Social cells: The slime mold Dictyostelium discoideum spends much of its time as an apparently typical microscopic single-celled amoeba, oozing around in wet soil as it grazes on bacteria. Something truly odd happens, however, when the food runs out. Starving D. discoideum band together to form a conglomerate organism. A multicellular slug of sorts, the group grows into a spore-making tower, a beacon for sending amoebae out to richer grounds. The sudden lifestyle change is interesting enough, but the real evolutionary puzzle is the cells that make up the delicate stalk. They die without reproducing, which means cells at the top of the tower can turn into more effective spores. This form of altruistic sacrifice has fascinated biologists for decades. (It appears that related amoebae are more likely to group together, so even the dying cells get to pass on their genes.) Here is an organism that is both solitary and fully, suicidally social, a nearly perfect model creature for understanding how multicellular life emerged from the amoebae. In this shot, a slice through a culture dish, you can see a progression of slugs into towers.
Image: Alex Wild