Beginning of life

Philosophers from ancient Greece believed that life was contained in matter itself and when conditions were favourable it would appear spontaneously. Aristotle synthesized in one of his theories all the ideas on spontaneous generation by the philosophers that came before him. According to the great philosopher, living beings are born from similar organisms, but sometimes they can also be generated from inert matter. All things, in fact, have a “passive element” which is their matter and an “active element” which is their shape, meaning a sort of inner force which gives the matter its shape. For example, clay is a non-living matter that has an active element which enables it to shape inert matter into a living being, such as for example a worm or a frog. The spontaneous generation theory was supported by famous scientists such as Newton, Descartes and Bacon and in 1500 there were people who still believed that geese were born from certain trees that lived in contact with the ocean and that lambs were generated inside melons. The first experiments to prove the spontaneous generation theory were done in the XVII century and a doctor called Jean Baptiste Van Helmont declared he had performed a unique experiment: he placed a dirty shirt together with some wheat and according to him, mice were born 21 days later. According to the Doctor the sweat in the shirt was the active element which gave life to the inert matter.

First experiments

Following the first incorrect experiments such as Van Helmont’s one, many others were performed. In 1668 Doctor Francesco Redi led a series of experiments that were supposed to prove that spontaneous generation does not exist. Redi placed in containers some samples of veal and fish, some he sealed while others he left in the open air. As time went by, he noticed that in the open containers on the decomposed meat there were worms (that were in fact larvae!), flies and other insects, while there were no signs of life in the sealed containers.

Life on Earth

There are two different theories on the origin of life on Earth: the autotrophy theory and the heterotrophy one. The first theory assumes that the first living being was autotrophic, meaning that it is able to produce organic substances from inorganic ones like green plants do, through a complex reaction called “chlorophyll photosynthesis”. In the second case instead, the first living organism would have been heterotrophic, meaning that it was not able to produce its own food, but had to feed on other living beings.

Primordial atmosphere

The sun and its planets formed about 5 billion years ago following the explosion of a supernova, which is a big star, that before bursting had generated heavy elements starting from Hydrogen and Helium. At the beginning Earth was an enormous incandescent ball composed primarily of Hydrogen and Helium, but also by heavy elements such as carbon, nitrogen, oxygen, iron and silicon which had been flung into space by the explosion. After that, the planet cooled down and part of the lighter gasses, such as hydrogen and helium, bonded with heavier elements and part got lost in space.

Miller’s experiment

In 1952 the American professor Harold Clayton Urey, Nobel prize for chemistry in 1934, asked a young researcher, Stanley Lloyd Miller, to perform an experiment. Inside a glass bottle, Miller put some extremely hot water and in another one he put a Hydrogen mix (H2). ammonia (NH3) and methane (CH4), which are all those gasses that combined with water vapour (H2O) were thought to have created the primordial atmosphere. Hot water, which according to scientists was in the primitive ocean, created vapour that passed through a tube and went to the container that held the gas mix.

The primordial soup

Similar experiments to those done by Miller have definitely proved that in high temperature conditions, with frequent storms and intense ultraviolet rays, which are similar to the kind of conditions present on Venus nowadays, simple inorganic molecules can transform into more complex substances which we call organic because they are part of living organisms. These organic substances spread in the sea and reacted between themselves and with inorganic salts.

Proteins’ ancestors

In 1957 Sidney Walter Fox, an American biochemist, invented an experiment that proved how proteins could be formed outside of living beings starting from amino acids. Fox simply warmed an amino acid mix on a metal plaque. Right after it cooled off it was possible to notice some complex molecules, very similar to proteins, which he called proteinoids so as not to get them mixed up. At that point it was believed that these new molecules had formed from the union of amino acids freed of the water through its evaporation.

The coacervates

Actually, we are still a long way away from what could be defined as a living organism also because nowadays it is surrounded by a wrap called “cellular membrane”, that separates it from the outer world. Starting from these suppositions, Oparin assumed that in the hot primitive seas organic molecules would gather in small drops, similar to the actual cells. These small drops wrapped up in water molecules are called “coacervates” (from cum acervo = gather together) and were already known before Oparin’s research.

The living cell

In 1665, while observing a sliver of cork, Robert Hook discovered the cell, that extremely complex building block in multicellular beings. In fact, each one contains an incredible number of structures which can be observed through an electron microscope and each one of them performs specific biological and biochemical activities that make the cell into a perfectly organized living “factory”. One can see the cell’s evolution by watching unicellular organisms through the microscope. Within these cells one can see complex structures and organs which are similar to other multicellular organisms.

Energy for life

All living beings need an energy source to activate chemical reactions. For example, to light a match requires some kind of energy source to trigger the reaction. In this case one simply has to rub the top on a rough surface to produce heat and make it light. This is a case of “activation energy”. Almost all the experiments we spoke about in the other chapters, used electric discharges, ultraviolet light and heat as energy sources. However, these energy sources can be harmful to living molecules, because too much heat can disintegrate the molecules and the coacervate in them, causing irreparable damage.

Fermentation and respiration

ATP or adenosyntrophosphate is a complex molecule formed by a nitro compound called adenine, by one sugar with five carbon atoms called ribose and three phosphoric groups. The phosphoric groups were present in the Earth’s crust as phosphates, that is, salts found in the rocks that the hot water of the primordial earth could have melted and carried to the sea. Adenine and ribose instead formed spontaneously and we have experimental proof of this. In 1960 the American biochemist Juan Oro made hydrocyanic acid (one of the products of Miller’s experiment) react with ammonia thus obtaining adenine.

