Uni cell to multi cell

For most of the history of life, which began roughly 3.9 billion years ago, there was only one way to be alive: as a lone cell. The first life forms were, in their entirety, single, clearly defined microscopic units that reproduced by dividing into two new cells, each of which went on its way. Life stayed that way for billions of years. But then some of these cells started to cooperate. They transitioned from solo existence into group life. When one cell became two and then more, they remained together and eventually came to function as a distinct kind of living assemblage: a multicellular organism.
 
Despite the ongoing success of single-celled life, multicellularity has proved to be a remarkably successful adaptation. Life invented it not once, but at least 20 times. Independent evolutionary events resulted in today’s multicellular plants, fungi and animals. Crucially, it allows for a division of labor: Within a single organism, cells fill specific roles to the exclusion of others. Our bodies, for example, are made up of trillions of cells with different identities and jobs. Immune cells fight invaders. Nerve cells help us move, feel and think. Heart cells pump blood around the body.
 
By divvying up jobs among cells, multicellular organisms could grow both larger and more complex, and develop new ways of living. But multicellularitycomes at a cost: Survival depends on the functioning of an interconnected system with high energy demands in which the deaths of some cells can kill the rest, too.
 
How multicellularity evolved is one of biology’s great mysteries. The timelines are murky. So are the reasons. And fossil evidence from most of the history of life simply doesn’t exist. The fossil record we have suggests that multicellular life started to become more common around 600 million years ago. But other evidence suggests that very simple multicellular organisms may have been around a billion years before that.