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1. How old is the earth?
It is believed that the earth is approximately 4.5 billion years old.

2. How old is the universe?
From analysis of data collected by the Hubble telescope the age of the universe is estimated to be about 12 billion years.

3. When did life appear on earth?
It is estimated that life on earth emerged about 3.5 billion years ago, thus 1 billion years after the formation of the planet.

4. Historically how has the origin of life on earth been explained?
The most recurrent explanation for the phenomenon of life on earth is the mythological. People from various parts of the world developed explanatory myths about the origin of animals and human beings. Some of those myths were incorporated into religions and almost all religions have metaphorical or transcendental explanations about the origin of life on the planet.

With the development of science new explanatory attempts have emerged. Notable among them are the spontaneous generation hypothesis, or abiogenesis, that asserted that living beings were created from nonliving material, the cosmic panspermia hypothesis, theory that life on earth is a result of seeding from the outer space, the autotrophic hypothesis, according to which the first living beings were autotrophs, and the heterotrophic hypothesis, the most accepted nowadays, that affirms that life emerged from heterotrophic cells.

At the end of the 1980s decade a new hypothesis known as the RNA world hypothesis was presented. This hypothesis asserts that primitive life had only RNA as genetic material and as structural molecules that later turned into DNA and proteins. The RNA world hypothesis is strengthened by the fact that RNA can play a catalytic role, like enzymes, and by the finding that some bacteria have ribosomes made only of RNA without associated proteins.

5. What is the spontaneous generation hypothesis?
The spontaneous generation hypothesys, or abiogenesis, asserts that life on earth has come from nonliving material. For example, the fact that with time rats appeared around waste was considered in the past a confirmation of this hypothesis. Some supporters of spontaneous generation associated it with the existence of an active principle (the vital elan) that would be the source of life, a theory known as vitalism.

Origin of Life - Image Diversity: abiogenesis

6. How did the experiments of Redi and Pasteur refute the hypothesis of spontaneous generation?
To refute the spontaneous generation hypothesis many experiments were performed. Francisco Redi, in 1668, verified that maggots appeared on meat only when there was exposition to the environment; within closed environments, they did not appear. In 1862, Louis Pasteur working with swan-neck flasks refuted the abiogenesis hypothesis definitively. In this experiment Pasteur demonstrated that boiled (to kill microorganisms) nutritive soups put in swan-neck flasks (with a curved down mouth so microorganisms could not enter easily) did not contaminate with microorganisms while the same soups within flasks with open upwards mouths were contaminated in a few days. The fact that both flasks were open refuted the argument of the vitalists that the vital elan could not enter the flasks. Pasteur broke the swan-necks of the flasks to demonstrate that proliferation of microorganism could happen if these beings were able to reach the broth.

Origin of Life - Image Diversity: Redi's experiment Pasteur's experiment

7. What is panspermia?
Panspermia is a hypothesis that describes life on earth as not originated from the planet. The idea is that the first living beings that colonized the earth came from outer space, from other planets or even from other galaxies by traveling in meteorites, comets, etc. According to this hypothesis even the type of life now existent on earth could have also been seeded intentionally by extraterrestrial beings in other stellar and planetary systems.

8. What is the autotrophic hypothesis on the origin of life?
The autotrophic hypothesis on the origin of life asserts that the first living beings on earth were producers of their own food, just like plants and chemosynthetic microorganisms.

9. What is the heterotrophic hypothesis on the origin of life?
According to the heterotrophic hypothesis the first living beings were very simple heterotrophic organisms, i.e., not producers of their own food, which emerged from the gradual association of organic molecules into small organized structures (the coacervates). The first organic molecules in their turn would have appeared from substances of the earth's primitive atmosphere submitted to strong electrical discharges, to solar radiation and to high temperatures.

10. What is the most accepted hypothesis about the origin of life on earth? How does it compare to the other main hypotheses?
The heterotrophic hypothesis is the strongest and most accepted hypothesis about the origin of life.

The spontaneous generation hypothesis has been excluded by the experiments of Pasteur. The panspermia hypothesis is not yet completely refuted but it is not well-accepted since it would be necessary to explain how living beings could survive long space journeys under conditions of extreme temperatures as well as to clarify the manner by which they would resist the high temperatures faced when entering the earth's atmosphere. The autotrophic hypothesis is weakened if one takes into account that the production of organic material from inorganic substances is a highly complex process requiring diversified enzymatic systems and that the existence of complex metabolic reactions on the primitive earth were not probable.

