Once the electron microscope became widely available, biologist were able to examine the internal structure of a wide variety of cells. It became apparent from these studies that were two basic classes of cells-prokaryotic and eukaryotic-distinguished by their size and the types of internal structures, or organelles, they contain (Figure 2.1). The existence of two distinct classes of cells, without any known intermediates, represents one of the most fundamental evolutionary separations in the biological world. The structurally simpler, prokaryotic cells include bacteria, whereas the structurally more complex eukaryotic cells include protist, fungi, plants, and animals.
We are not sure when prokaryotic cells first appeared on Earth. Compelling evidence of prokaryotic life has been obtained from rocks approximately 2.7 billion years of age. Not only do these rocks contains fossilized microbes, they contain complex organic molecules that are characteristics of particular types of prokaryotic organisms, including cyanobacteria. It is unlikely that such molecules could have been synthesized abiotically, that is, without the involvement of living cells. The dawn of the age of eukaryotic cells is also shrouded in uncertainty. Complex multicellular animals appear rather suddenly in the fossil record approximately 600 million years ago, but there is considerable evidence that simpler eukaryotic organisms were present on Earth more that one billion years earlier. The estimated time of appearance on Earth of several major group of organism is depicted in Figure 2.2. Even a superficial examination of Figure 2.2 reveals how "quickly" life arose following the formation of Earth and cooling of its surface, and how long it took for the subsequent evolution of complex animals and plants.