EVIDENCES OF EVOLUTION

        The word “evolution” is derived from the Latin word “evolvere” which means to “unroll” or “unfold”. Thus, the theory of Organic Evolution states that it’s a kind of gradual unfolding or formation of more complex, advanced and present day forms from simple and primitive organisms living in the past,millions of years ago. 

There are various evidences available to prove this point. viz.

(a) Morphological and Anatomical Evidences

(b) Embryological evidences

(c) Paleontological (Fossils) evidences: The study of fossils (paleontology) is the best and most direct conventional type of evidence, e.g., the fossil of Archaeopteryx*. (“Archaeo” means primitive and “ptera” means wings in Latin) is a classical example of evolution of birds from reptiles, because it contains features of both the forms. Similarly, the world famous fossil of “Lucy” (Australopithecus afarensis) shows that “bipedal” locomotion arose for the first time in ancestral man here and finally led to the development of erect “modern” man.

(d) Biogeographical evidences

(e) Biochemical evidences: The most convincing evidence of common ancestry comes from similarities of certain biomolecules viz. composition of protoplasm, enzymes, cytochrome ‘C’ (respiratory pigment present in all eukaryotic cells), blood serum proteins, hemoglobin, insulin, etc. The degree of molecular homology is more in closely related forms and vice versa, e.g.,amino acid sequence for cytochrome ‘C’ of humans and chimpanzee is identical.

(f) Genetic evidences or Molecular evidences: These are the latest observable tools in determining the phylogenetic relationships between organisms and allow molecular comparisons even between very uncommon relatives with no morphological, behavioural, physiological or ecological relationships e.g., bacteria and humans. Some of the molecular tools used to prove evolutionary relationships are:

•         Amino acid analysis or sequencing e.g., Hb, Mb sequencing in para and orthologous species.
•         Nucleotide sequence analysis (data from 5S ribosomal RNA)
•         DNA hybridisation
•         Restriction enzymes: which cleave DNA into fragments at particular sites like ECORI from                          E. coli. They allow comparisons among DNA of different species                                           
•         RFLPs (Restriction Fragment Length Polymorphisms) 

•         Mitochondrial DNA and microsatellites, etc.

 EVOLUTION OF LIFE

        The  theories explained how life must have originated on early primitive earth about 3.5 billion years ago, but it did not give any information about how the biological diversity arose. Again, there were different views about how this must have happened.

    Some of the dominant thoughts were: (i) Greek Theories (ii) Pre-modern Theories and (iii) Modern Theories. 

Among the Modern Theories, the following two are most popular. 

Theory of Inheritance of Acquired Characteristics

This theory was proposed by famous French naturalist Jean Baptiste Lamarck. It propounded that the characters were acquired by the organisms due to an “inner need” or “desire” of the organism and these acquired characters were then inherited. This theory was subsequently rejected.

Theory of Natural Selection

Charles Darwin, who is better known as the “Father of Evolution” was the architect of this world famous theory, which charmed both the scientists as well as laymen. This theory took shape when Darwin was aboard H.M.S. Beagle, the famous ship, for a voyage planned for world exploration.According to him, the struggle for existence, due to limited availability of resources, led to the survival of the fittest or natural selection

 ENVIRONMENTAL POLLUTION

Pollution is the introduction of contaminants into the environment that cause harm or discomfort to humans or other living organisms, or that damage the environment. India has been ranked as the seventh most environmentally hazardous country in the world. Brazil is the worst, United States is second worst and China is third. Singapore has been found to be the best.

There are three major types of environmental pollution:

Air Pollution

Water Pollution

Soil Pollution

Air Pollution

Some of the most important air pollutants are Sulphur dioxide, nitrogen dioxide, carbon monoxide, ozone, volatile organic compounds (VOCs) and airborne particles. Radioactive pollutants are the most destructive ones specially when produced by nuclear explosions. Carbon dioxide and water vapour present in the atmosphere, strongly absorb a large fraction of the earth’s emitted radiation. This absorbed radiation is partly re-emitted to the earth’s surface. The net result is that the earth’  surface gets heated up by a phenomenon called the Greenhouse Effect.

Deforestation and combustion of fossil fuels (oil, gas, coal) have a cumulative effect on the net increase in carbon dioxide content. Forests are the areas where a great deal of photosynthesis occurs, which helps in maintaining a balance in the atmospheric carbon dioxide level.