11. Before the emergence of life of what gases was the earth's primitive atmosphere constituted?
The earth's primitive atmosphere was basically formed of methane, hydrogen, ammonia and water vapor.

12. What are the main constituents of the earth's atmosphere in our time?
The present atmosphere of the earth is constituted mainly of molecular nitrogen (N2) and molecular oxygen (O2). Nitrogen is the most abundant gas, approximately 80% of the total volume. Oxygen makes up about 20%. Other gases exist in the atmosphere in a low percentage. (Of great concern is the increase in the amount of carbon dioxide due to human activity, the cause of the threatening global warming.)

13. Was there molecular oxygen in the earth's primitive atmosphere? How has that molecule become abundant?
The presence of molecular oxygen in the primitive atmosphere was probably at a minimum and extremely rare. Oxygen became abundant with the emergence of photosynthetic beings, approximately, 1.5 billion years after the appearance of life on the planet.

14. Which physical elements contributed to the great amount of available energy on the primitive earth at the time of the origin of life?
3.5 billion years ago the water cycle was faster than today, resulting in hard storms with intense electrical discharges. There was also no chemical protection from the ozone layer against ultraviolet radiation. The temperatures in the atmosphere and on the planet surface were very high. Electricity, radiation and heat constituted large available energy sources.

15. What was the experiment of Stanley Miller (1953) on the origin of life?
In 1953 Stanley Miller arranged an experimental apparatus that simulated the atmospheric conditions of the primitive earth. The experiment contained a mixture of methane, ammonia, hydrogen and circulating water that when heated was transformed into vapor. He submitted the mixture to continuous bombardment of electrical discharge and after days obtained a liquid residual within which he discovered organic molecules and among them surprisingly the amino acids glycine and alanine, the most abundant constituents of proteins. Other researchers reproduced the Miller experiment and noted also the formation of other organic molecules such as lipids, carbohydrates and nucleotides.

Origin of Life - Image Diversity: Stanley Miller experiment

16. What are coacervates?
Coarcervates are small structures made of the aggregation of organic molecules under water solution. By electrical attraction the molecules join into bigger and more organized particles distinct from the fluid environment forming a membrane-like structure that separates an internal region of the coacervate from the exterior. The coacervates might divide themselves and also absorb and excrete substances. It is believed that these structures may have been the precursors of cells.

Origin of Life - Image Diversity: coacervates

17. How can coacervates be formed of phospholipids or polypeptides?
Phospholipids are amphipathic molecules, i.e., they present a polar portion and a nonpolar portion. In contact with water these molecules tend to spontaneously unite and organize themselves forming membranes that create a closed interior space separated from the exterior environment. Polypeptide chains in their turn can attract water (by electrical attraction) forming a surrounding water layer and also creating an organized structure with delimited interior space.

18. How could coacervates have facilitated the emergence of life on earth?
Coacervates probably provided a nitid separation between an internal and an external environment and thus the organic material within was not lost to the ocean. The enzymatic action inside that internal environment could develop in different manners increasing the speed of specific chemical reactions. Coacervates also allowed the molecular flux across its membrane to be selective. Since containing different molecules and differently organized from each other, coacervates could have promoted a competition for molecules from the environment setting out an evolutionary selection.

19. What is the evolutionary origin of the internal membranous organelles of the cell?
It is accepted that the internal membranous organelles of the eukaryotic, like the Golgi apparatus and the endoplasmic reticulum, appeared from invaginations of the external membrane of primitive cells.

20. How have prokaryotic cells given origin to aerobic eukaryotic cells and to photosynthetic aerobic eukaryotic cells?
According to the most accepted hypothesis aerobic eukaryotic cells emerged from the association of aerobic prokaryotes engulfed by primitive anaerobic eukaryotic cells. This would have been the origin of mitochondria that thus would have primitively been aerobic bacteria engulfed by eukaryotic anaerobes. This hypothesis is called the endosymbiotic hypothesis on the origin of mitochondria.

Chloroplasts would also have appeared by endosymbiosis from the entry of photosynthetic prokaryotes into aerobic eukaryotes, both establishing a mutualist ecological interaction.

