Sun light is the primary source of energy for Earth's biosphere made available through photosynthetic organisms. In the cells of such organisms, so-called photosynthetic units form in which about a hundred interacting chlorophylls are assembled into a molecular network. The network (see figure) absorbs through its individual chlorophylls light and transfers the ensuing electronic excitation and with it the light energy efficiently along the network connections to a center where the energy is transformed into a membrane potential that subsequently fuels the machinery of the organisms' cells. The biological evolution that lead to such energy transfer networks had to obey physical constraints that act upon the network assembly and all stages of the light harvesting process. With the availability of atomic level structures of two networks, one in plants and one in cyanobacteria, it became possible to compare the energy transfer networks from two related species that are separated by more than a billion years of evolution. The comparison is reported in a recent paper and on our web site. It answers many questions about how living systems evolved extremely efficient solar cells for their energy needs, while still leaving a puzzle behind to be answered: despite a huge evolutionary distance the plant and bacterial photosystems maintained an amazing identity as the positions and geometry of about eighty chlorophylls remained unchanged for over a billion years. For most of these chlorophylls, repositioning would have had little effect on the efficiency of the light harvesting processes as calculations show, yet evolution chose to freeze the chlorophyll ensemble geometry-wise.