Biology – Module 3 – Life on Earth



Biology – Module 3 – Life on Earth

1) Analysis of the oldest sedimentary rocks provides evidence for the origin of life.

• Identify the relationship between the conditions on early Earth and the origin of organic molecules.

The approximate age of the Earth is 4.5 Billion years. During the Hadean Eon was the formation of the earth as it was transformed from a gaseous cloud into a solid body. The heavier molten iron sank down and became the core, whereas the lighter rock rose to the surface – forming the crust.

As a result of the high temperatures at the centre of the Earth and due to volcanic activity, there as an emission of gases, or out-gassing, of volatile molecules, such as water (H2O), methane (CH4), ammonia (NH3), hydrogen (H2), nitrogen (N2), and carbon dioxide (CO2).

The environment was also anoxic as there was no free oxygen. This meant that there was no ozone layer, thus the Earth was exposed to huge amounts of solar radiation (i.e UV).

Early Earth, with an atmosphere of water vapour, hydrogen, methane and ammonia, provided an environment in which the production of organic carbon-containing molecules would be fairly easy. The energy for driving these reactions could have come from a number of sources, in particular the sun. Ultraviolet light would easily have reached the Earth’s surface because no ozone layer existed. Other possible energy sources could have been lightning, hot springs and volcanoes, radioactivity in the crust, and impact from meteorites. At this stage, organic molecules would have most likely formed in the lower atmosphere or the Earth’s surface.

Stages of the Earth’s Development:

- Dense clouds of water formed in the atmosphere, which reflects the sun’s heat

- Earth cooled sufficiently to allow for the formation of a rocky crust.

- Volcanic gas eruptions increased the air pressure, forming clouds which allowed for the water vapour to condense and fall as rain.

- Water would have absorbed a lot of the carbon dioxide present in the atmosphere due to solubility.

- The cooling process by the rain continued until the Earth was cool enough to allow for the formation of rivers and oceans.

- Heat was dissipated into space, causing a more solid crust to form

- From then on, the process of reducing the amount of carbon dioxide ws occurring – ie transferring it into rocks like limestone.

• discuss the implications of the existence of organic molecules in the cosmos for the origin of life on Earth.

There is very little evidence towards the existence of organic molecules in the universe or cosmos. However due to meteorites, we have found that they contain organic molecules much similar to that we have on Earth today. This implied that perhaps the origin of organic molecules needed for the origin of life on Earth may have infact have originated from outside of Earth.

Urey and Miller through experimentation were able to replicate the conditions of Early Earth and show that organic molecules can be formed from inorganic reactants. This theory is commonly supported but there is no evidence to show the transformation of organic molecules into life.

• describe two scientific theories relating to the evolution of the chemicals of life and discuss their significance in understanding the origin of life

1 – Spontaneous generation – This theory was proposed by Aristotle who suggested that life arose spontaneously, assuming that certain particles of matter contained and “active principle” which could produce a living organism when conditions were suitable. He correctly assumed that the active principle was in a fertile egg, but his hypothesis that these principles were in meat, mud and wood were incorrect. This theory was disproven by Louis Pasteur. It’s significance however, in the understanding of the origin of life is that it shows us that it is extremely unlikely life simply generated itself from non-living substances.

2 – Panspermia – This theory suggests that life could have arisen once, or several times, at various times and in various parts of the universe. Materials found in meteorites and comets may have acted as ‘seeds’ falling onto the Earth. There is no evidence to support / disprove this theory due to lack of ability to explore space but the similarity of amino acids in meteorites and Earth do provide some inclination towards Panspermia. The theory is significant in the understanding of the origin of life as it opens the option of life may be existent outside the Earth – a hypothesis that will never be proven wrong.

3 – Biochemical Evolution Theory – This theory suggests that certain conditions of Early Earth generated the organic compounds and the right environment for the first production of a living organism.

Evidence to support the Biochemical Evolution Theory:

Oparin suggested that organic compounds could have formed in the early Earth’s oceans from more simple compounds, where the energy source was the excessive UV radiation of the sun. He argued that considering the amount of simple molecules in the oceans, the energy available and the time frame, it is feasible that the oceans would gradually accumulate organic molecules to produce a ‘primeval soup’ in which life may have arisen.

