The Story of Life
As soon as the first cell split into two competing individuals, natural selection began to operate. In the early environment, where the first cells were surrounded by energy–rich molecules and very few neighbors, competition would not be intense. However, genetic mutations would start and some cells would be different from others. Those differences involved the efficiency with which cells were able to utilize the molecules that they found in their environment. Certain cells, for example, might be able to get energy faster and therefore reproduce faster. Over time, the beneficial mutations dominate the entire population by the process of natural selection.
Over time you would expect the descendants of that first cell to spread around the Earth and occupy most of the oceans. Some scholars suggested that this spread may have only taken a few years, given the lack of competition for the environmental resources. In this process of spreading, some cells would wind up in different environmental than others. Some, for example, would be in tropical waters, while others in the Arctic. Some would be in deep oceans, while others would be next to the shore. Each, of these environmental would exert slightly different pressures on the cells. An adaptation that might be very advantageous in the tropics, for example, might not be advantageous near the poles. The driving force of natural selection, coupled with the fact that might different environments existed on our planet, would quickly have produced a number of very different living things. Thus we would expect the appearance of diversity—the process of speciation—bean quite early in the history of life.
Our knowledge of this early period of like limited by the fact that we have very little hard physical evidence that pertains to it. As you might guess, it is difficult to find fossils of single–celled or microscopic organisms, though scientists have found them—(595) shows, we have examples of fossil bacteria. We have even unearthed a few cases of fossil bacteria in the act of dividing.
The best guess as to what went on until about a billion years ago is that the new living things spread around the world and differentiated, driven all the while by natural selection and changes in the Earth's climate. We suspect that in the early part of this evolutionary process, all cells were prokaryotic, and scientists often speak of this as the period dominated by cyanobacteria— single–cell life forms that provide oxygen as a byproduct of photosynthesis. To an observer, the Earth would looked remarkably sterile. There was no life at all on land but the margins of the oceans were covered with collections of green scum that was going about the business of talking in carbon dioxide and returning oxygen to the atmosphere.
About a billion years ago, symbiotic relationships were set up between cells that eventually led to the development of eukaryotes. At some point, smaller cells found that they did better living inside their large neighbors than they could do on their own, and cells whose genetic materials were carried inside a nucleus were born. These cells, like their neighbors and ancestors the prokaryotes, remained as single–celled organisms.
Sometimes during this period as well, cells began to come together to large colonies. At first, these objects were probably nothing more than clumps of single–celled organisms living next to each other. Later, however, they developed into larger bodies. Indeed, by about 600 million years ago, the seas were probably full of large multicelluar animals and plants. You can think of some of them as resembling modern jellyfish. The stage was set for one of the most important developments in the history of life—the hard shell.
About 570 million years ago, a crucial development took place in living systems. By a process that we do not fully understand, but which may have involved a new enzyme that converted calcium in the ocean water into shell material, some animals began to grow hard shells. This new chemical trick was so advantageous that the seafloor was soon teeming with many different kinds of hard–shell animals. As always happens when a new evolutionary path develops, there was a great deal of competition and experimentation among living things as they evolved outer shells, body designs, and metabolisms suited for each environment.
From the scientist's point of view, one of the most important aspects of this developments was that , for the first time, living things left large numbers of fossils. In fact for most of the nineteenth and twentieth century, before discovery of the fossils that indicated the presence of primitive forms of life, it looked to scientists as if life suddenly exploded at the beginning of this period. The sudden change in life, therefore is often referred to as the Cambrian explosion. (Geologists refer to the time during which skeletons developed as the Cambrian period, after Cambria, the old Roman name for Wales, where rocks from this period were first studied.)
