For centuries, scientists have dreamt of discovering an underlying unity to nature. Science now offers powerful explanations for both the dazzling diversity and striking similarities seen in the living world. Life is complicated. It is truly the “entangled bank” that Charles Darwin described. But scientists are now discovering that energy is the unifying force that joins all life on Earth. Visionary biologists have advanced a new theory that explains how the natural world—from the tiniest amoeba to the greatest rain forest—is constructed, providing a fresh perspective on the essential interconnectivity of living systems. This revolutionary theory explains a variety of phenomena—helping us understand why a shrew eats its bodyweight in food each day, why a mammal’s heart beats about 1 billion times in its lifetime, why there are no trees as tall as the Eiffel Tower, and why more species live at the Earth’s equator than at its poles. By looking at how living things use energy, we can answer these and myriad other intriguing questions. In the Beat of a Heart combines biography, history, science and nature writing to capture the exciting advances— and the people who are making them—that are triggering a revolution as potentially important to biology as Newton’s insights were to physics.