Trees and animals contribute large quantities of hydrocarbons (mainly methane) to the atmosphere. Harmful effects of hydrocarbon pollution are due to the photochemical reactions in which they are involved. Large quantities of automobile exhausts in the atmosphere exposed to intense sunlight leads to the formation of photochemical oxidants. This gives rise to photochemical smog (Characterised by brown, hazy fumes) which irritates the eyes and lungs, leads to the cracking of rubber and extensive damage of plant life. Smog is a combination of smoke and fog. Smog containing high levels of sulphur dioxide is called reducing smog. Peroxyacyl nitrate (PAN) is a potent eye irritant found in photochemical smog.

Oxides of nitrogen and sulphur entering the atmosphere are converted into nitric acid and sulphuric acid respectively. They combine with hydrochloric acid from HCl emission to generate acidic precipitation known as acid rain. Acid rain is now a major pollution problem in some areas. It causes extensive damage to buildings and sculptural materials of marble, limestone, slate, mortar, etc. It is harmful for aquatic plants and animals, and causes soil acidification.

Small solid particles and liquid droplets are collectively called particulates. These are present in large amounts in the atmosphere and sometimes pose a serious air-pollution problem. Examples of processes which inject particulate matter into the atmosphere: volcanic eruptions, blowing of soil and dust by wind, formation of fly ash from power plants, smelters and mining operations, and smoke from incomplete combustion processes. Airborne asbestos (a fibrous silicate mineral) and toxic metals like beryllium are carcinogenic. Fine particles (less than 3 microns) are the worst causes of lung damage. Coal miners’ black-lung disease, and asbestos workers’ pulmonary fibrosis are associated with the accumulation of such small particles. Very fine particulate matter, which is less than 2.5 microns in size, is called PM 2.5 and is known to cause diabetes and cardiovascular diseases. 

Air in Delhi is choking with pollutant PM 2.5. Any kind of combustion, especially of vehicular origin, contains this particle. If PM 2.5 is not regulated it will increase the number of patients suffering from asthma and lung cancer. It brings about lead deposition in the lungs. Particulates accelerate corrosion of metals and cause damage to buildings, sculptures, paints, etc. Particulates in the atmosphere reduce visibility and influence the climate through the formation of clouds, rain and snow.

Burning CNG has the highest rates of potentially hazardous carbonyl emissions. Retrofitted CNG car engines emit 30% more methane than original CNG engines. Almost all CNG car engines in India are retrofitted. Coal pollution kills more than 300,000 people every year. Nitrogen and sulphur oxides emitted by aircrafts, at an altitude of 35,000 ft, combine with other gases in the atmosphere to create harmful pollutants.

Vehicle emission is responsible for 70% of the air pollution in India. Bangalore is called the asthma capital of India. The levels of suspended particulate matter are above permissible limits in Mumbai. 

Bhopal gas tragedy was the greatest industrial disaster in the world that took place at a Union Carbide pesticide plant in the Indian city of Bhopal, Madhya Pradesh. On 3rd December, 1984 (midnight), the plant accidentally released methyl isocyanate (MIC) gas. Within a week about 10,000 people died, 1,000 became blind while more than 1 lakh continued to suffer from various disorders. This was the deadliest man made environmental disaster in history.

On 14 July 2010, chlorine gas leaked from the Sewri industrial area on land owned by the Mumbai Port Trust and approximately 76 people were treated in hospital. Rice crop yields in Southern India are decreasing as brown clouds are increasingly blocking the sunlight. The brilliant white of the famous Taj Mahal is slowly fading to a sickly yellow. The Supreme Court ordered the closure of more than 200 factories to save the Taj Mahal from being polluted by fumes.

There has been a drastic fall in the number of butterflies in the Western Himalayas, famous for their biodiversity. In areas of Himachal Pradesh, and Jammu and Kashmir, the population of 50% of the 288 species has declined more than half in just 10 years. Over 700 million people in India suffer from high levels of indoor air pollution as 75% homes use biomass fuel like wood, crop residue and dung cakes.

