A script for life

"All life comes form pre-existing life";      "all cells are the descents of a primordial cell".

     As soon as the first cell divided into two cells, two competing individuals began to operate according to the Laws of Natural Selection. In these early cell's environment, there were few competitors, so competition for energy rich molecules was not very intense. Before too long some genetic mutations may have started to occur and some cells would have become slightly different from the other cells. Some of the differences may have involved the efficiency with which these cells were able to use the energy rich molecules of their environments. Some cells may have been able to get energy more quickly from some molecules, and consequently reproduce faster than other cells. Over time beneficial mutations would come to be shared throughout an entire population of cells by the processes of Natural Selection

    It is likely that the vast majority of early mutations were not beneficial, just as they are not today. Non-beneficial mutation and their resultant differences likely died out quickly, and it is probable that only energy beneficial mutations, which favored enhanced reproduction survived. The process may be a little like watching games of college football that have a history of over 100 years of being played. A great many poor or average games have been played over time, but we don't watch those games anymore. What we remember and preserve are the greatest games, such as the 1984 Miami v. Nebraska National Championship game in the Orange Bowl. In the same way, only the "greatest games" of all the cell mutations survive into the future.

     Over time the descendents of that first cell began to spread over the Earth's surface. Some researchers have suggested that the spread may have taken only a few years, given the lack of any real competition for environmental resources [see by the numbers]. Cells ended up in different environments, some in warm tropical waters, some in the cold Arctic,  some in deep oceans, while others would be in shallow pool near the shore. Each environment would exert different selection pressures on these cells. Molecular adaptations advantageous in the tropics might not be so in Antarctic, and vice versa. The driving force of Natural Selection coupled with a a multitude of different environments could quickly produce a vast number of different living cell types. The advent of diversity, i.e., the process of speciation, might have begun very early in the history of life.

     Life is estimated to have begun between 3.5 billion years ago (first cell), though little fossil evidence exists. During this period new varieties of single cells spread around Earth and differentiated, driven all the while by Natural Selection and changes in the Earth's climate. About 2.5 billion years ago (photosynthetic cells) the oceans became dominated by cyanobacteria, single cells that produce oxygen as a byproduct of photosynthesis. The margins of the world's oceans would have been covered with green scum, which was going about the business of reducing CO2 and pumping O2 into the atmosphere.

     About 1 billion years ago symbiotic relationships between cells probably led to the development of eukaryotes. Smaller cells found they might live better inside bigger ones, and cells whose genome was carried inside a nucleus may have been born, but remained single celled. Soon, however, cells began to clump together into chains or masses of cells forming colonies. By about 600 millions years ago the seas were probably filled with large multicelled animals and plants that probably looked like jellyfish.

     This development of colonies set the stage for the development of hard shells, an important milestone in the script of life. About 545 millions years ago a metabolic reaction that likely involved a new enzyme converted ocean water Ca into calcium carbonate, hard shell material. Being extremely advantageous, the sea beds were soon teeming with hard shelled animals. As with any new evolutionary path there was great competition and experimental design differences among animal group as they evolved their outer hard shells, new body designs, and metabolism, each suited for the many different environments. This evolutionary period is often described as a sudden explosion of life forms. It is the period when the basic animal body plan and skeletons developed, and it has not significantly changed since that period. The period is known as the Cambrian Explosion, after Cambria, the old Roman name for Wales, where rocks from  this period were first studied and the first fossil seen.

     Following the period of momentous development of shells, the last one-half billion years have seen enormous growth in both complexity and diversity of life (see table below). 

Major Steps in Evolution of life
Time (millions of years) Event
~3500 First cell
~2500 Photosynthetic cells
1000 Eukaryotes
700 Multicellularity
545 Animals with shells
450 Vertebrates and land plants
400 Amphibians
350 Reptiles
250 largest know mass extinction
140 Appearance of flowering plants
100 Placental mammals
65 primates; extinction of dinosaurs
7 Hominids
2.5 the genus Homo
0.2 Homo sapiens