| Life cannot exist without energy. All living things take in some kind of food or external chemical energy source, turn it into molecular energy (adenosine triphosphate, or ATP), and then spend that energy to power chemical reactions. These reactions include everything from building and transporting molecules to sending cellular messages — and in complex organisms like us humans, triggering nerve impulses in the brain and body and contracting muscles. The simplest organisms, such as bacteria, make ATP through processes such as respiration and fermentation. But more complex cells — including all big, multicellular organisms on Earth — have their own energy factories known as mitochondria.
Mitochondria have been an object of biological fascination and speculation since the discovery in 1963 that they have their own DNA. In 1967, the biologist Lynn Margulis theorized that this "powerhouse of the cell" was once a free-living proto-bacterium that merged with another ancient cell billions of years ago. Since then, biologists have argued that this partnership kicked off a new era of life on Earth. Having gained a dedicated energy-producing structure, cells were able to evolve new traits, form collectives and ultimately organize into complex multicellular superstructures made of many specialized cell types.
Over the past decade, tools that let biologists peer deep into cells have revealed that mitochondria are far more than our cells' captive energy generators. Indeed, studies are revealing that, in some ways, they have retained their ancient origins as free-living cells — and still behave that way.
Mitochondria can divide through fission or merge through fusion. They not only process food into energy, but also break down and synthesize many types of molecules. They function as signaling hubs, sharing messages within and between cells and tissues to set off stress, immune, metabolic and cell fate processes, including cell death. More recent studies show that mitochondria can even specialize for different functions, just like complex cells, and travel between cells through what's known as mitochondrial transfer.
All this work reveals that the "powerhouse" analogy has expired. And the more scientists discover about these influential organelles, the more central they appear to our physical and mental health.
What's New and Noteworthy Mitochondria can be targets of evolutionary processes, and research into birds shows how changes in the number, shape and behavior of mitochondria can support ecological traits — in this case, long-distance migration. Many birds perform athletic and physiological feats when flying long distances. For example, a one-ounce sparrow will fly hundreds of miles every spring and fall, flapping nonstop for days without stopping for food or water. Two labs independently pinned birds' seasonal sprints on differences in their mitochondria, which become "turbocharged" only during migration season — and then revert to their normal state during the rest of the year. It's a subcellular discovery that explains continent-spanning behavior, which I discussed with editor-in-chief Samir Patel in a recent episode of The Quanta Podcast.
What makes life tick? During the first stages of life, as an embryo divides into two cells, then four, eight, and so on, it does so at a certain pace. In addition to genetic differences, variation in developmental tempo can explain variability among species: For example, mice and humans use the same genes to create neurons or build a spine, but those genes are activated and deactivated on different timelines. What is the fundamental control center of an organism's developmental tempo? A body of work from different labs has pointed to mitochondria as the cellular metronome; these studies suggest that the rate at which mitochondria process molecules sets the rate of other cellular processes, such as gene expression and protein synthesis. In lab experiments, speeding up and slowing down the metabolic rates of mitochondria altered developmental rates too.
Along with producing ATP, mitochondria synthesize many other molecules: amino acids, nucleotides, carbohydrates, lipids and more, together known as metabolites. For many years, researchers in epigenetics have investigated how certain metabolites interact with DNA to turn genes on and off, regulating patterns of gene expression that determine cell types, for example differentiating a heart cell from a liver cell. Now developmental biologists are finding that a cell's metabolism and the products of its mitochondria help determine cell specialization during embryonic development, which shows how, at the molecular level, environment influences life in its earliest stages. "It really changes the way we think about developmental biology, the way we think about how our own life starts," said Berna Sozen, a developmental biologist at Yale University. | 
