Nothing lasts forever, Tam reminded herself, least of all man. The likelihood of life elsewhere in the Galaxy, the inevitability of her species being drawn into an interstellar extinction lottery, was all spelled out on the insides of Sargenti-Peterson. Even before her team had landed and started tunneling, Tam knew the comet to be a treasure chest of strange and thrilling discoveries. Arginine was venting from somewhere below the surface—and ethanol, purines, and even porphyrin molecules—actual precursors of chlorophyll and hemoglobin.

Inside they had found microscopic balls and filaments of protein, filled with enzymes that were able to make starch. A few of them even yielded up crude snippets of RNA. Tam’s best guess was that life on Earth had begun much like this, with billions upon billions of protocells forming and dissolving every day on the high seas and in underground slime pools, with no two exactly alike. Earth’s protocell era had left few traces of itself in the fossil record, but here was the same process frozen in mid-stride for all to see.

But not permanently frozen, Tam had discovered. Ever since some disturbance had slowed Sargenti-Peterson’s orbital motion in the Oort Ring, sending the comet sunward every two hundred years, warmth seeping down below the surface was periodically bringing the hydrothermal springs to life; and for a year or more evolution would pick up where it had left off—random variation providing the raw materials, natural selection sorting out the winners and losers—until the next deep freeze set in.

Most of her crew believed that comets full of protocells had seeded life on Earth, but Tam disagreed. She saw no reason why the Earth could not have cooked up its own life, to which the comets might simply have added their own two cents worth. That colonies of living cells had been found thriving near hydrothermal vents beneath the ice of Ganymede, Europa, Titan, and Enceladus—and that frozen protocells had been found inside Ceres, and were still evolving in the nucleus of Sargenti-Peterson— drove home the conclusion that wherever there existed a warm and wet place, sooner or later the process of life was bound to get started.

Tam floated upside down (by one set of references) in the microbiology chamber, intently studying a cartridge of slides. Everyone else had gone off to take care of more important concerns, such as watching the Valkyries explode or peering down through the clear eyes of Earth’s proliferating hurricanes to see what traces of humankind might still be left on the planet.

Tam’s wallscreen showed a sample of protocells from one of Sargenti-Peterson’s newly restarted vents. Scores of smaller proteinaceous sacks had surrounded and were absorbing a larger one, exchanging biochemistries as they fed. She could not decide whether to call this evolutionary step the invention of sex or the dawn of predation. She was about to decide that both descriptions were true when Second Officer Susan Skurla drifted into the room with something in her hand.

“Here’s that book you wanted,” Susan said.