Excessive ultraviolet radiation coming from the sun through the ozone layer in the upper atmosphere, which is eroded by some air pollutants, may cause skin cancer in wildlife and damage to trees and plants. Ozone in the lower atmosphere may damage lung tissues of animals, and adversely affect plant respiration and photosynthesis.

Water Pollution

Water pollutants include insecticides and herbicides, food processing waste, pollutants from livestock operations, volatile organic compounds (VOCs), heavy metals, chemical waste, disease causing agents, sewage, Detergents, oil, sediments and radioactive materials. Beryllium, boron, cadmium, chromium, cyanide, lead, mercury, selenium, silver, sulphide, vanadium and zinc are some of the toxic water pollutants.

Waterborne diseases caused by polluted drinking water are typhoid, amoebiasis, giardiasis, ascariasis, hookworm, hepatitis, encephalitis, gastroenteritis and Diarrhoea. Conditions related to water polluted by chemicals are cancer, hormonal problems, and damage to liver, kidney, nervous system and DNA. Water polluted by mercury can lead to Parkinson’s disease, Alzheimer’s disease, heart disease, etc.

According to a United Nations report released on 22 March 2010 on World Water Day, contaminated and polluted water kills more people than all forms of violence including wars. 80% of urban waste in India ends up in rivers.

The river Ganga is dying slowly due to unchecked pollution. Samples taken from the river Ganga near Varanasi showed that levels of fecal coliform, a dangerous bacterium that comes from untreated sewage, were some 3,000 per cent higher than what is considered safe for bathing.

Chemical contamination in water can cause declines in frog biodiversity. Persistent organic pollutants (POPs) may cause declines, deformities and death of fish life. Plants and animals may be killed by too much sodium chloride (ordinary salt) in water. Plants may be killed by herbicides in water. Water pollution may disrupt photosynthesis in aquatic plants.

Soil Pollution

Soil pollutants include hydrocarbons, solvents and heavy metals. Soi contamination causes cancer. Lead in soil damages the brain, and mercury leads to kidney damage. It causes headache, nausea, fatigue, eye irritation, skin rashes, and depression of the central nervous system. It may alter plant metabolism and reduce crop yields. It can alter the metabolism of microorganisms. Trees and plants may absorb soil contaminants and pass them up the food chain. This may lead to increased mortality rates and even animal extinction.

Fossil Fuel Pollution

Fossil fuels (oil, gas, coal) are used in vehicles, power-generating plants, and in the manufacture of products like plastics, solvents, detergents, lubricating oils, etc. Fossil fuels contribute to soil contamination and water pollution.

When oil is transported by pipelines, an oil leak from the pipeline may occur and pollute the soil, and subsequently the groundwater. When oil is transported by tankers in an ocean, an oil spill may occur and pollute ocean water. Transport and power-generating plants are the biggest sources of fossil fuel pollution. Fossil fuel combustion is also a major source of carbon dioxide emissions (air pollution) and perhaps the most important cause of global warming.

 CHEMICAL THREATS TO BIODIVERSITY

Chemistry has brought about a medical revolution which resulted in the average life expectancy rising from 47 in 1900 to 75 in the 1990s. The explosive expansion of world’s food supply has been because of development of chemicals that protect crops and enhance growth. In almost every area (food, clothing, shelter, transportation, communication, etc.) chemistry has brought about an improvement in the quality of life of people.

The Toxics Release Inventory (TRI), USA, established under the Emergency Planning and Community Right-to-Know Act, tracks the release of chemicals by different sectors of industry. Of the top ten industrial sectors tracked by the TRI, the chemical industry releases a greater amount of waste to the environment than the other nine industrial sectors combined.

 CHEMICAL TOXICOLOGY

There are various chemicals in the environment, some of which are toxic.These toxic chemicals are released by industries into air, water and soil. They upset the complex system of chemical reactions occurring in the human body. They can cause discomfort, illness, disability or even death. Chemical Toxicology is the science of the study of toxic chemicals and their modes of action.

Toxic Chemicals in Air Some of the extremely hazardous substances in the atmosphere are acrylonitrile, arsenic, asbestos, benzene, beryllium, cadmium, chlorinated solvents, chlorofluorocarbons, chromates, coke oven emissions, ethylene dibromide, ethylene oxide, lead, mercury, ozone, sulphur dioxide, vinyl chloride and toxic waste disposal emissions.  Toxic Chemicals in Water ,The toxic elements found in natural and waste waters are arsenic, cadmium, beryllium, boron, chromium, copper, fluorine, lead, manganese, mercury, molybdenum, selenium and zinc. The water bodies contain pesticides, mainly from the drainage of agricultural land.