21. What evidence strengthens the hypothesis that chloroplasts could have been photosynthetic prokaryotes and mitochondria could have been aerobic prokaryotes?
The fact that chloroplasts are the organelles responsible for photosynthesis in plants leads to the supposition that before symbiosis they were autotrophic prokaryotes. For the reason that mitochondria are the center of the aerobic cellular respiration, the powerhouse of the eukaryotic cell, it is supposed that they were once aerobic prokaryotes.

The endosymbiotic hypothesis to explain the emergence of aerobic and autotrophic eukaryotic beings is strengthened further by the following evidence: chloroplasts as well as mitochondria have their own DNA, similar to bacterial DNA; chloroplasts and mitochondria reproduce asexually by binary division, like bacteria do; both organelles have ribosomes and synthesize proteins.

Origin of Life - Image Diversity: endosymbiotic hypothesis

22. How did the first fermenting autotrophs appeared? What about the first aerobic beings?
The heterotrophic hypothesis asserts that the first living beings were the fermenting heterotrophs. Fermentation released carbon dioxide (CO2) and then the atmosphere became enriched by this gas. By mutation and natural selection organisms capable of using carbon dioxide and light to synthesize organic material appeared. These would have been the first photosynthetic beings (that were also fermenting beings since there was no abundance of oxygen).

Since photosynthesis is a reaction that releases molecular oxygen, with the emergence of fermenting autotrophs this gas became available. Some organisms then developed aerobic respiration using O2, a highly efficient method to produce energy.

23. Why is it more probable that the photosynthetic prokaryotes appeared before the aerobic eukaryotes?
It is more probable that photosynthetic prokaryotes appeared before the aerobic eukaryotes because without photosynthesis the earth's atmosphere would not be enriched with molecular oxygen, and without oxygen the existence of aerobic beings would not be possible.

24. What is an argument that shows that the emergence of photosynthetic beings was crucial for life to reach the marine surface and later the dry land?
Ultraviolet radiation from the sun was not disallowed to reach the surface of the primitive earth. Therefore the development of life on dry land or even near the aquatic surface was impracticable. Probably the first living beings lived submerged in deep water to avoid destruction by solar radiation. Only after the appearance of photosynthetic beings and the later filling of the atmosphere with oxygen released by them the formation of the atmospheric ozone layer that filters ultraviolet radiation was possible.

1. What is the problem that the theory of evolution and its rival theories try to solve?
The problem that the theory of evolution, or simply evolution, and its rival theories try to solve is to explain how the different living beings that live on earth have appeared.

2. What is the main theory opposed to evolution?
The main theory that opposes the evolution theory on the explanation of how species emerged (phylogenesis) is fixism.

3. What is fixism?
Fixism is the theory about the diversity of life on earth that affirms that the current existent species were identical to species of the past and came out already adapted to the environment without undergoing changes.

Fixism opposes evolutionism since evolutionism is the idea that current species emerged from gradual transformations suffered by ancestral and extinct species.

The religious version of fixism is called creationism. Many different forms of creationism are found in the mythology of various religions. Modernized religious interpreters teach creationism as a metaphorical wisdom and not as opposed to evolutionism.

Furthermore it is possible to make evolution compatible with creationism by considering that God in His perfection would not create a world so full of imperfections and sufferings like our world. One can maintain the creationist belief thinking that the world God created is another much better world or at least not the one that we see while admitting the imperfection of life that we see has emerged by evolution.

4. In the scientific competition against fixism what are the main arguments that favor evolutionism?
The main arguments in favor of evolutionism are: paleontological, from the study of similarities among fossils of different periods; of compared anatomy, the existence of structures with same origin and function and of residual organs, like the human appendix, that reveal relationships among species; of compared embryology, similarities of structures and developmental processes among embryos of related species; of molecular biology, larger percentage of similar nucleotide sequences in the DNA of species having common ancestors.

5. What are fossils?
Fossils are petrified vestiges of beings that lived in the past conserved by chemical and geological processes and found within rocks and sedimentary strata of the terrestrial crust.

Evolution Theory and Evidence: fossils

6. How does the study of fossils strengthen the theory of evolution?
The study of fossils reveals ancient and extinct species having many similar structures to others of the present and of the past. Fossils still allow radioactive dating to estimate the periods during which species lived and to establish a chronological relationship between them. Those evidences strengthen the hypothesis of relationship and common origin among species and that their features have modified gradually until the formation of the current species.

