b+Biology

The birth of life on our earth

= 1. Earth’s origin =

==== The origin of life on earth dates back to a period between 4.4 billion years ago, when liquid water appeared on the Earth. The concept of the origin of life has been treated since ancient times in different religions and philosophy. With the development of scientific models often in contrast with what is literally said in the sacred texts of religions, the origin of life has become the subject of debate among science and faith. From a scientific point of view, the explanation of the origin of life starts from the fundamental assumption that the first forms of life originated from non-living material, through reactions that, at present, are no longer in place on our planet. The problem was how it originated this simple, primordial form, probably a cell similar to the current prokaryotes, containing the genetic information stored in nucleic acids, proteins and other biomolecules as well as essential to their reproduction and survival. The first basic stage was the production of easy organic molecules, like amino acids and nucleotides, that set up “the life bricks”.The experiments of S.Miller and other people prove of that this event was possible in the chemical-physical condition of the Early Earth. The most difficult question to be expained is how from these easy organic compounds, concentrated in a primordial soup, some cells with the essential requirements could from to be considered living, that is the capacity of using materials present the environment to maintain its structure, organization and reproduce. Despite it, the reconstruction of the life history implies many question again the especially regarding the sequence of the events. ====




 * 1) Spontaneous generation theory



Before 1668 it was thought the bugs came to life for "spontaneous generation". For spontaneous generation (or a biogenesis) you mean the belief, widespread till the 17th century, that life could came into being "spontaneously" from inert or inanimate matter, by the effect of "vital flows". It was believed that God had directly created only superior living beings (like people and large animals), while "lower" beings (such as worms and insects) could arise spontaneously from the mud or rotting carcasses. This theory was objected in the 17th century, thanks to some experiments of Francesco Redi and Lazzaro Spallanzani. In 1668 Francesco Redi carried out an effective experiment, which represents a classic example of the application of the experimental method in biology.

I am going to describe this experiment:



A piece of meat is put into a glass jar, in the jar open (1a and 1b) there is the formation of larvae and flies, while in the closed jar (2a and 2b) neither flies nor larvae appear in it. Francesco Redi took eight jars, in each of which introduced pieces of different animals: a snake, fish, eels and a piece of beef, and he divided them into two groups of four:

>
 * Without a cap (where the conditions were reproduced in the places such as butchers, etc..)
 * Capped (experimental group)

In the jars without a cap some flies could be observed, which were in direct contact with the meat and then developed into different larvae. In closed jars neither larvae, nor flies were found. From these results Redi concluded that flies could be generated only by other flies: in the open jar, in fact, the flies had entered and laid their eggs.


 * 1) Creazionism



Creationism is the belief that the universe,the earth,life and man are the results of a direct intervention of a god or gods (religious creationism,typically referred to God),or,more rarely,of extraterrestrial entities. All ancient civilizatious have,in their mythology,a story that explains the origin of the world in these terms.Proponents of the "Old Earth creationism" accept geological discoveries but reject the evolution, claiming that all living species, including humans, originally were created by God. Whereas the supporters of the "Young Earth Creationism" assert that the earth is about 6000 years old.


 * 1) Oparin’ s hypothesis and Miller’s experiment



Oparin (born in Oakland on 7th March 1930, died in Nation City on20th May 2007) was an American biochemist. In confirmation of Oparin’s theories on the primordial soup, he created an experiment where he was able to produce amino acids generating electrical discharges in an abiotic environment, greatly reducing (for the presence of hydrogen) and in presence of water, methane and ammonia inside a bowl, then called “Miller’s bowl” (during the experiment called Miller-Urey). The experiment consisted in simulating the climate events that occurred at the primordial birth of life on Earth: some water symbolizing ocean was made to evaporate in a container water, the water vapor went up along a tube and to it, there were added methane, ammonia and hydrogen that were (the hypothetical gases which the atmosphereis thought to be composed)the whole then passed into a container where electric discharges simulating very frequent lightning of that time were given, finally, after cooling, simple organic compounds such as amino acids were obtained. Miller suggested that organic molecules could have been formed from the gases of the atmosphere, which then would be collected in the seas and lakes on the planet, giving origin to a soup called primordial soup. This experiment didn’t demonstrate that these organic compounds were formed spontaneously on the primitive earth, but only that they might have been formed.




 * 1) Endosymbiotic Theory

Prokaryotic organisms were the only form of life on our planet before the birth of organisms of eukaryotic cells. Many biologists think that the transition from prokaryotic to the eukaryotic cell was a highly significant event in the history of life on Earth, second only for biological importance , to the birth of the first simple life forms. Currently, there are several theories on how this step might have happened. One of the most reliable theory is the endosymbiotic one formulated in the late eighties of last century by the American geneticist Lynn Margulis, according to whom , mitochondria and chloroplasts might derive from ancient prokaryotes that have been introduced in larger cells. Here prokaryotes might have given rise to a symbiotic relationship, that is a mutual exchange of favors: the larger cell would provide biomolecules and minerals, while the prokaryotes would provide energy. The theory is called endosymbiotic precisely because it provides a symbiosis, that is an advantageous relation between two organisms which live within each other. The processes that led to the appearance of the first eukaryotic cells were certainly very slow, it is estimated, in fact, that it took 2 million years to move from the first prokaryotic cells to the formation of a cell that presents a core delimited by a membrane(about 1.5 million years ago).