Why did we ever leave the single-celled world? Why did “we” become “we”—that is, multicellular organisms? Take a yeast cell or some species of single-celled algae. These single cells, or modern cells, as biologist Nick Lane calls them, possess virtually all the features of the cells of vastly more complex organisms, including humans. They are abundant, fiercely successful in their environments, and can thrive in diverse places on Earth. They communicate with one another, reproduce, metabolize, and trade signals. They possess nuclei, mitochondria, and most of the cellular organelles that make
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