The Making Of The Cat Essays - Evolutionary Biology,

The Making Of The Cat R. Roger Breton Nancy J Creek ------------------------ Soup or Sandwich IN THE VERY BEGINNING, about 4.6 billion years ago (give or take a few years), a small ball of rock, water and gas had come to be and immedi- ately set about the process of combining its atoms into more and more complex arrangements. Thus began that most wondrous story, the evolu- tion of life on Earth. For the first 2.1 billion years of the Earth's existence, the Archeo- zoic Era, life very slowly evolved. The Earth's crust was still in flux and covered for the most part by shallow seas. The atmosphere was composed primarily of methane, ammonia, carbon dioxide and water vapor. From these primitive chemicals life evolved. There are two primary schools of thought on the processes involved: the "soup" theory and the "sandwich" theory. According to the more-popular soup theory, chemical evolution first took place in the upper atmosphere, where ultraviolet radiation from the sun could generate an assortment of simple and complex organic (carbon-based) molecules out of the basic components of the atmos- phere. As these molecules slowly rained into the early oceans, a kind of primordial soup was created. Via the ultraviolet radiation, light- ning, volcanic action, and other forms of heat and energy, this soup was able to slowly combine the organic molecules into ever more com- plex forms: first simple amino acids, then organic macromolecules, then single-strand RNA molecules, and finally simple viruses. The only trouble with the soup theory is that is almost definitely wrong! The time required for it to work is statistically greater than the lifetime of the Earth. The time is only statistically greater, however, and anything is possible... Various explanations have been put forth to account for this time discrepancy. The most popular of these is the seeding of the early seas by organic molecules from space. This seeding could have been either through organic molecules present in the original formation of the Earth, or from later bombardment by meteors or more likely comets containing the organic compounds (a cosmic soup mix). None of the compensatory theories put forth are very likely, however. This brings us to the sandwich theory. The sandwich theory states that complex organic molecules formed on the surface of undersea crystalline rocks, such as those surrounding volcanic vents. The name "sandwich theory" comes about because the active area is sandwiched between the sea and the rock. Besides, what scientist could resist the "soup and sandwich" pun! Free-floating molecules in the water tend to cling to smooth surfaces. This surface effect allows various molecules to gather in one place. Ultraviolet energy from the sun or, more likely, heat from volcanic vents, would allow this gathering of simple molecules to combine into more complex organic molecules rather easily. Some of the simplest organic molecules are scums, easily formed on flat surfaces, which themselves are sticky and gather more simple molecules. Within these scums, ever more complex molecules are easily formed. These more complex molecules tend to be three-dimensional, and bulge outward from the rock surfaces. This allows them to be easily washed away by the sea, forming a primordial soup not of basic simple mole- cules, but of the far more complex and already evolved RNA macromole- cules and possibly even viruses. Viruses are fundamentally RNA and amino-acid conglomerates with many life-like properties. Although it is open to debate as to whether or not they are themselves alive, viruses are definitely right on the edge: simpler things are clearly not alive, while more complex things clearly are. One aspect of the sandwich theory is that at undersea volcanic vents today life may still be evolving from basic components! This exciting possibility is being carefully investigated and holds great promise for the future. The Great Pollution After the virus, life was off and running. During the next 500 mil- lion or so years, viruses evolved into simple prokaryotes, single- celled living beings without a cellular nucleus. In this case, blue- green algae, the first plants. This marked the beginning of the Proterozoic Era, about 2.5 billion years ago. Blue-green algae are blue-green because they possess that truly wondrous molecule, chlorophyll. It is chlorophyll which makes possible the production of food directly from sunlight and the carbon dioxide in the atmosphere. This is the process of photosynthesis. A side-effect of photosynthesis is the generation of free oxygen as a waste product. Free oxygen combined with itself and the methane and ammonia in the atmosphere to form ozone, water, free nitrogen, and more carbon dioxide. Over the next