WONDER OF THE MOMENT

     During the last third of the 20th century, biologists sorted life into five kingdoms. These were Animalia, Plantae, Fungi, Protista, and Monera. We’re all familiar with animals, plants, and fungi, but what are protista and monera? Protista are all the one-celled creatures whose cells contain a nucleus, and Monera are all the one-celled creatures whose cells do not contain a nucleus.
      One problem with this system of categorizing living things is that it is a human system, based more on what humans see than on what is actually true about the living organisms. For instance, four of the kingdoms, animals, plants, fungi, and protists, consist of creatures made of eukaryotic cells, that is, cells with a nucleus. Yet these are grouped as if they were as different from one another as all of them are from the monerans.
      For a long time, the members of monera were thought to be “bacteria and blue-green algae.” Eventually, researchers realized that those “blue-green algae” were actually photosynthetic bacteria. So actually, Kingdom Monera should simply have been called “Kingdom Bacteria.” But then, in the late 1970’s, a biologist named Carl Woese announced a startling discovery. He had compared RNA from a number of different living types and had found that Monera actually contained two quite different forms of life.
      Woese called the two forms of monerans “eubacteria” and “archaebacteria.” Furthermore, Woese could show that the eukaryiotic forms of life were more closely related to the archebacteria than to the eubacteria. Woese proposed that above the “Kingdom” category in the hierarchy of life forms, there should be a “Domain” category. The Domains would be Eubacteria, Archebacteria, and Eukarya, with Archebacteria and Eukarya closer together than to Eubacteria.
     For quite a long time, the rest of the biological community disbelieved and actively ostracized Woese. But now his domains are accepted and taught as a regular part of biology courses. The names of the Domains have been shortened to Bacteria, Archaea, and Eukarya. Further research has turned up intriguing similarities between eukarya and bacteria, and between eukarya and archaea. Obviously, clues about the evolution of eukaryotic cells provide plenty of work for evolutionary biologists!

Woese, Carl R. and Fox, George E. PNAS, 11/1/77, vol. 74, #11, pp. 5088-90.

         Since the 1960’s, we have discovered a lot about the evolution of cells.

         Fossil evidence indicated that bacteria had not only been the first living creatures, but they had had the earth to themselves for two billion years. Bacteria are single-celled organisms. Each one carries its genes, made of DNA, in a ring-shaped chromosome folded up in a special region of the cell. Smaller rings of genes, called plasmids, sometimes accompany this chromosome.

         Over two billion years, plenty of mutations took place in bacterial genes, resulting in vast numbers of different bacterial species. Also, being single-celled, bacteria were, and are, capable of picking up chromosome fragments from one another, introducing even more new species.

         About a billion and a half years ago, a new type of organism appeared in the fossil record. Like bacteria, they consisted of single cells. But unlike bacteria, these cells carried their chromosomes enclosed within a special membrane. These membrane-enclosed chromosomes formed a “nucleus” in the new cell type. To distinguish bacteria from the new cells, biologists call bacteria “prokaryotic,” meaning “before the nucleus;” and they called nucleated cells “eukaryotic,” meaning “true nucleus.” Besides the nucleus, the new eukaryotic cells contained a number of infinitesimal organs, called “organelles.” Some of these organelles were photosynthetic and made sugar from light energy. Some did the opposite, extracting energy from sugar to run cell processes.

         Over the next billion and a half years, mutations and gene trading resulted in vast numbers of new eukaryotic species. In some cases, eukaryotic cells joined into multicellular species, such as plants, animals and fungi.

         As François Jacob famously wrote, evolution acts like a tinkerer. Old devices and mechanisms get put to new uses. So it was unlikely that eukaryotic cells had sprung up on their own. It was much more likely that they had somehow evolved out of prokaryotic cells.

         In 1967, Lynn Margulis at Boston University suggested that the first eukaryotic cell could actually have been a group of prokaryotic cells that began living together. In fact, she found that the photosynthetic organelles, called “chloroplasts,” are quite similar to certain photosynthetic bacteria. She also found that the energy-harvesting organelles, called “mitochondria,” are quite similar to certain oxygen-using bacteria. And it turned out that chloroplasts and mitochondria have their own genes, exactly as we might expect, if they were actually bacteria that just happened to be living inside another cell. Margulis’ idea is called the “endosymbiont hypothesis” or the “endosymbiont theory.” It is the beginning of some interesting stories about cell evolution. Stay tuned!