THE INFINITE VARIETY: THE BEGINNING OF LIFE
Foreword
/ This material was originally prepared for a first year tutorial system at the Zoology Department and is built around the David Attenborough film series “Life on Earth”. At the time of development there was no intention to develop it as eLearning content. With the introduction of the Ecological Informatics courses at the University of the Western Cape, it became necessary to provide a basic review of how Biodiversity evolved? In updating extensive use of hypertext is used so you can dip and out at various points of the material and get additional information. This material should be used in conjunction with viewing of the material “Life on Earth” by David Attenborough, but we also encourage you to follow the hyperlinks and examine the classifications provided.It should be realized that science and especially our understanding of how biodiversity evolved is continuously changing – so we have had to correct certain information and provide text boxes with the latest information and access to recent scientific papers.
/ For both updating and providing supplementary information I have used the public domain Wikipedia Encyclopaedia, and unless otherwise stated all images and nomenclature/classifications were sourced from Wikipedia and Wikispecies.
/ At the end of each chapter are some self-study questions plus the link to the really excellent Public Broadcasting Service (PBS) “Shape of Life” series which is available for viewing from the following url:
http://www.pbs.org/kcet/shapeoflife/episodes/index.html. It is very important that this material is reviewed in combination with this online course and in the absence of access to the “Life on Earth” is a good substitute.
This resource was developed using standard html and ccs and should work under all platforms and browser configurations, but extensive use of pop-up is made which means that you must enable pop-ups and your browser is java-enabled.
For registered UWC students assessment criteria will be provided separately through continuous assessment (using electronic quizzes), discussion forum and a course-project.
Good Luck with your Biodiversity studies.
Richard Knight
Coordinator: National Information Society Learnership- Ecological Informatics
c/o Department of Biodiversity and Conservation Biology
University of the Western Cape
Private Bag X17
Bellville 7535
South Africa
Phone: 27 + 21 + 959 3940
Email:
Darwin and the Giant Tortoises
The world is rich in animals and plants, some of which still remain to be discovered. A small area of the Tropical Forests of South America will still yield insects that have never been described, the difficulty is finding a specialist whose is able to classify them. The understanding of such biodiversity would have been almost impossible, if it had not been for Charles Darwin and his trip around the world. For example Darwin described the adaptations of the Giant Tortoises (Geochelone nigra) that occur on the Galapagos Islands in the South Pacific.
Tortoises occurring on the well-watered islands, with short, cropped vegetation had gently curved front edges to their shell.
An example of dome-shell Galapagos Tortoise that occurs on the well-watered parts of the islands.
Tortoises occurring on more arid islands had to stretch their necks to reach branches of cactus and other vegetation. Consequently, these later individuals had longer necks and a high peak to the front edges of their shells, which enabled them to stretch their heads almost vertically.
A “saddle-back” Galapagos Tortoise that inhabits drier areas of the islands and has a longer neck and a high peak to the front edge of its shell, this enables it to stretch it neck further out and obtain food higher up off the ground.
Observations such as these were the foundations for the theory of evolution, which suggests that species were not fixed for ever but changed with time and thereby contribute to the immense diversity of life.
Geochelone nigra
Galápagos Tortoise
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Family: Testudinidae
Genus: Geochelone
Binomial name Geochelone nigra (Quoy & Gaimard, 1824)
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Darwin's argument for the evolution of different necks in these tortoises was as follows:- all individuals of the same species are not identical. In a single clutch of eggs there will be some hatchlings, which, because of their genetic constitution, will develop longer necks than others. In times of drought such individuals will be able to reach leaves higher off the ground than their siblings and therefore will survive. The brothers and sisters in the clutch who possessed shorter necks would be unable to stretch and reach food and therefore would starve to death. Since this time natural selection has been debated and tested, refined, quantified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and animal behaviour have developed the theory of natural selection still further. It remains the key to our understanding of the natural world and it enables us to recognize that life has a long and continuous history during which organisms, both plants and animals, have changed, generation by generation, as they colonized all parts of the world.
Evidence of Evolution in the Rocks
Occasionally some animals after dying may be covered in mud, where their bones can be preserved. Dead plant material may also accumulate and is turned to peat, in time peat is compressed and turned to coal. Great pressure from overlying sediments and mineral-rich solutions that circulate through them cause chemical changes in the calcium phosphate of the bones. Eventually they are turned to stone giving an accurate representation of the original bones. This process is called fossilization.
A fossil Ammonite
The most suitable sites for fossilization are in seas and lakes were sedimentary deposits like sandstone and limestone are slowly accumulated. Fossils are exposed when such deposits erode away. Fossils can often be dated with the discovery of radioactivity in the surrounding rocks. Some chemicals in rocks decay with time producing radioactivity, for example potassium turns to argon, uranium to lead and rubidium to strontium. The amount of change from one chemical to the other depends on the amount of elapsed time. Consequently the proportion of the second element to the first can be used to calculate the time when the rocks were first laid down around the fossil.
Layers of Rocks give us clues to their age
When rocks occur as undisturbed layers, we find that the lowest layers will be the oldest and topmost layers will be the youngest. Frequently rivers cut incisions into the earths's surface and expose such layers. The Grand Canyon in the U.S.A. is the deepest cleft on the earth's surface.
The Grand Canyon, Western United States of America
The upper rocks of this canyon are about 200 million years old and contain traces of reptiles, impressions of fern leaves and wings of insects. Halfway down the canyon you find limestone of about 400 million years old which contains the remains of primitive armoured fish. Further down the canyon there are no traces of vertebrates. Three-quarters way down there are no apparent traces of life. Close to the bottom of the canyon the rocks are more than 2 000 million years old.
