BIODiversity 1 Lecture 1 Origins of life on earth Course presentations 1) Discuss the following theories on the origins of life on earth: a) special creation theory b) spontaneous generation theory c) the Cosmozoan theory d) the biochemical evolution theory Date {TBA} NB. ALL MEMBERS OF THE GROUP SHOULD PARTICIPATE. ALL QUESTIONS ARE WORTH 50 MARKS. CATEGORY Marking scheme- presentations MAXIMUM POSSIBLE MARK Presentation Groups Question 1a Question 1b Question 1c Question 1d Self presentation and general conduct 5 Organisation of presentation 5 Clarity of presentation 5 Group participation 5 Time management 5 Evidence of research / Research depth 25 Total 50 Characteristics of living organisms 1) auto conservation-the main function of every living organism is making sure that it can continue its existence 2) auto reproduction-any living system can reproduce or proceed from a reproduction process 3) storage of information-each organism contains genetic information. This appears stored in the DNA and is read and translated by proteins according to a universal genetic code 4) breathing-every living organism must have a metabolism that will transform energy and matter taken from the environment into energy and compounds that can be used by different parts of the living organism 5) stability- through the creation and control of its own internal environment, all creatures remain stable in front of perturbations of the external world 6) control- the distinct parts of an organism contribute to the survival of a group and therefore to the conservation of its identity 7) evolution- the mutations in the hereditary material and natural selection permit the perfection, adaptation and complexity of living beings. For many, life is a mere product of evolution 8) death- it is determined by the genes and aggressions from the exterior.it marks the final phase of a living creature 1
HOW ARE THEORIES AND LAWS RELATED? Scientific Theory Scientific Law Subject to change as new information is available. Explains why things behave the way they do. Explanation of a set of related observations or events based upon proven hypotheses and verified multiple times. Generally accepted to be true and universal. Explains how things behave. Description of what happens in nature. HOW ARE THEORIES AND LAWS RELATED? In general, both a scientific theory and a scientific law are accepted to be true by the scientific community as a whole, and represent the tools and products of science. INTRODUCTION Several attempts have been made from time to time to explain the origin of life on earth. As a result, there are several theories which offer their own explanation on the possible mechanism of origin of life. Theories that attempt to explain the origin of life on earth are diverse and uncertain but the major ones are as follows: Theory of Special Creation Theory of Spontaneous Generation Theory of Biochemical Evolution Theory of Panspermia The special creation theory Special creation theory- the earth is only a few thousand years old populated by unchanging forms of life that had been individually made during the single week in which the creator formed the entire universe. A supernatural created life at a particular time. 2
All the different forms of life - created by God. HINDU CONCEPT :- Lord Brahmacreated the living world in one stroke. CHRISTIAN & ISLAM BELIEF :- God created this universe, plants, animals and human beings in about six days. 3 main postulates : All different kinds of animals & plants were created at once. All organisms were created in the same form in which they exist today. Their bodies & organs have been designed to fully meet the needs of the environment. It has no scientific basis. THEORY OF SPONTANEOUS GENERATION Living things originated spontaneously from inanimate objects. It is also called abiogenesis or biopoesis. Supporters :- Aristotle» Epicurus» Von Helmont They believed» Insects arise from dew» Fish & frog from mud Redi s experiment» Fly maggots from meat. Opposers» Fransisco Redi» Spallanzani» Louis pasteur The opposers disproved this theory. Pasteur s experiment 4) Spontaneous generation- Life arose from non- living matter on numerous occassions. theory was advanced by Aristotle who lived from 384-322BC and assumed that certain particles of matter contained an active principle which could produce a living organism when conditions were suitable. Theory fell from favour when Christianity was starting to spread. Experiments by the Italian biologist, Francesco Redi (1688) demonstrated the concept of BIOGENESIS, that life can arise only from pre-existing life. Other experiments by Lazaro Spallanzani (1765) disproved the theory In 1860, Louis Pasteur, modifying Spallanzani s experiments demonstrated biogenesis and finally disproved the theory. Proposed by Richter in 1865 and supported by Arrhenius. Life had come to earth from other planets of the universe, in the form of resistant spores. British astronomers Fred Hoyle and Chandra Wickramasinghe proposed: interstellar dust and comets contain organic compounds. comets, which are largely made of water-ice, carry bacterial life across galaxies and protect it from radiation damage along the way. Dr. Richard B. Hoover, an astrobiologist with NASA s Marshall Space Flight Center published a paper on March 4, 2011, claiming to have found fossil evidence for cyanobacteria in an extremely rare class of meteorites. 3