Although the transition from inorganic to organic molecules is accepted. The transition from inorganic molecules to life is widely debated. Oparin suggested that life emerged through random molecular interactions, but some scientists find this impossible to conceive.

This is the most probable theory that we currently have today. Thus it is significant to our limited understanding of the life.

• discuss the significance of the Urey and Miller experiments in the debate on the composition of the primitive atmosphere

As hypothesized by Oparin (and Haldane), the conditions of early earth included little free oxygen, volcanic eruptions, lightning and torrential rain. It was believed that these conditions favoured chemical reactions which caused the synthesis of organic molecules from inorganic substances.

Miller (along with Urey), set up a flask containing the combination of four gases – methane (CH4), ammonia (NH3), hydrogen and water. The flask was air tight. The gases were then exposed to a continuous electric discharge from tungsten electrodes with represented ‘lightning’. The gases were circulated for one week and then the liquid in the apparatus was analyzed. The electric current had caused the gases to interact, resulting in the formation of a variety of amino acids and other organic compounds.

Urey and Miller experiment was conducted to test the hypotheses about the origin of the Earth. The experiment proved that in an atmosphere containing methane, ammonia, hydrogen gas and water vapour, with a supply of energy, organic compounds can form. This provided evidence for the hypotheses to turn into a theory about the origin of life.

Their experiment proved the hypotheses that organic compounds could be synthesised from abiotic conditions. After the publishing of their experiment, variations of the gases that were present in the early earth atmosphere have been tested and have also produced organic compounds. It has been recently found that it was a lot easier for the organic molecules to form in the atmosphere of the early earth than in the experiment. Therefore their experiment was the starting point of the all the research and discoveries that were later found.

To show that the results of the experiment were not caused by the contamination of the mixture by micro-organisms, the experiment was repeated without electric discharge (less organic compounds) and then by sterilising entire experiment (same as first).

The work of Urey and Miller gas shown that the major building blocks for living organisms could have formed on Early Earth with its primitive atmosphere and energy sources of lightning and UV radiation. This is called chemical evolution.

On Earth today, many of these organic molecules break down easily and are highly perishable. They break down because they are either oxidised by oxygen in the atmosphere or are decomposed by organisms. Since there was little oxygen and no organisms present on Early earth, these complex molecules would not have broken down like they do today.

• Identify changes in technology that have assisted in the development of an increased understanding of the origin of life and the evolution of living things

Early Technologies

- Glass jar and cotton – used by Francesco Redi for a spontaneous generation with flies and meat, testing the idea that organisms directly appear from non-living matter.

- Swan-necked flasks - Used by Louis Pasteur in order to disprove the theory of spontaneous generation. This was done when he was able to trap bacteria and other micro-organisms in the swan neck tube

- Light Microscope – This led to the discovery of many simple biological processes, and allowed us to view things not visible to the naked eye.

Recent Technologies

- electron microscope development - this led to the understanding of structures at the molecular level, the remains of micro-organisms and the mineral nature of early rocks

- Radiometric Dating – This is the process by which the age of the fossils can be determined due to the matter that is remaining in them.

- X – ray crystallography – This is the process by which X-ray images are taken of molecules at the atomic level. This allowed for the identification of DNA as a double helix by Rosalind Franklin

- Engineering Developments – Allowed us to explore both the ocean depths and the vast space that surrounds us.

2. The fossil record provides information about the subsequent evolution of living things.

• Identify the major stages in the evolution of living things, including the formation of:

▪ Organic molecules

▪ Membranes

▪ Prokaryotic heterotrophic cells

▪ Prokaryotic autotrophic cells

▪ Eucaryotic cells

▪ Colonial organisms

▪ Multicellular organisms

The following steps are believed to be the major stages in the evolution of life on Earth:

1) The Formation of organic molecules

Complex organic molecules formed in water on Early Earth. All organisms are composed of complex organic molecules.

2) The formation of Membranes

A membrane was developed and this separated a group of molecules from the external environment, providing protection and the ability to maintain itself as a separate identity. For example – liposome.

3) The formation of heterotrophic procaryotic cells

After organic molecules were being separated due to the formation of membranes, it is believed that prokaryotic cells appeared from these. These cells obtained their energy by consuming the organic ‘soup’ in which they lived or other smaller cells. They did not have any membrane bound organelles. These cells may have been similar to bacteria (evidence of microfossils).

