The first method to cause a culture of human and other primate stem cells to directly and uniformly differentiate into a committed cell lineage is disclosed. Treatment of primate stem cells with a single protein trophoblast induction factor causes the cells to transform into human trophoblast cells, the precursor cells of the placenta. Several protein factors including bone morphogenic protein 4 (BMP4), BMP2, BMP7, and growth and differentiation factor 5 can serve as trophoblast-inducting factors.