WONDER OF THE MOMENT

         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!

In a follicle in the body of a maternal fruit fly or mouse or human being lies an egg cell.

This egg cell is immense compared to the sperm that may eventually fertilize it. A sperm must race vast distances to arrive at the egg in time to compete with millions of other sperm for the fertilization prize, the chance to pass its genes to future generations. So a sperm carries only necessities: a nucleus with the genes packed into chromosomes, high energy molecules and energy-releasing structures to power the long race, a flagellum to swim with, enzymes to dissolve the egg’s protective covering and to signal that this sperm is now fertilizing the egg, and no other sperm may enter.

Once fertilized, the egg cell will divide repeatedly to produce a hollow ball of many identical cells, which will then layer themselves to start developing into an embryo. These early cell divisions happen so fast, the new cells have no time to grow before they divide again. Therefore, except for their chromosomes, which duplicate in full sets before each division, the new cells’ substance and internal structures are all portions of the original egg. So a mother animal must produce an egg large enough to provide all this material.

Next, how do the hundreds of identical new cells wind up becoming different parts of an embryo? How do they turn into head, middle, tail, top, bottom, sides, limbs? We don’t know for sure, but certain clues suggest that the maternal reproductive system conveys this body architecture. Before the egg cell leaves the follicle, molecules from the mother’s body diffuse into it. These molecules are concentrated at one side of the egg, so when the fertilized egg repeatedly divides, some of the new cells will contain a lot of the maternal molecules, and some will contain few or none. So already, the many new cells are not identical. Inside the new cells which contain them, the maternal molecules may produce proteins, and the proteins may signal the new cells to specialize in being the head-end of the embryo. Once these head-end cells specialize, they manufacture their own signals to send to other new cells farther downstream in the incipient embryo, telling them to become the embryo’s mid-section. Then the mid-section cells signal the tail-end cells. Once all these groups of cells start to specialize, they can signal within the group for finer and finer anatomical details.

So like it or not, we owe a lot to Mom.