4) The formation of autotrophic procaryotic cells

The ozone layer began to form and due to this cells may have developed the pathway to make their own food (photosynthesis). There is evidence of cyanobacteria in stromalites.

5) Eucaryotic Cells

First cells appeared that had a nucleus, using cyanobacteria as chloroplasts and bacteria as mitochondria. Some of the simple procaryotic cells may have engulfed other cells, which became internal structures or organelles. Eucaryotic cells may have also evolved to produce membrane bound organelles such as mitochondria.

6) Colonial Cells

Multicellular organisms may have originated when daughter cells became bound together. These individual cells that continued to live and function may have been the ancestors of multicellular organisms eg. Stromalites.

7) Multicellular organisms

These organisms represent the most advanced of life forms; where cells in the body show differentiation to carry out specific functions as a coordinated unit, for examples humans.

• Describe some of the palaeontological and geological evidence that suggests when life originated on Earth.

Paleontology is the scientific study of fossils and all aspects of extinct life.

Geology is the scientific study of the origin, history and structure of the Earth as recorded in rocks.

Palaeontological Evidence

- Fossils of single-celled prokaryotes or microfossils, were found that were believed to be 3500 million years old. These were very similar to those found living today.

- Fossilized remains of stromalites were found and provided information on the structure of early organisms. Simple bacteria existed in these structures were very similar to present-day stromalites.

- Discovery of nanobacteria were found in a meteorite from Mars. It is not yet conclusive if these are actual organisms or just crystals.

In general, using the fossil evidence from different rock layers, it has been found that the more primitive cells and marine organisms are found in the lower layers of rock compared to the more complex and land-dwelling organisms. This trend suggests that simple organisms are marine organisms preceded land-dwelling organisms/

Geological Evidence.

2500 million year old Archaean rocks from north Western Australia were found in 1999. The found biomarkers, or chemical evidence, for the existence of cyanobacteria. Biomarkers are chemicals that are produced by only one group of organisms providing evidence of their existence in the past.

Oxidised rocks such as banded ion and red bedrock formations provide geological evidence towards the origin of photosynthetic life. Oxygen produced by photosynthetic organisms accumulated in the rocks until fully saturated building up as a gas in the atmosphere.

Uraninite (UO2) provides evidence for an increasingly oxygenated atmosphere. Uraninite readily combines with oxygen and no longer forms. The deposits are at least 2300 million years old. Calculations using known breakdown rate estimate that atmosphere at that time has less than 1/100 of oxygen compared to today.

• Explain why the change from an anoxic to an oxic atmosphere was significant in the evolution of living things.

An anoxic atmosphere is one defined as being deficient, or lacking in oxygen.

An oxic atmosphere is one where oxygen is readily available.

A change from an anoxic atmosphere free of oxygen to an oxic atmosphere with plenty of available oxygen had a significant influence on the conditions of Early Earth and hence the evolution of living things. This major change to the atmosphere and increase in oxygen inhibited the growth of anaerobic organisms and caused them to decline, while photosynthetic organisms became more abundant.

The next significant change as a result of the increase in oxygen in the atmosphere was that aerobic organisms became more efficient in energy production (respiration), providing a large energy source for increased activity and eventually this led to an increased complexity and size of the organisms.

As the oxygen began to accumulate in the atmosphere, it reacted with the sun’s UV radiation to produce ozone (O3). As the amount of oxygen increased, so did the amount of ozone produced, until a layer was formed around the Earth. This layer inhibited the UV rays to reach the surface of the Earth and this had a significant impact on organisms as it protected them from dangerous UV radiation. This meant that organisms more successfully inhabitated the land.

• Discuss the ways in which developments in scientific knowledge may conflict with the ideas about the origins of life developed by different cultures.

Developments in scientific knowledge about the origins of life are constantly occurring as discoveries are made and new technologies provide more advance approaches to unanswered questions. These changes may conflict with the ideas about the origins of life held by different cultures.

Science and religion can never have any intellectual conflict. For many people, cultural beliefs about life’s origin form part of their religion, where the belief is that God is the creator of the Universe.

Thus due to this many conflicts will occur, and when attempting to resolve or avoid such conflicts, careful consideration must be taken for the beliefs of individuals and science, culture and religion must be balanced.

