Right became distressed, as she so often did. She dropped the pea pods and folded over on herself, rocking, keening softly, her hands over her small face. Noth took her in his arms and carried her to the crook of a branch, where he began to groom her. He worked carefully through the sparse fur on her back, neck, head, and belly, removing dirt, bits of leaf, and dried feces, untangling knots, picking out parasites that were attempting to feast on her young skin.

Right quickly calmed. The grooming’s mixture of pleasure, attention, and mild pain flooded her system with endorphins, her body’s natural opiates. Before she grew much older she would be addicted, literally, to this pleasurable scratching — as her brother already was. Noth badly missed the strong, nipping caress of adult fingers on his back.

But Noth was worried about her, on deep levels he could not understand.

Right’s bewildering grief served a purpose. It was a signal to her that she had suffered a loss, that there was a hole in her world that she must fix. And though Noth was not capable of true empathy — if you didn’t really understand that other people had minds and thoughts and feelings like yours, you couldn’t possibly be empathetic — still the signs of grief in his sister triggered a kind of protectiveness in him. He wanted to put the world right for his sister: The instinct to help the orphaned went very deep.

But in the end obsessive grief was maladaptive. If Right was unable to recover, in the end there would be nothing he could do for her. He would have to abandon her, and then she would surely die.

As day followed day, the sun, at the lowest point of its arc in the sky, began to slip beneath the southern horizon. At first the brief nights were like twilight, and on clear nights purple-red curtains of light climbed into the tall sky. But quickly the sun’s excursions into invisibility became longer, and there were increasing intervals when stars shone in a deepening blue. Soon true darkness would return to the polar forest.

The weather quickly became colder and drier. Rainfall was scarce now, and on some days the warmth of the sun barely seemed to penetrate the lingering mist. Already many of the birds of the forest canopy had departed, skein after skein of them flitting over the sky to the warmer lands to the south, watched by uncomprehending primate eyes.

Noth became exhausted, ragged, and his dreams were full of flashing teeth and biting claws, visions of his scrap of a sister taken by gigantic mouths.

Now their biggest problem was thirst. It had been so long since the last rain that the treetops were becoming parched. And already the trees were starting to shed; the last leaves were withered and brown. Soon Noth was reduced to licking the bark each morning for the cold dew.

At length, driven by their thirst, the siblings went in search of ground water. Near the closest large lake they scurried down a tree trunk, eyes wide.

Approaching the water, the primates crept past a pair of what looked like miniature deer. The size of small dogs with long, trailing tails, these fast, solitary runners, browsing on leaves and fallen fruit, were ancestors of the mighty artiodactyl family, which would one day include pigs, sheep, cattle, reindeer, antelope, giraffes, and camels. Right disturbed a frog, which hopped away, croaking in protest. She cowered back, eyes wide at its strangeness. Soon they saw more amphibians, frogs and toads and salamanders. Birds crowded the bushes, raising shrill cries that filled the dank air.

Noth was uneasy. The shore was too crowded: Noth and Right were not the only thirsty creatures in this shivering jungle.

A meter-long creature like a long-tailed kangaroo ran past; this was a leptictidium, a hunter of small animals and insects. Exploring the ground with its mobile nose, it disturbed a pholidocercus, a spiky-haired ancestor of the hedgehogs, that indignantly hopped away like a rabbit. Here was a close-packed herd of horses. They were tiny: no larger than terriers, with perfectly formed equine heads. Shyly these exquisite little creatures picked their way through the undergrowth. They walked on pads, like cats, and on each foot they had several hoofed toes. Their genus had emerged in Africa only a few million years earlier. A rough growl from an impatient carnivore startled the little horses, making them stir into sudden flight.

Through this exotic crowd the two primates proceeded cautiously, moving in scurries, in fits and starts.

The water itself was a languid sheet, dense with matted vegetation, dead reeds, and algal blooms. In places ice had already formed in thin gray slices. But on the open water birds waded, ancestors of flamingos and avocets, and huge water lilies rested languidly on the surface.

Over the open water a spider was suspended on a thread of silk, and huge ants flew, each as large as a human hand, on their way to found new nests. Through this crowd of insects flapped a family of delicate bats. Recently evolved, as huge and fragile as paper kites, the new flying mammals snapped at the insects. Primitive bony fish broke the surface and gulped at the aerial fodder, as did a twisting eel.

The primates found a place far enough from any of the predators to be able to drink unhindered. They bent and plunged their muzzles into the chill water, sucking it up gratefully.

The largest animals of all wallowed at the muddy fringes of the lake.

A pair of uintatheres stood side by side. These great animals looked like gargantuan rhinos, each with a set of six bony horns on its head and long upper canine teeth like a saber-toothed cat’s. Their thick hides were coated with mud, which helped keep them cool and kept off insects. They cropped placidly on the soft vegetation of the lake bottom, sucking at water stained green by algae, while a fat youngster, more agile and lively, played around his parents’ legs, barging their tree-trunk knees with a head ladened with stubby, unformed tusks. Noth watched their huge feet fearfully. Closer to the shore there walked a family of moeritherium. No more than a meter tall, the adults moved through the water with a stately calm, rumbling reassurance to each other, while their round-bodied infants splashed at their feet. They worked the lake bottom vegetation efficiently with their long noses. These were among the first proboscideans, the ancestors of elephants and mammoths. They were still more piglike than elephantine, but they were already clever and social animals.

Around the herbivorous herds circled carnivores. These were mostly creodonts; they looked like foxes and wolverines. And there was one pack of hoofed predators — like carnivorous horses — bizarre, terrifying creatures with no analogies in human times.

Many of these creatures looked slow and lumbering, oddly ill-formed, the results of nature’s first experiments in producing large herbivores and predators from the mammalian stock that had survived the dinosaur extinction. Open grasslands still lay millions of years in the future, along with the fleet, long-legged, graceful herbivorous forms that would adapt to their open lush spaces, and the cleverer, faster carnivores that would arise to prey on them. When that happened most of the species around Noth would succumb to extinction. But the orders that would be familiar to humans — the true primates, the hoofed animals, the rodents and bats, the deer and the horses — had already made their entrance on the stage.

And there was no more complex and crowded an ecology anywhere on Earth right now than here on Ellesmere Island. This place was a pivot on the great migratory routes up through the Americas and over the roof of the world to Europe, Asia, and Africa. Here, pangolins from Asia, carnivores from North America, hoofed creatures from Africa, European insectivores like ancestral hedgehogs, and even anteaters from South America mingled and competed.