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 immediately set about the process of combining its atoms into more and more complex arrangements. Thus began that most wondrous story, the evolution of life on Earth.
For the first 2.1 billion years of the Earth's existence, the Archeozoic 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 atmosphere. As these molecules slowly rained into the early oceans, a kind of primordial soup was created. Via the ultraviolet radiation, lightning, volcanic action, and other forms of heat and energy, this soup was able to slowly combine the organic molecules into ever more complex 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 molecules, but of the far more complex and already evolved RNA macromolecules 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 million 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 billion years, blue-green algae polluted the Earth with enough free oxygen to completely change the entire chemistry of the world. Gone was the pristine methane, ammonia, and carbon-dioxide early atmosphere, to be replaced by a corrosive mixture of free nitrogen and free oxygen, surrounded by a thin layer of ozone.
It is this corrosive nitrogen/oxygen atmosphere that allowed the evolution, about 1.5 billion years ago, of chlorophyll-less creatures such as bacteria and protozoans. These creatures were active, like the oxygen they consumed. They preyed on the algae (and each other) for food, and were the first animals: very early proto-cats. The production of free oxygen also altered the structure of the very rocks themselves, causing a slow but radical geologic change.
Protozoans are eukaryotes (cells with a central nucleus). The secret of all but the simplest lifeforms is locked in that nucleus: the chromosome.
Virtually all living things have several different chromosomes in each cell. These chromosomes comprise a set, which is itself a blueprint. In a multi-celled creature, each cell contains an identical set of chromosomes. A cat, for example, has 38 chromosomes per set, with an identical set in each and every cell, except sex cells. Each cell of a cat contains within itself the code for the complete cat.
A chromosome is itself composed primarily of a thin protein membrane enclosing a bit of water and a single molecule of DNA (deoxyribonucleic acid). The DNA molecule is composed of two long strands wound around each other in a double helix (like two intertwined springs), with each component of a strand connected to the opposite strand by a crossbar or rung. If the double helix were laid flat, DNA would be ladder-like in appearance.
The evolution and concept of DNA is awesome in its potential, and awe- inspiring in its simplicity and beauty. There are only six simple compounds that go together to make up DNA, phosphate and deoxyribose alternate to form the helixes while four amino acids make up the rungs.
It is not the number of differing compounds that provide the secret of DNA's success, but rather the number of rungs in the ladder (uncounted millions) and the order of the amino acids that make up the rungs. The four different amino acids are arranged in groups of three, forming a 64-letter alphabet. This alphabet is used to compose words of varying length, each of which is a gene (one particular letter is always used to indicate the start of a gene). Each gene controls the development of a specific characteristic of the lifeform. There is an all-but-infinite number of possible genes. As a result, the DNA of a lifeform contains its blueprint, no two alike, and the variety and numbers of possible lifeforms has even today barely begun.
There was a small problem with evolution up to this time: it was asexual. A cell multiplies by dividing! That is, once it has accumulated enough material to make another cell, it does--by dividing in half. This process is called mitosis.
In highly simplified form, when a cell undergoes mitosis, its chromosomes duplicate, move to opposite sides, and the cell divides in two. Each daughter cell is an exact copy of the parent cell, barring mutations.
Since evolution depends upon change, asexual evolution is wholly dependent upon random mutation, and thus very slow. It took almost 4 billion years, about 85% of the Earth's existence so far, to evolve up to the complexity of protozoans. What was needed was a means of speeding up the process. What was needed was sex!
At first, sex had nothing to do with reproduction, not directly, anyway. The protozoans would get together, merge, swap a few genes, the separate and go their ways. This chromosome-swapping allowed them to pass around and share an advantageous characteristic.
In order for the sexual merge to occur efficiently, the concept of a double chromosome evolved. In this form, chromosomes are doubled and paired. This gives each lifeform two of each chromosome (so far), and hence two of each gene. Thus, after a sexual encounter, a protozoan had two of any given gene. They may both be the genes it originally possessed, both be the genes the other protozoan possessed, or one of each. If, due to a mutation somewhere along the line, one of a pair of genes had a slightly different code than the other, the protozoan would assume the characteristics of the dominant gene (unless they are identical, one gene is always dominant over the other). It would, however, keep the recessive gene, and may pass it on (or not) at its next encounter. The tendency is then for dominant genes to quickly spread through a community.
This effect was clearly demonstrated in a recent experiment wherein a small group of a penicillin-resistant strain of the bacterium gonococcus was merged with a much larger group of normal gonococci. After a short while, all bacteria in the test were penicillin-resistant. The bacteria had sexually interfaced and shared the genes that contributed to penicillin resistance.
After the discovery of sex, the protozoans would occasionally merge and share protoplasm. They would then separate and go their individual ways, reproducing asexually.
At some point in time, a mutation occurred in which a cell would divide not into two daughter cells, but into four half-cells, or gametes. Each of these gametes contained half of each pair of chromosomes, comprising a half-set. The urge to merge was all powerful, and quickly carried out. The mutation, however, was dominant. As a result, so a whole colony of protozoans was dividing into gametes, a process call meiosis, and quickly merging in a mix and match fashion.
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