Effect of Toxic Chemicals on Enzymes

Toxic chemicals attack enzymes, inhibiting their essential function. Heavy metal ions like Hg2+, Pb2+ and Cd2+ act as effective enzyme inhibitors.

Classification of Toxic Substances

1. Corrosive Poisons Corrosive poisons are toxic substances which destroy tissues, e.g., strong acids and alkalies, and many oxidants which can destroy tissues. Concentrated mineral acids such as sulphuric acid and bases like sodium hydroxide attack cellular structures. The cell dies because the proteins are hydrolysed by the acid or the base

2. Metabolic Poisons Metabolic poisons cause illness or death by interfering with a vital biochemical mechanism to such an extent that it ceases to function or does not function efficiently.

3. Neurotoxins Some metabolic poisons limit their action to the nervous system. These include poisons such as strychnine and curare. Neurotoxins act at the point where two nerve fibres come together (called a synapse).

Neurotoxins such as atropine and curare occupy the receptor sites on nerve endings of organs that are normally occupied by the impulse carrying acetylcholine. When atropine or curare occupies the receptor site, no stimulus is transmitted to the organ. Neurotoxins of this type are useful in medicines. For example, atropine is used to dilate the pupil of the eye to facilitate examination of its interior. When applied to the skin, atropine salts relieve pain by deactivating sensory nerve endings on the skin. Curare is used as a muscle relaxant. Nicotine is a natural compound that blocks receptor sites as is done by atropine and curare. This powerful poison causes stimulation and then depression of the central nervous system. Nicotine is present in tobacco and insecticides

4. Mutagens Mutagens are chemicals that can change the genes and chromosomes (hereditary pattern) to cause abnormalities in the offspring. They alter the structures of DNA and RNA, which compose the genes that transmit the traits of parent to offspring. Nitrous acid is a potent mutagen in bacteria, viruses, moulds and other organisms. It reacts with nitrogenous bases of DNA to produce new compounds. The result is an alteration in the genetic coding of DNA, so that a different protein is formed. Sodium nitrite is used as a preservative, colour enhancer or colour fixative in meat and fish products. Sodium nitrite is converted to nitrous acid by hydrochloric acid in the human stomach. Some examples of mutagens: Aflatoxin (from mould), Benzo (α) pyrene (from cigarette and coal smoke), Caffeine, Captan (a fungicide), Chloroprene, Dimethyl sulphate (used for methylation), LSD (lysergic acid diethylamide), Maleic hydrazide (plant growth inhibitor), Mustard gas (dichlorodiethyl sulphide), Ozone, solvents in glue (like toluene, acetone, cyclohexane, hexane and ethyl acetate), and TEM (triethylene melamine, an anticancer drug).

5. Teratogens The chemicals which affect reproduction are called teratogens. The study of birth defects produced by chemical agents is called Teratology. There are three known classes of teratogens: radiation, viral agents and chemical substances. Any chemical that can cross the placenta is a potential teratogen. Smoking a cigarette results in higher-than-normal blood levels of such substances as carbon monoxide, hydrogen cyanide, cadmium, nicotine and benzo (α) pyrene. Many of these substances are present in polluted air as well. Other examples of teratogens: Arsenic, Cobalt, Gallium, Lead, Lithium, Mercury,Thallium, Zinc, Caffeine, DES (diethyl stilbestrol) and PCBs (polychlorinated biphenyls).

6. Carcinogens Carcinogens are chemicals that cause cancer in animals and humans. An abnormal growth is classified as cancerous or malignant when examination shows it is invading neighbouring tissue. A growth is said to be benign if it is localised at its original site. Some compounds cause cancer at the point of contact. Other compounds cause cancer in an area remote from the point of contact. The liver, the site at which most toxic chemicals are removed from the blood, is particularly susceptible to such compounds.

7. Hallucinogens Hallucinogens can produce temporary changes in perception, thought and mood. They destroy one’s sense of judgement. LSD has been linked with birth defects.

