Biologists have discovered that primitive, predatory lampreys have structures within their gills that play the same role as the thymus, the organ where immune cells called T cells develop in mammals, birds and fish. The finding suggests that in vertebrate evolution, having two separate organs for immune cell development — the bone marrow for B cells and the thymus for T cells — may have preceded the appearance of the particular features that mark those cells, such as antibodies and T cell receptors.