Panspermia Panspermia = life originated elsewhere and migrated to Earth Life began in rock, then kicked off the planet by an impact Support: organic material is everywhere, and some bacteria can withstand large amounts of radiation and go dormant under low atmospheric conditions Panspermia Martian meteorites Both have possible fossil evidence of life on Mars The biochemical evolution theory Theory stipulates that life arose according to physical and chemical laws. The first organisms were products of a chemical evolution in 4 stages: a) The abiotic (non-living) synthesis and accumulation of small organic molecules or monomers such as amino acids and nucleotides b) The joining of these monomers into polymers including proteins and polynucleotides c) The aggregation of abiotically produced molecules into droplets called protobionts that had chemical characteristics different from their surroundings. d) The origin of heredity Stages in the biochemical evolution 1. The earth and its atmosphere are formed The primordial atmosphere originated from outgassing of the molten interior of the planet (through volcanos) and consisted of CO, CO 2, H 2, N 2, H 2 O, S, HCl and HCN, but little or no O 2 2. The primordial seas formed As the earth cooled, gases condensed to produce primordial seas consisting of water and minerals 3. Complex Molecules Synthesized Energy catalyzed the formation of organic molecules from inorganic molecules Organic soup formed Complex molecules included acetic acid, formaldehyde, and amino acids These molecules are monomers that would become the building blocks for the synthesis of polymers 4
4. Polymers and self-replicating molecules were synthesized Monomers combined to form polymers. Some made by dehydration condensation (removal of water) Protenoids are abiotically produced polypeptides. They can be experimentally produced by allowing amino acids to dehydrate on hot dry substances 5. Organic molecules were concentrated and isolated into protobionts Protobionts were the precursors of cells. They could carry out chemical reactions enclosed within a border across which materials can be exchanged, but couldn t reproduce Microspheres and coacervates are experimentally (and abiotically) produced protobionts that have some selectively permeable qualities 6. Primitive Heterotrophic Prokaryotes Formed Heterotrophs are living organisms that obtain energy by consuming organic substances (i.e. pathenogenic bacteria) The organic soup was a source of organic material for the heterotrophic cells to consume for energy As the cells reproduced, competition drove natural selection to favor those cells most successful at obtaining food 7. Primitive Autotrophic Prokaryotes were Formed As a result of mutation, a heterotroph gained the ability to produce its own food. As an autotroph, it had a successful advantage Autotrophs make their own organic compounds using light energy or the energy of inorganic substances (i.e. cyanobacteria) 8. Oxygen and the Ozone Layer Formed and Abiotic Chemical Evolution Ended As a by-product of the photosynthetic activity of the autotrophs, oxygen is released and accumulates in the atmosphere Ozone layer formed when O2 interacts with UV light. Ozone layer blocks UV light and ends a major source of energy for the abiotic synthesis of organic molecules Origin of Life Eukaryotes Formed (endosymbiotic theory) Endosymbiotic Theory states that eukaryotic cells originated from a mutually beneficial association (symbiosis) among various kinds of prokaryotes Mitochondria, chloroplasts, and other organelles established residence inside another prokaryote, producing a eukaryote 5
Evidence A) Mitochondria and chloroplasts possess their own DNA (circular and naked ) B) Ribosomes of mitochondria and chloroplasts resemble those of bacteria C) Mitochondria and chloroplasts reproduce independently of their eukaryotic host cell (binary fission) D) Thylakoid membranes of chloroplasts resemble the photosynthetic membranes of cyanobacteria Mitochondria and chloroplasts Early eukaryotic cells engulfed aerobic bacteria in a process similar to phagocytosis in amoeba Having been surrounded by a plasma membrane, the bacteria were not digested but, instead, entered into a symbiotic relationship with the host cell. The bacteria would have continued to perform aerobic respiration, providing excess ATP to the host eukaryotic cell,which would have continued to seek out and acquire energy-rich molecules from its surroundings. Endosymbiotic bacteria, benefiting from this chemical-rich environment, would have begun to reproduce independently within this larger cell. This process is referred to as endosymbiosis Endosymbiosis theory (Lynn Margulis, 1970 s) Endosymbiosis Evidence which supports this theory is that both mitochondria and chloroplasts: have their own DNA undergo division independently of their own cell s division contain two sets of membranes (outer and inner membranes) 6
