For 3 billion years, life was unicellular. Why did it start to collaborate?

After the dawn of life some 4 billion years ago, one of the most fascinating and mysterious turning points in the history of life on Earth is its leap from single-celled organisms to multicellularity about 3 billion years later. Most scientists believe that this transition occurred only once in animals. So how and why, after so many aeons, did cells begin to cooperate and specialise to ultimately form animal life? This video from Nature uses microscopy to explore five unicellular microbes that display limited multicellular behaviours. Scientists hope that studying these species may help explain how life made the extraordinary leap towards complexity, giving rise to the rich diversity of animals on the planet today – including, of course, the complex creatures whose cells are collaborating to read these words.