In the first weeks of an embryo's life, a dense crowd of cells crawls across its body. These cells will later form the facial bones, parts of the heart, and nerves. The crowd doesn't move randomly: a few leader cells take the front, and the rest follow them, like cyclists in a peloton.

For a long time, it was believed that leaders are a special breed of cells, born with a drive to lead. But live observations show otherwise: a cell that leads today may end up at the back tomorrow. So the leader role isn't hardwired into the cell; it emerges along the way.

The new model describes each cell as a tiny compass. The compass needle—'polarity'—points where the cell wants to crawl. The compass spins depending on how long the cell has been sensing the chemical signal. If the signal has been coming from the front for a while, the needle points forward, and the cell breaks into leadership. If the signal is weak or erratic, the cell sticks with its neighbors.

Running thousands of such cells in a computer simulation, scientists saw how a flock with leaders emerges from complete uniformity. No one appointed the leaders: they arose from shared rules, like a stadium wave from individual stands.

The model was compared with real footage of cell movement in a chicken embryo. The behavior matched: both in life and in simulation, leaders go ahead and the rest follow, but roles can switch at any moment.