The primitive atmosphere The earth is about 4.5 billion years and life probably began only a few hundred million years after the formation of the earth. Metabolic activities of single celled organisms have been recorded in rocks 3.8 billion years old in Greenland. For the stages in the biochemical evolution theory to take place, the conditions in the atmosphere would have to be different from what they are today. In the 1920 s, A.I Oparin of Russia and J.B.S Haldane of Britain separately suggested that the primitive earth favoured chemical reactions that synthesised organic compounds from inorganic compounds present in the early atmosphere and seas. Modern earth s atmosphere is rich in oxygen from photosynthesis. The lack of oxygen is a necessity because it is easier to generate organic molecules in an atmosphere lacking oxygen. Oxygen attacks chemical bonds, so its presence is not conducive to the spontaneous synthesis of complex molecules. The early earth was probably hot (about 4000-8000 C). A cooling carbon and other less volatile metals condensed to form the earth s core. The lighter gases hydrogen, helium, nitrogen, oxygen and argon would have escaped. Single compounds containing these elements (amongst others) would have been retained e.g water, carbon dioxide and methane. The earth had a much less oxidising atmosphere derived mainly from volcanic vapours. The reducing atmosphere (electron adding) enhanced the joining of simple molecules to form complex ones. The considerable energy needed for the synthesis was probably provided by lightning and intense UV radiation that penetrated the primitive atmosphere. The modern atmosphere has an ozone (O 3 ) layer which keeps most of the UV radiation. The joining together of smaller molecules may have produced complex organic molecules. The transition from complex organic molecules to simple living organisms is debatable. In 1953, Stanley Miller and Harold Urey, tested the Oparin Haldane hypothesis in the laboratory by creating conditions comparable to those of early earth. Their apparatus produced a total of 15 amino acids and other organic compounds found in living organisms. The Miller-Urey Experiment In the 1950 s, Stanley L Miller working in the laboratory of Harold C. Urey at the University of Chicago did the first experiment designed to clarify the chemical reactions that occurred on primitive earth. In the flask, (at the bottom) he created an ocean of water which he heated forcing water vapour to circulate through the apparatus. The flask at the top contained an atmosphere consisting of methane (CH 4 ), ammonia (NH 3 ), hydrogen (H 2 ) and circulating water vapour. He then exposed the gases to continuous electrical discharge lightning causing the gases to interact. Water soluble products of these reactions then passed through a condenser and dissolved in the mock ocean. The experiment yielded many amino acids and enabled Miller to explain how they had formed. Primordial Earth Miller-Urey experiment showed the spontaneous formation of macromolecules was possible with the conditions in the earth s early atmosphere. 7
The Miller-Urey Apparatus The atmosphere in the Miller-Urey model was made up of methane and ammonia gases thought to have prevailed in the primitive atmosphere. It has since been suggested that CO 2 was present in relatively high concentration in the primitive atmosphere. Experiments elsewhere using Miller-Urey s apparatus but containing mixtures of CO 2, H 2 O and traces of other gases have produced similar results. Oparin considered that protobionts (protein molecules) were crucial to the transformation of complex organic molecules to simple living organisms. Living cells may have been preceeded by protobionts, aggregates of abiotically produced molecules. Oparin called the protobionts coacervates Each coacervate is an aggregate of macromolecules, mostly hydrophobic surrounded and stabilised by a shell of water molecules. The chemical building blocks of life may have been accumulating as a natural stage in the chemical evolution of the planet. Therefore the incoperation of a pre-existing molecule capable of self replication into the coacervate and an internal arrangement of a now lipid coated coacervate may have produced a primitive cell. HOW DID CELLS FORM? Sidney Fox and other scientists have done extensive research on the physical structures that may have given rise to the first cells. Cell like structures, including microspheres and coacervates, form spontaneously in certain kinds of solutions. Coacervates and microspheres like cells can take up certain substances from their surroundings. Coacervates can grow Microspheres can bud to from smaller microspheres. However, microspheres and coacervates do not have all of the properties of life. Unlike cells, microspheres and coacervates do not have hereditary material. Formation of Protocells The next important step in formation of cells is to develop a membrane to protect those selfreplicating molecules. Phospholipid molecules naturally arrange themselves into spherical shapes called liposomes. Evolution of the genetic code Once the genetic information became incorporated inside the membrane and closed compartments, coacervates would have acquired heredity and the ability to evolve as units RNA was probably the first genetic material. The first genes may have been abiotically produced RNA, whose base sequences served as templates for both alignment of amino acids in polypeptide synthesis and the alignment of complementary nucleotide bases in a primitive form of self replication. The self replicating ability of RNA has been proved under laboratory conditions. Evolution and Variation Along with amino acids and proteins, experiments have shown the spontaneous development of other macromolecules such as lipids and nucleic acids (such as RNA). As soon as a molecule formed which could selfreplicate, the possibility then exists for errors in those copies. Errors in replication produces variation. And natural selection acts on variation. Any molecule which could copy itself more efficiently or faster, could be selected for in an environment. 8
