The basic unit of life — the smallest thing that is itself alive, a membrane-bounded packet of chemistry that takes in energy, responds to its surroundings, grows, and reproduces. Cell theory holds that all living things are made of cells, that the cell is life's basic unit, and that every cell comes from a pre-existing cell. That last claim, with later experiments by Pasteur and Tyndall, helped end the old belief in spontaneous generation and made biology a single science: the same machinery runs a bacterium, an oak, and a human, all of them descended in an unbroken line from the first cells.
In 1951, doctors at Johns Hopkins took a sample of a cervical tumor from a thirty-one-year-old Black woman named Henrietta Lacks. They did not ask her. They did not tell her. Her cells did something almost no human cells had done before in a laboratory dish: they kept dividing, and dividing, and never stopped. Named HeLa from the first letters of her name, they became the first immortal human cell line, and they went on to underpin much of modern medicine, from the polio vaccine to cancer research to later COVID-19 vaccine research. Lacks died that same year, in a segregated ward, of the cancer those cells came from. Her family learned what had happened only decades later, and shared in none of the wealth her cells helped generate. The single living unit that all biology is built on turned out to carry a question biology could not answer on its own: who owns a life once it has been reduced to a cell in a dish?