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Multicellular strings of bacteria were born around 3.5 billion years ago. Filamentous bacteria\u2014so named because they form chains\u2014kill themselves to yield precious nitrogen for the good of their sisters. Every tenth or so cell commits suicide for the benefit of this communal thread of bacterial life. Another, quite different kind of microbial cooperation was revealed by the dogged efforts of Lynn Margulis at the University of Massachusetts, Amherst. She proposed that more complex \u201chigher\u201d cells were the result of symbiosis, when single-celled creatures became so closely associated that they worked as one. For example, around 1.8 billion years ago, there was an important moment when one kind of wriggling bacterium invaded another. Perhaps the former was seeking food. But this particular parasitic infestation suited both parties and evolved so that the participants formed a long, harmonious, and fruitful truce. This is what Margulis calls symbiogenesis and it led to the formation of higher cells, known as the Eukarya.<\/p>\n\n\n\n
Thanks to this cooperation, a new, more complex kind of cell appeared on Earth. Whereas bacterial cells are relatively simple and known as prokaryotes, the newer cellular consortia, known as the eukaryotes, are the building blocks of plant and animal bodies, including our own. These cells contain organelles\u2014 which divide up the task of cellular life as organs do for a body\u2014including a nucleus where their DNA resides. These organelles are the leftovers of earlier episodes of microbial mergers and acquisitions.<\/p>\n\n\n\n
Peek inside your own cells and you will find these Russian doll coalitions. The most obvious example of symbiogenesis comes in the form of little lozenge-shaped structures called mitochondria. Not only do they look like bugs, they even have their own separate DNA that is passed down the maternal line, from mother to child. Our cells are driven by these descendants of bacteria that hundreds of millions of years ago traded chemical energy for a comfortable home. These organelles now power our muscles, our digestion, and our brains.<\/p>\n\n\n\n