Although the cell is the smallest unit of life, it is by no means simple. The human body is made up of tens of trillions of cells like this one, which have developed a highly synchronized set of components to carry out the processes that keep the organism alive, allow it to reproduce and adapt to changing environments. New research suggests such complex cells, which scientists call "eukaryotes," arose from the fusion of two simpler kinds of single-celled organisms in a symbiotic effort to survive. An ostrich egg is the largest known single cell, but most individual cells are too small to see without a microscope.