, is built for long, long walks through unforgiving lands, of desert and scrub. Australia is the kind of land we’ve been evolved for.”

“So we’ve been evolved to be refugees,” Madeleine said sourly.

“If you like. Looking at the crowd that seems to be on the way along the spiral arm, maybe that’s a good thing. What do you think?”

Walking, he said, was the basis of the Dreamtime, the Aboriginal Genesis.

“In the beginning there was only the clay. And the Ancestors created themselves from the clay — thousands of them, one for each totemic species…” Each totemic Ancestor traveled the country, leaving a trail of words and musical notes along the lines of his footprints. And these tracks served as ways of communication between the most far-flung tribes.

Madeleine had heard of this. “The song lines.”

“We call them something like ‘the Footprints of the Ancestors.’ And the system of knowledge and law is called the Tjukurpa… But, yes. The whole country is like a musical score. There is hardly a rock or a creek that has been left unsung. My ‘clan’ isn’t my tribe, but all the people of my Dreaming, whether on this side of the continent or the other; my ‘land’ isn’t some fixed patch of ground, but a trade route, a means of communication.

“The main song lines seem to enter Australia from the north or northwest, perhaps from across the Torres Strait, and then weave their way southwards across the continent. Perhaps they represent the routes of the first Australians of all, when they ventured over the narrow Ice Age strait from Asia. That would make the lines remnants of trails that stretch much farther back, over a hundred thousand years, across Asia and back to Africa.”

“From Africa,” she said, “to Triton.”

“Where the land is unsung. Yes.”

They climbed a little farther, through clumps of the wiry yellow-white grass, which was called spinifex. She reached out to touch a clump, feeling nothing; Ben snatched her hand back. He turned it over. She saw spines sticking out of her palm.

Patiently he plucked out the spines. “Everything here has spines. Everything is trying to survive, to hold onto its hoard of water. Just remember that… Look. ”

There was a crackle of noise. A female kangaroo, with a cluster of adolescents, had broken cover from a stand of bushes.

The kangaroos looked oddly like giant mice, clumsy but powerful, with rodentlike faces and thick fur. Their haunches were white against the red of the dirt. When the big female moved, she used a swiveling gait Madeleine had never seen before, using her tail and forelegs as props while levering herself forward on her great lower legs. There was a cub in her pouch — no, Ben said it was called a joey — a small head that protruded, curious, and even browsed on the spinifex as the mother moved.

The creatures, seen close up, seemed extraordinarily alien to Madeleine: a piece of different biological engineering, as if she had wandered into some alternate world. The Chaera, she thought, are hardly less exotic.

Something startled the kangaroos. They leapt away with great efficient bounds.

Madeleine grinned. “My first kangaroo.”

“You don’t understand,” Ben said tightly. “I think that was a procoptodon: a giant kangaroo. They grow as high as three meters…”

Madeleine knew nothing about kangaroos. “And that’s unusual?”

“Madeleine, procoptodon has been extinct for ten thousand years. That’s what makes it unusual.”

They walked on, farther from the car, sipping water from flasks.

“It’s the Gaijin,” he said. “Of course it’s the Gaijin. They are restoring megafauna that have long been extinct here. There have been sightings of wanabe, a snake a meter in diameter and seven long, a flightless bird twice the mass of an emu called genyornis… The Gaijin seem to be tinkering with the genetic structure of existing species, exploring these archaic, lost forms.”

They came upon an area of bare rock that was littered with bones. The bones were broken up and scattered, and had apparently been gnawed. Few of the fragments were large enough for her to recognize — was this an eye socket, that a piece of jaw?

“We think they use parsimony analysis,” Ben was saying. “DNA erodes with time. But you can deconstruct evolution if you have access to the evolutionary products. You track backward to find the common gene from which all the products descended; the principle is to seek the smallest number of branch points from which the present family could have evolved. When you have the structure you can recreate the ancient gene by splicing synthesized sequences into modern genes. You see?” He stopped, panting lightly. “And, Madeleine, here’s a thought. Australia has been an island, save for intervals of bridging during the Ice Ages, for a hundred million years. The genetic divergence between modern humans is widest between Australian natives and the rest of the population.”

“So if you wanted to think about picking apart the human genome—”

“ — here would be a good place to start.”

She thought of the Gaijin she had seen undoing itself, decades back, in Kefallinia. “Perhaps they are dismantling us. Taking apart the biosphere, to see how it works.”

“Perhaps. You know, humans always believed that when the aliens arrived, they would bring wisdom from the stars. Instead they seem to have arrived with nothing but questions. Now, they have grown dissatisfied with our answers, and are seeking their own… Of course, it might help if they told us what it is they are looking for. But we are starting to guess.”

“We are?”

They walked on, slowly, conserving their energy.

He eyed her. “For somebody who has traveled so far, you sometimes seem to understand little. Let me tell you another theory. Can you see any cactus, here in our desert?”

No, as it happened. In fact, now she thought about it, there were none of the desert plants she was accustomed to from the States.

Ben told her now that this was because of Australia’s long history. Once it had been part of a giant super-Africa continent called Gondwanaland. When Australia had split off and sailed away, it had carried a freight of rain-forest plants and animals that had responded to the growing aridity by evolving into the forms she saw here.

He rubbed his fingers in the red dirt. “The continents are rafts of granite that ride on currents of magma in the mantle. We think the continents merge and break up, moving this way and that, under the influence of changes in those currents.”

“All right.”

“But we don’t know what causes those magma currents to change. We used to think it must be some dynamic internal to the Earth.”

“But now—”

“Now we aren’t so sure.” He smiled thinly. “Imagine a huge war. A bombardment from space. Imagine a major strike, an asteroid or comet, hitting the ocean. It would punch through the water like a puddle, not even noticing it was there, and then crack the ocean floor.” His lips pursed. “Think of a scum on water. Now throw in a few rocks. Imagine the islands of dirt shattering, convulsing, whirling around and uniting again. That was what it was like. If it happened, it shaped the whole destiny of life on Earth. The impact structures wouldn’t be easy to spot, because the ocean floor gets dragged under the continents and melted. After two hundred million years, the ocean floor is wiped clean. Nevertheless there are techniques…”

A huge war. Rocks hurled from the sky, battering the Earth. Tens of millions of years ago. The hot dusty land seemed to swivel around her.