Extra: Fossils

A fossil is anything that indicates the existence of past life. It can be part of or a whole organism or a trace left behind.

Usually the soft parts of an organism decay, leaving only the hard parts such as bones, teeth and shells. If the body of the organism leaves an impression the cavity it made may form a mould and it the cavity is filled with material a cast is formed.

If an organism lives for a short time and is found over a wide area it can be used as an index fossil. For example trilobites are used as index fossils. Index fossils are used to correlate different rock layers so that the age of the layers can be determined.

Comparing fossils with one another and with present-day species provides an important evidence to show how organisms have changed throughout the history of the Earth. The data can also be used to reconstruct ancient climates, environments and geography.

The study of fossil evidence shows several trends in the origin of life on Earth:

- A trend from simple to more complex structures

- A trend from aquatic to terrestrial lifeforms

- A trend from asexual to sexual reproduction.

- An increase in the degree of protection or care of embryo as it develops

Exam type questions on Fossils

1) Describe three ways in which organisms can become fossilized.

- burial in ice – similar process to that in sedimentary rock. It provides also for little decomposition of flesh as well.

- Amber – insects are caught in the sap of trees which solidifies to become amber thus trapping the insect and also drowning them

- Mummified – Where organisms are carefully wrapped and stored to prevent their decomposition.

2) Explain why vertebrates are more likely to be fossilized than invertebrates.

Invertebrates have no indo or exoskeleton thus when they die, their body fluids simply spread out quickly and no remains are left. Vertebrates however have skeletons which are found today as they remain intact if preserved in rock / ice for millions of years. Thus it is more likely that vertebrates will become fossilized.

3) List the events that occur in the fossilization of an organism.

- organism dies

- It is immediately buried (or submerged) in rock/ice.

- There is no route for bacteria to enter the grave thus preventing decomposition

- The environment becomes anoxic

- The organism is subject to a long period of tome without disturbance

- It is found due to excavation.

4) Explain why igneous rock does not contain fossils.

Igneous rocks are formed by volcanic activity, the lava that flows out would destroy any life forms in its path, so no fossils can be preserved in this rock

5) How is igneous rock important to the interpretation of the fossil record.

Igneous rock is important in dating the age fossils because it provides a reference point for fossils bearing rock layers above and beneath it. For example, if a layer of sedimentary rock lies directly beneath a layer of igneous rock that has been dated at 200 million years old, then this indicates the relative age of the fossils in the old sedimentary layer to be at least 200 million years old.

3. Further developments in our knowledge of present-day organisms and the discovery of new organisms allows for better understanding of the origins of life and the processes involved in the evolution of living things.

• Describe technological advances that have increased knowledge or procaryotic organisms

Structural methods of classifying procaryotic organisms in the past have been very valuable however, these methods did not always reflect the organisms’ possible evolution. New technological advances have changed the way procaryotic organisms are now classified, and increased our understanding and knowledge of biological structures, chemical composition and biochemical (genetic) characteristics of procaryotic organisms.

Procaryotic organisms do not have a membrane bound organelle, ie, they do not have a nucleus, mitochondria, chloroplasts, etc. They are the simplest of organisms on Earth, are very small, are extremely abundant and survive in a wide variety of environments.

Up until the 20th century people classified organisms into two groups – the plant kingdom and the animal kingdom. However the invention of the electron microscope and new biochemical techniques has led to changes in classification, especially with respect to procayotes.

• Describe the main features of the environment of an organism from one of the following groups and identify its role in that environment.

▪ Archaea

▪ Bacteria

Archaea are commonly referred to as archaebacteria. Archaea are single celled, microscopic organisms which do not contain any membrane bound organelles. They do not require any sunlight in order to conduct photosynthesis, nor do they require oxygen.

