A freak event created the ancestor of all multicellular life on Earth. Without this unconventional genesis, we might never have become more than bacteria
Life is what you make of it. For the first organisms on the newly watery Earth, that was little. For a billion years or more, single-celled entities simply morphed, multiplied and colonised the oceans. Photosynthesis was one innovation: from about 2.5 billion years ago, blooms of sea bacteria supplied Earth's atmosphere with its first whiff of oxygen. But when the next turning point came, it was in a rather unexpected direction.
Life on Earth stands either side of a chasm. On one side are the prokaryotes - bacteria and archaea - whose simple cells are not much more than tiny bags of chemicals. On the other are the eukaryotes, whose complex cells have internal membranes, skeletons and transport systems.
The world's largest bacterium is less than a millimetre long, but a single eukaryotic cell can stretch for well over a metre. And while bacteria never form anything more complex than strings of identical cells, eukaryotic cells cooperate to make everything from brains and leaves to bones and wood.
The countless simple cells living in many different environments on Earth have had over 3 billion years to evolve complexity. It could have happened repeatedly - and yet it appears to have happened just once, perhaps 2 billion years ago. All complex life is descended from a single common ancestor.
Why is that so? Because, says Nick Lane of University College London, natural selection normally favours fast replication, keeping simple cells simple. Then a freak event occurred: an archaeon engulfed a bacterium and the two cells formed a symbiotic relationship. That transformed the dynamics of evolution, leading to a period of rapid change that produced innovations such as sex. The incorporated bacterium eventually evolved into mitochondria, the energy generators of complex cells.
Such "endosymbioses" are now common in complex cells.The chloroplasts that carry out photosynthesis in plant cells, for example, were originally a photosynthesising bacterium. But we only know of a couple of other examples of a simple cell playing host to another. So it seems there was nothing inevitable about the rise of the complex cells from which we evolved. "The unavoidable conclusion is that the universe should be full of bacteria, but more complex life will be rare," says Lane.