13.13–13.15: Most protists are single-celled eukaryotes.

Diatoms, single-celled photosynthetic algae that form an important part of the plankton, have mineralized cell walls that provide protection and support.

For the first 2 billion years of life on earth, organisms were extremely small—bacteria and archaea less than 10 micrometers across, one-eighth the diameter of a human hair. But in rocks about 1.9 billion years old, we find fossils of new kinds of organisms that are 10 times larger (FIGURE 13-20). These are a group of organisms called acritarchs (a name that can be translated as “confusing old things”). They were the first eukaryotes.

Figure 13.20: Ancient protists. These acritarch fossils represent the first eukaryotes on earth. The oldest fossils of this type were found in rock 1.9 billion years old.

The larger size is the first thing you notice about these fossil cells, but the internal changes in the cells are even more significant: they were the basis for the success of the entire eukaryote lineage, including humans. For the first time in the history of life, cells had internal structures that carried out specific functions. These structures, the cellular organelles, perform the specialized activities that make eukaryotic cells more complex than prokaryotes.

The nucleus is an evolutionary innovation that appeared for the first time in protists. The nucleus is a region inside the cell that is enclosed by its own double-layered membrane. The nuclear membrane was probably formed by the fusion of folds of the plasma membrane that surrounded the cell. We know that many modern prokaryotes have complex infoldings of their plasma membranes—these increase the total surface area of the cell and allow better exchange of material between the cell and the external environment—and infoldings of this type may have played a role in endosymbiosis and the formation of the first organelles (see Figure 3-8). Infoldings probably fused around the DNA, for example, creating a membrane-enclosed compartment containing the cell’s DNA. Thus, the cell nucleus was formed, separated by a double membrane from the cytoplasm of the cell.

Subsequently, these first nuclear membranes developed two specializations: they incorporated proteins that controlled the movement of molecules into and out of the nucleus, and they extended outward from the nucleus to form a folded membrane called the endoplasmic reticulum. The endoplasmic reticulum is the part of a eukaryotic cell where some proteins are assembled. Further development of internal membranes produced the Golgi apparatus, where newly synthesized proteins are given some final processing steps, and sac-like structures called lysosomes, which contain enzymes that break down damaged molecules. Finally, a lineage of protists took in a guest—a bacterial cell that subsequently became the mitochondrion, the organelle that produces most of the ATP synthesized by a eukaryotic cell.