Evolution & the Environment
Evolution is defined as the gradual change of any object, in our case, the biological system, through time. We discussed briefly in the previous section some of the aspects of the complex intricate system of the chemistry of biomolecules. We now shift to the level of the organism--cells organized into tissues, organs, and organism--which have persisted and changed through time. Broadly speaking, the first single-celled organism did not have a nucleus. These prokaryotes persist today as microbes, such as bacteria and viruses.
Protein synthesis was an early step in the evolution of organisms who were then capable of reproducing themselves using their RNA. It is not clear when the first cells in the form we know now were formed. Somewhere, in the primordial soups, some molecules got enclosed inside a membrane to form a cell. RNA molecules within this cell could then facilitate reproduction of the cell. All we can surmise is that this aggregation of molecules started about 3.5 billion years ago. It is believed that RNA came after proteins and enzymes evolved. Once a cell was established, RNA evolved into DNA, making a subcellular structure we now call the nucleus. The DNA, which is a coil of two strands of RNA (with the bases A, C, G, and T, whereas RNA has A, C, G, U), is the molecule that enables protein synthesis.
Some of the prokaryotic bacteria we know like mycoplasms, as well as different varieties of bacillus, e-coli, and staphylococcus, still abound on Earth. Bacteria live in an enormous variety of environments, often symbiotic with other organisms. Thus benign forms of e-coli live in our guts and numerous bacteria live in soil and in plants. Species of bacteria can use a variety of organic molecules such as sugars and polypeptides as their food. Bacteria are the most abundant type of cell on earth. The early bacteria seemed to have used ATP to store the energy of sunlight. Blue-green algae is a type of bacteria that converts CO2 and N2 into its food.
As plants developed the capability of photosynthesis, the composition of the early atmosphere (mostly CO2) began to change over billions of years to this current composition of 20% oxygen and only traces of CO2 as the plants "fixed" the carbon. Many of the early bacteria were used to an oxygen-poor environment and several of them then became extinct. Others formed a symbiotic association with oxygen--using (aerobic) types of cells to form the present day cells with nuclei (eucaryotes). The different organelles in the cell such as the chloroplasts and mitochondria are now believed to have been separate organisms that originally associated with each other to form mutually beneficial colonies we call cells.
Origin of Life
As we noted before, life on Earth has evolved around the chemical versatility of a few atoms, especially carbon. Some special features of chemistry are used by living systems. These features are:
A live organism is an open system, continuously exchanging energy and matter with the environment. It is "self-organizing," meaning it takes raw material and reassembles it into complex vital molecules. During this process, life increases internal order (decreases local entropy). Thus life builds up information (order) which is then duplicated.
In the Beginning
The early environment on Earth is a matter of conjecture. Piecing together evidence, it is believed now that the environment consisted of high energy events such as volcanic eruptions, continuous torrents of rain, and large amounts of lightning. It is believed that there was little if any oxygen in the atmosphere, and certainly no ozone layer. Therefore, ultraviolet from the sun could reach all the way to the Earth's surface.
Some of our models for the early environment of Earth come from the observation of the atmospheres of Mars and Venus, made by NASA.
Table X shows the major gases in the atmosphere of Venus, Mars and the Earth. Note the difference between the estimate of the composition with and without life on earth.
It has been shown in laboratory experiments that simple carbon-based (organic) molecules are formed under these early conditions that prevailed on Earth. In 1953, Stewart Miller, a graduate student of the famous chemist, Harold Urey, simulated the early (prebiotic) atmosphere on Earth--a mixture of ammonia (NH3), water vapor (H2O), hydrogen, and methane (CH4). He bombarded the mixture with electrical discharges to simulate lightning. In a week, he saw some spectacular results: alanine and glycine, two amino acids that form proteins in life forms today (including humans) were formed in the resulting mixture. Under the conditions provided, more complex molecules such as formaldehyde (HCHO), formic acid (HCOOH), and hydrogen cyanide (HCN) had formed. In a water solution these molecules had then reacted with each other to form more complex organic molecules such as acetic acid (CH3COOH), glycine (NH2CONH2), alanine (NH2CHOHCOOH).
The richness of carbon chemistry and the plethora of carbon compounds form the basis of life on Earth. Carbon chemistry (called organic chemistry) and the function of biomolecules are explained in detail in the section on carbon compounds.
