Even in a multicellular organism, individual cells display a surprising amount of, well, individuality. Every cell has a distinct identity, and the majority are able to perform most, if not all, of the functions of free-living unicellular organisms . So distinct are cells that the cell doctrine of biology—the concept that all organisms are made up of cells—has been described as the fundamental paradigm of modern biology and medicine. The focus of biologists upon cell biology (there is a whole subfield of biology which goes by this name, and is perceived by its proponents as absolutely crucial to an understanding of life) has led to major breakthroughs in biology, but as we have seen, cell architecture is only fundamental to life at one particular scale . Vital interactions take place at subcellular levels, while multicellular organisms are very much more than the sum of their cells.
It is generally accepted that the cells of multicellular organisms evolved from independent single celled bacteria. Further, most biologists now believe that eukaryotic cells arose from the fusion of several different types of single celled organisms. This theory, known as endosymbiosis, was originally proposed in the nineteenth century, after the development of microscope allowed biologists to observe that cellular organelles bear a remarkable resemblance to free-living bacteria. The theory was not widely accepted amongst biologists until after it was espoused and publicized by the biologist Lynn Margulis in the 1960s. Since then, genetic analysis has confirmed that the DNA of organelles is in many ways more similar to that of bacteria than to that of the nucleus of the cell in which the organelle resides.
Basically, the theory suggests that eukaryotic cells evolved as a result of “lunch gone wrong” about 1.4 billion years ago. Bacteria prey upon smaller bacteria by engulfing them in an invagination of the cell membrane, which eventually pinches off to form a sac, or vacuole, inside the cell. Digestive enzymes then break down the contents of the vacuole and the hapless prey is consumed. It appears, however, that this process is not always foolproof. Some small bacteria, originally destined to be lunch, seem to have sidestepped the digestion process and taken up residence in the cytoplasm of the would-be diner. Whether they were originally harmful to their unwitting host is unknown; most probably the vast majority were. However, some tenants possessed abilities which were usefully complementary to those of the host—the ability to convert sunlight into useable energy via photosynthesis, for example, or to carry out other chemical reactions which were not in the host’s repertoire. The engulfed bacteria also benefited from the relationship, gaining advantages such as being part of a larger organism and therefore safer from predation, and having access to a steady supply of food. A partnership was established, and over time became not just beneficial but mutually necessary to the participants. From an uneasy admixture of predator and prey, the eukaryotic cell was born.
The specific sequence of steps that Margulis proposes to have occurred during evolution are as follows:
1. A sulfur and heat-loving archaebacterium merged with a swimming bacterium to form the first nucleated single celled organism (protist). This swimming protist eventually evolved mitosis, the process of cell division;
2. The protist merged with an aerobic bacterium (a purple bacterium or proteobacterium, whose descendants eventually evolved into mitochondria, the energy factories of the cell);
3. In some lineages, that commune engulfed photosynthetic bacteria (cyanobacteria) to form swimming green algae, the ancestors of today's plant cells.
The basic eukaryotic cell had been born, and continued to evolve and diversify into the whole spectrum of eukaryotes—single-celled and multicellular—that surround us today.
This sequence of events is now widely accepted. Eukaryotic cellular organelles still carry their own DNA, although often in a fairly rudimentary form, and the genes they carry have been demonstrated to be closer in sequence to genes from free-living single-celled organisms than to the genes in the nucleus of the cell in which they dwell. Further, there are minor differences in codon usage between organelle and nuclear DNA . The evidence appears to be clearly on the side of endosymbiosis.