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A model of Dunkleosteus telleri a highly evolved Placoderm which were armoured and jawed fish. Instead of actual teeth, Dunkleosteus possessed two long, bony blades that could slice through flesh and snap and crush bones and almost anything else. It was a vicious hunter, and probably ate whatever it could find, including sharks.
Dunkleosteus
Dunkleosteus
Conservation status: Fossil
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Placodermi
Order: Arthrodira
Family: Dinichthyidae
Genus: Dunkleosteus
Species: D. telleri
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Rocks as old as those of the bottom of the Grand Canyon have been found to contain a fine-grain flint-like substance called chert. Contained in this chert are simple organisms some of which resemble algae filaments others resemble bacteria.
Chert similar to that found at the bottom of the Grand Canyon
These were thought to be until recently the earliest known organism (see further down and for a time-line of life go here) and are referred to as cyanobacteria or blue-greens. These organisms are able to extract hydrogen from water and thereby produce oxygen which is essential for other organisms to survive. The chemical agent responsible for this process is called chlorophyll and process is called photosynthesis, and occurs in true algae and higher plants.
The Anabaena is a genus of filamentous-cyanobacteria, or blue-green algae, found as plankton. It is known for its nitrogen fixing abilities, and they form
Anabaena
Anabaena
Anabaena sphaerica (Nostocales)
Scientific classification
Kingdom: Bacteria
Division: Cyanobacteria
Class: Cyanophyceae
Order: Nostocales
Family: Nostocaceae
Genus: Anabaena
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Pre-Cambrian-stromatolites in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the scientific journal:Nature arguing that geological formations such as this possess 3.5 billion year old fossilized-algae microbes.
Recent News: Original article
Minik & Frei (2004) wrote a paper that concluded that “Planktonic organisms lived in the Isuan oceans where they produced large amounts of highly 13C-depleted organic matter. The aquatic environment of these organisms comprised relatively oxidized compartments, which allowed solute transport of U. The high biomass productivity of planktonic organisms, the strongly 13C-depleted carbon isotopic signature and the evidence for the presence of oxidized aquatic environments all suggest that oxygenic photosynthesis had developed before 3700 Ma.”
How life started?
How did life begin? Even before these blue-greens existed organic molecules must have evolved. The original atmosphere (see separate page) of the earth was very thin and contained hydrogen, carbon-monoxide, ammonia and methane, but no oxygen. This chemical mixture, together with ultra-violet radiation and frequent electrical discharges causing lightening was simulated in the Miller Urey experiment in the 1950s.
This experiment used water (H2O), methane (CH4), ammonia (NH3) and hydrogen (H2). The chemicals were all sealed inside a sterile array of glass tubes and flasks connected together in a loop, with one flask half-full of liquid water and another flask containing a pair of electrodes. The liquid water was heated to induce evaporation, sparks were fired through the atmosphere and water vapor to simulate lightning, and then the atmosphere was cooled again so that the water could condense and trickle back into the first flask in a continuous cycle.
At the end of one week of continuous operation, Miller and Urey observed that as much as 10-15% of the carbon within the system was now in the form of organic compounds. Two percent of the carbon had formed amino acids, including 13 of the 21 that are used to make proteins in living cells, with glycine as the most abundant.
Miller-Urey
Miller-Urey
The original Miller-Urey experiment that recreate the chemical conditions of the primitive Earth in the laboratory, and synthesized some of the building blocks of life.
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A diagramatic representation of the Miller-Urey experiment which attempted to synthesized some of the building blocks of life based on our understanding of the earths first environmental conditions.
Archives: Original article
Miller S. L., (1953). Production of Amino Acids Under Possible Primitive Earth Conditions, Science, 117: 528.
Interpretation of the Miller-Urey Experiment
The molecules produced form this experiment were relatively simple organic molecules, far from a complete living biochemical system, but the experiment established that natural processes could produce the building blocks of life without requiring life to synthesize them in the first place. With time these substances probably increased and interacted with each other to form more complex molecules. Eventually one substance essential to life as we know it appeared. This substance was called deoxyribonucleic acid or DNA.
This experiment inspired many experiments in a similar vein. In 1961, Joan Oro found that amino acids could be made from hydrogen cyanide (HCN) and ammonia in a water solution. He also found that his experiment produced a large amount of the nucleotide base adenine. Experiments conducted later showed that the other RNA and DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere.
Conditions similar to those of the Miller-Urey experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the driving force for chemical reactions. On September 28, 1969, a meteorite that fell over Murchison, Victoria, Australia was found to contain over 90 different amino acids, nineteen of which are found in Earth life. Comets and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such as tholins) formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black as asphalt. The early Earth was bombarded heavily by comets, possibly providing a large supply of complex organic molecules along with the water and other volatiles they contributed. (This could also imply an origin of life outside of Earth, which then migrated here. See: Panspermia)
How valid was the Miller Urey Experiment?
There have been a number of objections to the implications derived from these experiments. The following are extracts from Wikipedia:
Originally it was thought that the primitive secondary atmosphere contained mostly NH3 and CH4. However, it is likely that most of the atmospheric carbon was CO2 with perhaps some CO and the nitrogen mostly N2. The reasons for this are (a) volcanic gas has more CO2, CO and N2 than CH4 and NH3 and (b) UV radiation destroys NH3 and CH4 so that these molecules would have been short-lived. UV light photolyses H2O to H· and ·OH radicals. These then attack methane, giving eventually CO2 and releasing H2 which would be lost into space.