At present, most Archaea live in extreme environments and are known as extremophiles. However, there are still some Archaea that live in ordinary conditions such as normal temperature ranges, salinities and acidity.

|Type of Archaea Extremophiles |Current Environment that they prefer to habituate |

|Thermophiles |Water temperature must exceed 50 degrees celicus and be less than 110 degrees celcius. If in a |

| |volcanic habitat they prefer the higher temperature environments. |

|Halophiles |Hypersaline (high salinity) habitats that exceed nine percent salt concentration are preferred by |

| |these Archaea but the environment cannot be extremely saline (i.e. must be less than 32% |

| |concentration). |

|Acidophiles |As the name infers, these organisms prefer extremely acidic environments that have a pH that is |

| |lower than 2 but greater than about 0.8 |

|Thermoacidophiles |These Archaea prefer both high temperatures and extremely acidic environments. |

|Methanogens |Anaerobic environments such as deep soils or bogs, the digestive system of herbivores, and in |

| |sediments of marine and freshwater ecosystems. |

When the Earth was still at its early stages of formations, the environment was extremely hostile. However these conditions were suitable for certain bacteria. For example the first fossils of photosynthesising bacteria were observed at 3 billion years ago. Eukaryotes started to appear around two billion years ago with more advanced multicellular life appearing a billion years later. It was not until only two million years ago that it is believed that humans were present on the Earth.

Life has been around for the majority of the Earths history, but the circumstances back then were extremely different to now. There was intense heat, radiation from the sun and a severe lack of oxygen. All qualities that allow a certain type of organism called Archaea to thrive.

All cellular life on the Earth is believed to have originated from one of three main categories: Eukarya, Bacteria or Archaea. Eukarya includes all organisms whose cells contain a nucleus. The other two (Bacteria and Archaea) are unicellular and are also prokaryotes (meaning that they do not contain a nucleus).

However, contradictory to certain perceptions, many Archaea are not extremophiles. For example many are found in natural environments such as seawater and soil. This has resulted scientists to believe that Archaea are a significant component of the biosphere.

Archaea, however, are also found in several extremely harsh and hostile environments. They have adapted to endure these harsh abiotic conditions and have thus survived in extreme temperatures such as from superheated water to the icy cold environment of the polar ice caps. Furthermore they have been found in areas of extreme pressure, the extremes of alkalinity and acidity, and also in extremely salt environments (such as in the Dead Sea which is inhabitable to almost any organism).

Archaea comprise one of the main sections of the biosphere, and are also seen in soil, temperate sea water and even the human digestive tract. In order for them to survive in these types of extreme environments, all aspects of their cellular structure must be adapted in order to survive.

Due to these initial factors, scientists have begun to believe that these Archaea were present in the initial stages of the Earth’s development as they believe the conditions that were present at that stage were perfect for the Archaea to thrive. This theory is based on the fact that currently the Archaea live in such extreme and hostile environments, and these hostile conditions would have been present during the development of the Earth.

Chosen group of organism: Thermoacidophiles

|Past environments |Present Environments |

|Archaea are believed to one of the most primitive species |As can be inferred from the term “Thermo”, these extremophiles |

|present on the Earth, with fossils found dating back to 2.75 |prefer extremely high temperatures, thus they can be found in |

|billion years ago. During this time the Earth was in its stages|places such as geysers where these conditions are present. |

|of formation. This meant that the temperature of the |Furthermore, they are also found in hot lakes and springs, |

|environment would be extremely high, similar to the current day|where a layer of Archaea can be seen. Eg. They have been |

|environments in which the Thermoacidophiles prefer to live. |spotted in the Hot springs of the Yellowstone Nation Park, in |

| |the U.S.A. |

| | |

|All Archaea including thermoacidophiles display properties of |Thermoacidophiles currently reside in environments such as |

|autotrophism are able to produce their own food using materials|acidic hot springs, geysers and deep ocean vents. These |

|that are present in the Earths’ crust (i.e. Hydrogen, sulfur |environments provide similar characteristics to those that were|

|and carbon dioxide). The environment of early Earth would not |present in the past. For example all three named locations are |

|provide for any photosynthesising or heterotrophic organisms, |anaerobic, extremely acidic and reach high temperatures, which |

|therefore it shows how the conditions present during those |are the conditions to which these organisms are suited. Thus |

|stages of Earths development favoured the Archaea and in |scientists believe that Archaea may be one of the last common |

|particular, the Thermoacidophiles. |ancestors of all current organisms and species that are present|

| |on Earth. |

| | |

|The oldest known fossil of Archaea has been found in the |This is similar to current environments of Thermoacidophiles. |

|cavernous mines of Ontario. At the time when this would have |For example: |

|been fossilised, the area is believed to be submerged and also |In Iceland, scientists recently identified the first |

|inundated with hydrothermal vents. This means that there would |ammonia-oxidizing extremophiles that inhabited several acidic |