7. Historically what were the two main evolutionary theories?
The two main evolutionary theories were lamarckism and darwinism.

Evolution Theory and Evidence: Lamarck Darwin

8. What is meant by the law of use and disuse and by the law of the transmission of acquired characteristics?
According to the law of use and disuse the characteristics of a body vary as it is more or less used. This rule is valid for example for features like the muscular mass and the size of the bones.

The law of the transmission of acquired characteristics in its turn established that parents could transmit to their offspring characteristics acquired by the law of use and disuse.

9. What is lamarckism?
Lamarckism is the theory that unites the law of use and disuse with the law of the transmission of acquired characteristics, i.e., that asserted that acquired characteristics, for example, the muscular mass, could be transmitted from a parent to its offspring.

The theory was proposed by the French naturalist Lamarck in the beginning of the 19th century. At that time the idea was not so absurd since nobody knew how the transmission of hereditary characteristics occurred. (Lamarck had great merit in introducing an evolutionary theory based in natural law at a time dominated by fixism.)

10. Who was Charles Darwin?
Charles Darwin was an English naturalist born in 1809 and considered the father of the theory of evolution. At the end of the year 1831, before turning 23 years of age, Darwin embarked as volunteer scientist on the ship the Beagle for a five year expedition to the South American coast and the Pacific. During the voyage, whose most famous passage was the stop in the Galapagos Islands, Darwin collected data that he used to write his masterpiece “The Origin of Species” (1859). In this book the principles of the common ancestry of all living beings and of natural selection as the force that drives the diversity of species were described. Darwin died in 1882.

(The original name of the most famous book written by Darwin was “On the Origin of Species by Means of Natural Selection”.)

Evolution Theory and Evidence: the Beagle ship Galapagos

11. What is the mechanism described by Darwin that eliminates species less adapted to environmental conditions?
The mentioned mechanism is the natural selection.

12. How did Darwin reach the principle of natural selection from the observation of differences among individuals of the same species?
Darwin recognized that in a same species there were individuals with different characteristics. He also realized that those differences could lead to different survival and reproduction chances for each individual. Therefore he discovered the importance of the environment acting upon organisms and preserving those having more advantageous characteristics for survival and more able to generate offspring and so he described the basis of the principle of natural selection.

13. How did the industrial revolution in England offer an example of natural selection?
One of the classic examples of natural selection is regarding the moths of industrial zones of England in the end of the 19th century and the beginning of the 20th century. As the industrial revolution advanced the bark of the trees that moths landed on became darker due to the soot released from factories. The population of light moths then decreased and was substituted by a population of dark moths since the mimicry of the dark moths in the new environment protected them from predators, i.e., they had an adaptive advantage in that new environment. Light moths in their turn suffered the negative effect of natural selection for becoming more visible to predators and were almost eliminated. In the open forest far from factories however it was experimentally verified that light moths maintained their adaptive advantage and the dark moths continued to be more easily found by predators.

Evolution Theory and Evidence: moths

14. What are the fundamental similarities and differences between lamarckism and darwinism?
Both lamarckism and darwinism are evolutionary theories as opposed to fixism, both admit the existence of processes that caused changes in the characteristics of the living beings in the past.

They have however different explanations for those changes. Lamarckism combines the law of use and disuse with the law of the transmission of acquired characteristics to explain the changes. Darwinism defends the action of the natural selection.

15. In the time of Darwin the results of Mendel’s research on biological inheritance had not been published, Genetics was not yet developed, neither DNA nor the concept of genetic mutation were known. What is the modern darwinist theory that incorporates these bodies of knowledge?
The modern darwinist theory that incorporates knowledge from Genetics and Molecular Biology is called neodarwinism, or synthetic theory of evolution.

16. How does the synthetic theory of evolution incorporate knowledge from Genetics and Molecular Biology into the darwinism?
Today it is known that variation of inherited characteristics is created by alterations in the genetic material of the individuals, more precisely by modifications or recombinations of DNA molecules. Small changes in the genetic material accumulate and new phenotypical characteristics emerge. The carriers of these characteristics then are submitted to natural selection. From modern Biology its recognized that natural selection generates in a given population an increase in the frequency of alleles and genes more favorable to survival and reproduction; less advantageous genes and alleles tend to be eliminated.