Environment and Life
In the late 1970's, Elso Barghoom of Harvard University was looking for the earliest evidence of life, and found it eventually in Swaziland, Africa. He found evidence of bacteria in 3.4 billion year old fossils. This means that life started very early on our 4.5 billion year old planet. The time it took to move from inanimate matter to the first forms of life was actually shorter than that to move from bacteria to larger organisms--the earliest of which appear to be only 570 million years old, as evidenced by hard-shelled fossils of that age that appear all over the Earth.
Early life then probably came from mixtures of materials combining to form biomolecules with the energy provided by ultraviolet light and lightning. Replication of DNA and mutation in rapidly dividing bacteria, as well as local variations in environment, then provided a route to diverse populations of bacteria. Development of metabolic pathways to store and convert energy--mechanisms of fermentation to break down sugars--was an early step. Along the way the bacteria also began to capture atmospheric nitrogen to begin the manufacture of amino acids and other organic compounds. To this day, we need bacteria to take the stable nitrogen gas N2 from the atmosphere and convert it into usable compounds. This "nitrogen-fixing" is discussed under the nitrogen cycle in the Materials System.
Margulis and Sagan also state that "the evolution of photosynthesis is undoubtedly the most important single innovation in the history of life on the planet" (p. 78). The first photosynthetic organisms were bacteria that used H2S rather than H2O. H2S must have been plentiful, emitted from volcanoes. The development of the successive stages of bacterial development is fascinating as described by Margulis and Sagan. Early adaptations included developing pigments to protect against ultraviolet, then top layers protecting the layers below and developing repair enzymes. Repair enzymes persist in us today. When ultraviolet or other ionizing radiation damages part of our DNA, these enzymes remove the damaged portion and replace it with new healthy DNA. Despite the fact that we have had an ozone layer to filter out the almost all ultraviolet for over 2 billion years now, we still have this repair system.
Xeroderma pigmentosum is a rare genetic defect inhibiting DNA repair mechanisms against ultraviolet radiation damage. It is characterized by severe sensitivity to all sources of UV radiation (especially sunlight), and often results in cutaneous lesions, premature aging of the skin, cataracts, increased risk of ocular benign and malign tumors, and sometimes neurological disorders such as mental retardation. To learn more about XP, visit the Xeroderma Pigmentosum Society, or read its disease description at the Atlas of Genetics and Cytogenetics in Oncology and Haematology.
Gaia: Co-Evolution of Climate and Life
Gaia is the Greek goddess of the Earth. While designing experiments for NASA to detect life on Mars, the atmospheric chemist James Lovelock developed the theory called Gaia. Gaia refers to the system of all life on Earth and the atmosphere which mutually regulates prevailing conditions to continue life on Earth. The name "Gaia" was suggested to Lovelock at his request by his neighbor, William Golding, author of Lord of the Flies.
The Gaia hypothesis states that the biota (group of all living organisms) regulate the temperature and gas composition of the atmosphere. Lovelock came to this conclusion because the 20-80 composition of O2- N2 in our atmosphere can not be explained by laws of physics and chemistry alone. If we were to make a simple mixture of these gases in the laboratory along with some of the other materials on Earth, the gases would react quickly and become compounds, and not remain as O2 and N2 in the gaseous state. Lovelock therefore postulated that this unlikely mixture must be aided by the continuous production of these gases by live organisms! If this were not true, our atmosphere would be a mixture of N2, NH3, SOx, CH4, methyl chloride (CH2Cl), and others. These are indeed present but only in minute quantities.
In addition, the Earth's average temperature has remained relatively stable (around 22° C) despite the increase in the sun's temperature over the past 4 billion years. This too has been attributed by Lovelock to the feedback effects of life on the atmosphere.
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Several scientists disagree with Lovelock's hypothesis. Lovelock has actually modeled a simple system, "Daisy World," a planet with black and white daisies circling a sun-like star. He and his co-author, Andrew Watson, have demonstrated the Gaia-like character of this world. The daisies act as thermostats, stabilizing the temperature. In our world, microbes can play the role of the daisies. Margulis and Sagan cite the discovery that about 20,000 years ago there was only two-thirds the amount of CO2 that we have now, and that the rise of CO2 to pre-industrial levels took place abruptly in a 100 year span. This cannot be explained by geophysical or chemical processes alone, but could be the result of a sudden species death of algae.
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