|be no direct sunlight; it would be an anaerobic environment, |hot springs where temperatures reach 80 C (176 F). Thus the |

|with extremely high temperatures (this is due to the fact that |environmental situations were similar to the past. It was hot, |

|currently a gold mine is known to exist there). |acidic, anaerobic, and shut out from the open world. Thus |

| |showing how the Archaea were able to reside in extreme |

| |environments. |

| |Archaea, and also Thermoacidophiles do not require any oxygen |

|During the Early formations of Earth, there was no or a very |or any direct sunlight to survive. Vents located on the ocean |

|limited atmosphere. This meant that all oxygen dependant |floor do not receive any sunlight, and there is little or no |

|organisms would perish. Thermoacidophiles and Archaea in |dissolved oxygen present in the water. These conditions are |

|general do not require oxygen to survive thus they are believed|similar to their supposed past environments, thus allowing the |

|to have existed during this period of Earths history. |Thermoacidophiles to survive. |

Note: Every planet and moon in the solar system, displays environmental conditions that only extremophiles, such as the Archaea can endure. Thus, it is also being hypothesised, that Thermoacidophiles and other forms of Archaea may be present on other planets – thus furthering the limits of thought when it comes to extraterrestrial life.

Eubacteria:

These consist of two main types of bacteria – cyanobacteria and nitrogen fixing bacteria. Bacteria are an enormously diverse group that share many environments with, or live on and in humans and other animals and plants. These are habitats of moderate temperature with water freely available, low in salt or other solutes, and where sunlight or organic compounds are plentiful. Oxygen is not so important since many of the Bacteria have powerful fermentation capabilities, producing ATP (adenosine triphosphate) under anaerobic conditions.

Cyanobacteria are photosynthetic; since they contain chlorophyll, and also have the ability to fix nitrogen from the air. An overabundance of cyanobacteria in a lake or river can lead to eutrophication when the decomposition of the dead cyanobacteria uses up all the available oxygen in the water thus killing all animal life. They occur naturally in wet or damp situations: ponds, streams, wet rocks and soil. They also flourish in warm conditions, particularly where the water contains dissolved organic material.

Nitrogen fixing bacteria include both cyanobacteria and bacteria. They have mutualistic relationships with certain plants. Eg: they can live in the roots of legumes. They fix nitrogen in the air into compounds that are used by the plant. The plant receives these nutrients and the bacteria have a protected environment They usually occur naturally in the soil.

Deep-Sea bacteria have been found in many locations, especially near the rift vents at mid-ocean ridges. Both Eubacteria and Archaea have been found. Many of these bacteria can also be found in the guts of deep dwelling organisms since they break down organic compounds.

|Features |Procaryotic Cells |Eucaryotic Cells |

| |Examples: Archaea and Eubacteria |Examples: Plants, animals and fungi |

|Membrane-bound organelles |NO |YES |

|Chromosome |Single strand |More than one |

|Nucleolus |NO |YES |

|Ribosomes |Very Little |Prominent |

|ER |NO |YES |

|Microtubules |NO |YES |

|enzyme for respiration |Attached onto cell membrane |Attached onto cristae (mitochondria) |

|Photosynthetic pigment |Attached onto cell membrane |Attached on chloroplasts (in plants) |

|Cellular nature |Unicellular |Uni or multicellular |

|Cell Division |Not mitosis |Mitosis |

|Average cell size |1 micrometre |10-100 micrometres. |

4. The study of present day organisms increases our understanding of past organisms and environments

• Explain the need for scientists to classify organisms.

Classification sorts things into groups and assists scientists in several ways:

- It makes communication between scientists more precise and simpler. Eg. Rather than describe an animal with a three part body and three pairs of legs, we can just use the word “insect”,

- It provides a quick and accurate description of a particular organism eg mammal immediately provides information about the way it feeds its young, body temperature regulation etc.

- It assists in the identification of an unknown organism

- It shows trends in the development within the group.

- It shows evolution form simple to more complex structures.

- Predictions: classification serves also to predict information that we do not yet have. For example, if a set of organisms share characteristics A, B, C and D that no other organisms have, and we find another organism of which all we know is it has characteristics A and B, we can predict that it will also have C and D.

- conservation: classification provides us with information about the relationship of groups of organisms with their environment.