17. Using the concepts of variability, environmental pressure and natural selection how does the synthetic theory explain the darwinian natural selection?
Genetic variability occurs from recombination of chromosomes during sexual reproduction and from DNA mutations in germ cells and gametes. Such variability creates individuals who are carriers of some new phenotypical characteristics compared to their ancestors. These individuals are submitted to environmental pressure and can be more or less well-succeeded concerning survival or reproduction. Those better succeeded transmit their genetic patrimony to a larger number of descendants increasing the frequency of their genes in the population; those less well-succeeded tend to transmit their genes to a small number of descendants decreasing the frequency of their genes in the population or even becoming extinct. This process is called natural selection (preservation of organisms that present more adapted phenotypes for the environmental pressure they face).

18. In hospitals where many tuberculosis patients are treated the population of the tuberculosis mycobacteria may be constituted of multiresistant (to antibiotics) strains. How does the synthetic theory of evolution explain this fact?
The appearance of multiresistant strains of pathogenic parasites in hospitals, for example, of multiresistant tuberculosis bacteria, can be explained by the synthetic theory of evolution.

As in any environment, TB bacteria in hospitals undergo changes in their genetic material. In the hospital environment however they suffer continuous exposition to antibiotics. Many of them die by the antibiotic action but carriers of mutations that provide resistance to those antibiotics proliferate freely. These resistant microorganisms when submitted to other antibiotics again undergo natural selection and those which became resistant to these other drugs are preserved and proliferate. Thus strains of multiresistant (nontreatable) mutant bacteria emerge in hospitals.

The use of antibiotics is a factor that promotes natural selection and the emergence of multiresistant bacteria. This is the reason why hospitals often have committees that control the use of antibiotics.

19. What is reproductive isolation?
Living beings are considered under reproductive isolation when they cannot cross among themselves or if they can cross but cannot generate fertile offspring.

20. What is the relationship between the concept of reproductive isolation and the concept of species?
Reproductive isolation is an important concept because it defines the concept of species: only living beings that can cross among themselves and generate fertile offspring, i.e., that are not under reproductive isolation, belong to the same species. For example, humans and chimps are under reproductive isolation and are not of the same species.

21. What is speciation?
Speciation is the process by which different species emerge from a common ancestor species. Speciation generally begins when populations of the same species become geographically isolated, i.e., when they are separated by some physical barrier that disallows crossing between individuals from one population and individuals of another population.

Groups that for a long time are kept under geographical isolation tend to accumulate different phenotypical characteristics from each other by means of genetic variability (mutations and recombination) and natural selection. When those differences reach a point that makes the crossing of individuals of one group with individuals of the other group impossible or the generation of fertile offspring no longer happens it is said that speciation has occurred.

Evolution Theory and Evidence: speciation

22. Why does geographical isolation lead to speciation?
Geographical isolation between groups of the same species leads to formation of a new species since it disallows crossing among isolated individuals. Distinct characteristics from the other groups are incorporated by genetic variability and natural selection into the isolated groups until the emergence of a new species. So the geographical isolation creates the reproductive isolation.

23. How can the fact that fishes and dolphins have similar organs and similar general shape be explained?
Fishes and dolphins have similar organs and shape because although they have phylogenetically distant ancestors they face similar environmental pressures since they share the same habitat (water). So by undergoing genetic variability and natural selection some similar features, for example, the hydrodynamic body and the presence of fins, were incorporated into these animals.

24. What is adaptive convergence?
Adaptive convergence is the phenomenon by which living beings facing the same environmental pressure (problems) and undergoing genetic variability and natural selection incorporate similar (analogous) organs and structures (solutions) into their bodies during evolution. For example, the fins and the hydrodynamic body of fishes and dolphins, phylogenetically distant animals.

Evolution Theory and Evidence: adaptive convergence

25. What is adaptive radiation?
Adaptive radiation is the appearance of several other species from one common ancestral species that have spread to various regions or environments. The different characteristics among the species correspond to the adaptive necessities of the ecological niches each one occupies, i.e., to different environmental pressures.

Evolution Theory and Evidence: adaptive radiation

26. What is the difference between analogous and homologous organs?
Characteristics of different species are said to be analogous when having the same biological function, for example, the wings of bats and the wings of insects.

Characteristics of different species are said to be homologous when having the same biological origin, i.e., when they are products of differentiation of a same characteristic from a common ancestor, like cat paws and human feet. (Characteristics of different species may be analogous and homologous.)

Evolution Theory and Evidence: analogous organs homologous organs