For more than 150 years scientists have primarily relied on fossilized bones and teeth to reconstruct creatures from deep time. Skeletons divulge the sizes and shapes of long-ago animals; muscle markings on bones indicate how brawny the creatures were and how they may have moved; tooth shape and wear attest to the kinds of food eaten. All in all, researchers have managed to extract extraordinary quantities of information from these hard parts. On rare occasions, they have chanced on exquisitely preserved mummies and frozen carcasses that have allowed them to add more detail to their reconstructions, such as the length of the fur, the shape of the ears, the specific contents of an animal's last supper. Yet for all that scientists have been able to deduce about the physical characteristics of life-forms from past eras, we know very little about the physiological processes that sustained them.
That gap is closing, however. Recent advances in biotechnology now allow us to reassemble ancient genes from extinct animals and resurrect the proteins those genes encode—proteins that both form and drive the cellular machinery that underlies life-giving processes. The work heralds the dawn of a thrilling new scientific discipline: paleophysiology, the study of how the bodies of bygone organisms functioned in life. We are still in the earliest days of this research, but already we have gained stunning insights into how one iconic beast of prehistory—the woolly mammoth—adapted to the brutal conditions of its Ice Age world. Although the Jurassic Park dream of cloning prehistoric animals remains out of reach, our work has demonstrated the feasibility of observing key physiological processes that took place in creatures that have long since vanished from the face of the earth.