• Describe the selection criteria used in different classification systems and discuss the advantages and disadvantages of each system

Classification systems use structural characteristics to distinguish different organisms and split them into groups. Structural characteristics include features such as the presence of an internal skeleton.

There are several reasons on why structural characteristics are used:

- Structural features are usually more constant and usually do not dramatically change over time.

- Organisms with one structural feature in common frequently have other features in common. Eg. Vertebrates have a backbone but also have a digestive system with two openings and a closed circulatory system.

- There is a wide range of structural characteristics which allows divisions into groups to be based on precise details.

• Explain how levels of organisation in a hierarchical system assist classification

There are now seven basic levels of classification system – kingdom, phylum, class, order, family, genus, species.

A species is usually defined as a group of living things that can interbreed to produce fertile offspring. However it has been found that problems arise with this definition when studying a particular species which may change across a geographic region. To overcome this problem, a species has been defined as a group of organisms that share a common gene pool.

Having simple levels of hierarchy for classification makes it an easy and systematic way to name organisms according to their characteristics.

• Discuss, using examples, the impact of changes in technology on the development and revision of biological classification systems

The development of different classification systems has followed the development of technology as new instruments and methods of investigation have provided greater details about the structure and biochemistry of organisms. This new knowledge requires a revision of previous systems and new classification systems are constructed to take account of the new knowledge.

Early classification systems placed living things into two groups – Plants and Animals. The invention of the electron microscope showed that bacteria were different as they do not have a nucleus or and other membrane bound organelles. The bacteria were therefore placed into the kingdom – Monera.

Traditionally fungi was classified as a plant, but due to the fact that they do not contain chlorophyll and are heterotrophs, they were placed in their own kingdom called – Fungi

Later on the kingdom Protista was formed to include all organisms that did not fit in the previous four kingdoms. It was a mixed group containing both unicellular and multicellular organisms with differing characteristics. In recent years, the analysis of DNA and proteins in different species has shown which organisms are most closely biochemically related. This has led to modifications of evolutionary paths and relationships and of classification systems.

• Describe the main features of the binomial system in naming organisms and relate these to the concepts of genus and species

Binomial nomenclature is a two name system. It avoids the problem of different countries and different languages having different common names for the same organism. There are several rules to follow when using the binomial system:

1 – The genus comes first and begins with a capital letter

2 – The species name follows and begins with a lower case letter.

3 – Both names are italicised or underlined.

The ranks or categories of genus and species have a special significance because they form the basis of the binomial system. In the binomial system, the nature of each kind of organism is described by two parts. Eg: Homo sapiens (humans) and Gorilla gorilla (gorilla). The taxonomic category, species, is the lowest rank in the hierarchy to which all organisms must be classified.

• Identify and discuss the difficulties experienced in classifying extinct organisms

The concept of species is often debated, although most agree that the basic principle involved interbreeding and producing fertile individuals. However when studying extinct species it is impossible to check whether the offspring were fertile or not, or even if they shared a common gene pool.

Many fossils are only a cast or mould with no actual remains of the original organism. This makes it very hard to classify different fossils at a species level. If hard parts are recovered we have very little detail about body covering and colours. These details have to be inferred by studying modern-day descendants.

The classification of extinct species can also depend on the number of fossils that have been recovered. If there are many fossils it can be difficult to determine if the variations are natural variations or different species.

As technology improves more data is able to be collected. However different scientists will use the same evidence to support differing theories. Fossilized organisms do not give the same level of detail as modern day analysis of species thus classification is difficult.

• Explain how classification of organisms can assist in developing an understanding of present and past life on Earth.

There are many alternative views on the evolutionary relationships between different groups of organisms. Although often based on the same evidence, different interpretations have arisen and are constantly changing and conflicting. However, by attempting to arrange and order groups of organisms scientists are able to look at possible evolutionary pathways and relationships.

Our present-day organisms are much easier to classify than those that only lived in the past. However, we can look at similarities between modern-day organisms and those from the past to develop an understanding of the differences between present and past life. Some organisms may now be living in similar environments to those in which they lived in the past; however, many organisms have had significant changes in the environments and demonstrated changes in response to this.

Furthermore, using the dates of known fossils it is possible to construct a timeline to show how life on Earth has changed and the relative times at which this has occurred. However fossils show only 15% of total history of Earth.

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