| For most of the history of life, beginning 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, 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 multicellularity comes 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.
This uncertainty hasn't deterred scientists who want to trace the origins and history of complex life. Some search for fossils of tiny organisms or even molecules, while others try to coax multicellular strategies out of single-celled organisms in the lab. Theories abound, and most don't fit together. But given that multicellularity evolved so many times, we don't need to limit ourselves to a single explanation. Each new study and thought experiment gets us closer to understanding a critical evolutionary moment that made us possible.
What's New and Noteworthy
The physics of frigid seawater hundreds of millions of years ago may have played a role in the evolution of animal multicellularity, Veronique Greenwood reported for Quanta in July. Cold water, such as the water on Earth when the planet was blanketed in ice around 700 million years ago, is more viscous and therefore more difficult for single-celled organisms to swim through. In an experiment, the paleobiologist Carl Simpson of the University of Colorado, Boulder saw swimming, single-celled algae begin to behave collectively across many generations as they traversed increasingly viscous gels in petri dishes, simulating conditions that could have given rise to at least one form of multicellularity.
At the Georgia Institute of Technology, Will Ratcliff is also probing multicellularity in modern organisms. In 2021, he published work showing that in only two years, single-celled yeast can grow into multicellular snowflakes big enough to be seen with the naked eye. Ratcliff also takes a theoretical approach. Recently he wondered why prokaryotic cells such as bacteria — which, unlike our eukaryotic cells, lack a nucleus and organelles — never evolved their own version of multicellularity. Based on a computational model, his lab's research suggests that prokaryotic genomes may be too small to support the necessary complexity, as Greenwood reported for Quanta in May.
Many scientists assume that the first multicellular organisms started out as collectives of identical cells before they began to specialize. But recent evidence suggests that ancient single-celled creatures were already surprisingly complex. They carried the potential to specialize all along, differentiating into novel forms to perform certain tasks before reverting to a standard form — similar to our own stem cells — as Jordana Cepelewicz reported for Quanta in 2019. Those findings suggest that specialization might have been in place well before multicellularity made it more permanent. | 
