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.