Prokaryotic cells The oldest known fossils on Earth are dated to 3.5 billion years old from Western Australia layered in formations called stromatolites These microscopic fossils resemble present-day cyanobacteria Although the oldest fossil bacteria resemble photosynthetic cyanobacteria, which use oxygen, the very first prokaryotic cells would certainly have been anaerobic, as the atmosphere would then have contained little or no free oxygen. These first prokaryotic organisms would likely have relied on abiotic sources of organic compounds. They would have been chemoautotrophic, using compounds like H 2 S Changing the atmosphere Although the first photosynthetic organisms may have also used hydrogen sulfide as a source of hydrogen, those that used water would have had a virtually unlimited supply. As they removed hydrogen from water, they would have released free oxygen gas into the atmosphere a process that would have had a dramatic effect. The accumulation of oxygen gas, which is very reactive, would have been toxic to many of the anaerobic organisms on Earth. While these photosynthetic cells prospered, others would have had to adapt to the steadily increasing levels of atmospheric oxygen or perish. Some of the oxygen gas reaching the upper atmosphere would have reacted to form a layer of ozone gas, having the potential to dramatically reduce the amount of damaging ultraviolet radiation reaching Earth. At the same time, the very success of the photosynthetic cells would have favoured the evolution of many heterotrophic organisms. Multicellularity For the first 3 billion years of life on Earth, all organisms were unicellular. Eubacteria gave rise to aerobic and photosynthetic lineages, while archaebacteria evolved into three main groups: methanogens, extreme halophiles, and extreme thermophiles. Once eukaryotic organisms evolved complex structures and processes, including mitosis and sexual reproduction, they would have had the benefit of much more extensive genetic recombination than would have been possible among prokaryotic cells. Photosynthesis continued to increase the oxygen concentration in the atmosphere to the benefit of aerobic organisms. Multicellular organisms, including plants, fungi, and animals, are thought to have evolved less than 750 million years ago. Diversification The oldest fossils of multicellular animals date from about 640 million years ago. However, during a 40-million-year period beginning about 565 million years ago, a massive increase in animal diversity occurred, referred to as the Cambrian explosion. Fossil evidence dating from this period shows the appearance of early arthropods, such as trilobites, as well as echinoderms and molluscs; primitive chordates which were precursors to the vertebrates also appeared. Animals representing all present-day major phyla, as well as many that are now extinct, first appeared during this period, a time span that represents less than 1% of Earth s history. The history of life Major episodes in the history of life The earth was formed 4.5mya and life probably began a few hundred million years later. The relatively simple structure of the prokaryotic cell suggests that they were the earliest organisms and the fossil record now supports this presumption. 9
Clock analogy for some key events in evolutionary history The evidence of ancient prokaryotic life has been found in rocks called stromatolites. Stromatolites are banded domes of sedimentary rocks that are similar to layerded mats formed today in salt marshes by colonies of bacteria. The layers are sediments that stick to jelly-like coats of motile microbes, which continually migrate out of one layer of sediment and form a new one above producing the banded pattern Bacterial mats and stromatolites Although some stromatolites were formed from mineral sedimentation, without the presence of life, fossils resembling spherical and filamentous prokaryotes have been found in 3.5 billion year old stromatolites in Southern Africa and Western Australia. These are currently the oldest known fossils. 10
CONCLUSION All the evidence gathered thus far has revealed a great deal about the origin of life, but there is still much to learn. Numerous scenarios have been explored for many years, but there is still a large gap between what is known and what is unknown. Because of the enormous length of time and the tremendous change that has occurred since then, much of the evidence relevant to origins has been lost and we may never know certain details. Nevertheless, many of the gaps in our knowledge (gaps that seemed unbridgeable just 20 years ago) have been filled in recent years, and continuing research and new technologies hold the promise of more insights. The end Questions..!! 11