8. Alcohols Methyl alcohol is highly poisonous and is a cumulative poison in human beings. It has a toxic effect on the optic nerve and large doses can cause blindness. In the body, it gets oxidised first to formaldehyde and then to formic acid, which is eliminated in the urine. The toxic effect on the optic nerve is caused by the oxidative products.

 Ecosystems

A community and its physical environment is called its Ecosystem. An ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the non-living (abiotic), physical components of the environment with which the organisms interact, such as air, soil, water and sunlight.

What is an eco system?

Ecosystem is an open system with inputs, internal transfers and outputs of energy and nutrients. Sunlight is the initial energy source for nearly all ecosystems. 

Types of Ecosystem

The different types of ecosystems are

(i) Natural Ecosystem

(a) Terrestrial ecosystem

(b) Aquatic ecosystem

(c) Lentic (lake, pond or swamp)

(d) Lotic (river, stream or spring)

(ii) Artificial Ecosystems

ecological community

The species that live and interact in an area constitute an ecological community. Ecological communities are loose assemblages of organisms.

The organisms in a community can be divided into trophic levels (the position of an organism in food chain) based on the source of their energy.

Primary producers get their energy from sunlight. Herbivores that get their energy by eating primary producers are primary consumers; organisms that get their energy by eating herbivores are secondary consumers; and so on.

 A sequence of interactions in which a plant is eaten by a herbivore, which in turn eaten by a secondary consumer, and so on, is known as food chain. Food chains are usually interconnected to make a food web because most species  in a community eat and are eaten by more than one species. Most communities have three to five trophic levels. 

Energy and biomass decreases as it flows from lower to higher trophic levels and thus can be represented in the form of pyramid. Most of the energy ingested by organisms that is converted to biomass is eventually consumed by decomposers.

Ecological pyramids An important feature of energy flow is that most of the energy going from one trophic level to the next, in a food chain and food web, dissipates into the environment as a result of the second law of thermodynamics. Ecological pyramids often graphically represent the relative energy values of each trophic level. There are three main types of pyramids— a pyramid of numbers, a pyramid of biomass and a pyramid of energy.

Ecosystem productivity The gross primary productivity (GPP) of an ecosystem is the rate at which energy is captured during photosynthesis in a given period of time. In addition, a plant respires to provide energy for its own use; this acts as a drain on photosynthesis. Energy in plant tissues after cellular respiration has occurred is net primary productivity (NPP). 

Both GPP and NPP are expressed as energy per unit area per unit time. Humans consume (32%) far more of earth’s resources than any other of animal species. If we want our planet to operate sustainably, we must share terrestrial photosynthetic product with other organisms.

In environments that show variations in salinity, temperature and other environmental conditions, food webs tend to have short chains. In stable environments, such as parts of the deep ocean, food chains are longer. In addition to energy inputs, primary productivity and ecosystem structure require a cycling of nutrients. Water and minerals move slowly through the physical environment, rapidly through organisms, and back to the environment in biogeochemical cycles. 

Water moves through a hydrological cycle. In land ecosystems, plants stabilise soil and minimise nutrient loss during the cycle as runoff. In atmospheric cycles, a nutrient prevails mainly in gaseous form (such as carbon, in carbon dioxide). 

In the carbon cycle, carbon dioxide is the main gas in the atmosphere. The ocean is carbon’s main reservoir. Burning of fossil fuels, logging and conversion of natural ecosystems for farming disrupt the global carbon budget and may be responsible for global warming.

Nitrogen is a limiting factor in the total net productivity of ecosystem on land. Gaseous nitrogen is abundant in the atmosphere. Nitrogen–fixing bacteria convert N2 to ammonia and nitrates, which producers take up.

Mycorrhizae and root nodules, two symbiotic interactions, enhance the nitrogen uptake. Sedimentary cycles interact with the hydrological cycle to move mineral nutrients to and from ecosystems.

Certain human activities are depleting minerals from ecosystems, as when weathered soil of tropical forests is cleared for agriculture.

Some human activities are accelerating the process of eutrophication. They are adding nutrients such as nitrates and phosphates to aquatic ecosystems.

This promotes growth and decay of destructive algal blooms. The decomposition of these plants leads to the depletion of oxygen in the water, which threatens fish and other animal populations.