Protein or DNA?

Today, proteins are formed following instructions given by DNA (deoxyribonucleic acid) which in turn is synthesized by specific enzymes that are proteins. So, which came first, protein or DNA? Nucleic acids (DNA and RNA) are made up by nucleotides which are molecules formed by one sugar with 5 carbon atoms, one phosphoric acid molecule and a nitro base. Sugars with 5 atoms of carbon are ribose, which is found in the RNA (ribonucleic acid) and the deoxyribose in the DNA. Nitro bases are compounds with basic proprieties (which means that they can receive protons) that have nitro atoms and are: adenine, cytosine, guanine, thiamine, and uracil.

The origin of photosynthesis

The first cells fed on organic substances in the primordial soup as its concentration gradually diminished. It is likely that the scarcity of energy resources imposed a selection. Some cells acquired the ability to feed on others, while other cells developed the ability to synthesize new organic substances by using energy from oxidation. Even today there still are prokaryotes (cells that don’t have an actual nucleus but a nuclear “equivalent”) that draw energy in this way to live, they are the so called chemosynthetic bacteria.

The first living organisms we have proof of thanks to fossils, are three and a half billion years old. They are the so called “stromatolytes”; structures made of several layers piled one on top of the other like a stack of pancakes. Today one can find organisms similar to the fossilized stromatolytes in Australia’s hot seas. These present day stromatolytes are made up by the growth of bacteria communities and blue algae on which grains of sand deposit. Both bacteria and blue algae are prokaryote organisms, meaning that they do not have a differentiated nucleus, and therefore they are more primitive. But blue algae are able to perform photosynthesis and so we can believe that the first forms of life on Earth date even further back than 3 and a half billion years ago. We don’t have eukaryote cell fossils over a billion years old, so we can assume that life’s evolution in the first two or three billion years was very slow and affected only unicellular organisms. Instead researchers believe that the step from unicellular to multicellular organisms happened very quickly, because the first fossils of complex organisms were already plentiful six hundred million years ago. Precisely six hundred million years ago the Precambrian Era finished and the Palaeozoic era began, of which we have sure fossil proof, when life existed only in the sea. Subsequently algae made their first attempts to colonize the land. After the appearance of the first plants on dry lands came the first herbivores, of which some subsequently evolved into carnivores.

Darwin’s theory

Pasteur’s simple and remarkable experiment put a definite end to all controversy among abiogenesis and biogenesis believers, but new questions about life’s origin arose. If to create a living being there had to be another living being, who created the first one? Furthermore, if an organism creates only identical beings, how can it be that on Earth one can find such a large variety of living beings? Fortunately, the fossils that were found and Darwin’s theory on evolution are able to give some answers to these new questions.

The development of evolution theories

Towards the end the 19th century Weismann (1834 –1914) debunked Lamarck’s belief according to which features acquired during the course of a generation were transferable to descendents and identified in the mix of different heritages, as happens with sexual reproduction, which is the main cause of biological variability. Thus a relationship between evolution and genetics was established. The first genetic contributions to evolutionism came from two researchers, Hardy and Weinberg who in 1908, each one of them on his own, did a statistic study on gene distribution among a population and on the necessary conditions to avoid variations between one generation and the next one.

The first fossils

The oldest sedimentary rocks that we know of date back to 3 and a half billion years ago and they probably contain traces of life. These rocks were found in Canada, in South Africa and in Australia and have been given a precise age thanks to the radioisotope method. The chemical analysis of these rocks has revealed the presence of certain compounds that could be considered as “chemical fossils” because apparently they come exclusively from the metabolism of living beings.

We may say that every day, palaeontology researches, active worldwide, present some new discovery: it may be a new species, or totally unknown organisms, that are not represented in the modern flora and fauna, at times they are so “strange” that it is difficult to understand their anatomy and way of living, at times they show small “flashes” of day to day life, trapped forever in the geological layers. A general picture of the evolution of life on Earth is now quite clear, at least the general lines, however every new discovery forms a small piece of the puzzle of a picture that is becoming increasingly complete and greatly detailed. At times a small detail, preserved by chance in the sediments, that surfaces equally by chance, and is recognized, brings a fundamental contribution, and at times also generates a small “revolution” in the way of thinking and interpreting the long history of life on our Planet. This is the case, for example, of a recent discovery in Utah, that shows how a ferocious predator, Falcarius, a dinosaur covered with feathers, similar to a Velociraptor, evolved subsequently into an herbivorous dinosaur, approximately 125 million years ago, in a sort of “counter-evolution”.

What is a fossil?

When an organism dies, the parts that normally are not affected by decomposition are the hard, mineralized parts: shells and exoskeletons, bones and teeth, scales and plates. When the soft tissues have dissolved, the mineralized parts may be transported by water or by gravity and accumulated in fossil deposits where they become part of the sediments that include them and may be preserved for millions of years. In these cases, complete skeletons are rarely found, and fossil remains are generally mixed together, often with organisms of different species.

The beginning of life

The following era, the Mesozoic Era, is characterized by the extraordinary evolution of reptiles, the undisputed masters of this Era. Reptiles evolved from amphibians, with the “invention” of the egg, that made them independent from water also for reproduction. Some reptiles returned to the water and became good swimmers. Most reptiles were herbivorous, however many carnivorous species evolved. Some, like the feathered dinosaur of the Falcarius genus, which has been recently the subject of much attention, returned to an herbivorous style of life.

The history of life

The birth of life on the Earth required a very long “incubation” period. In a period between 4.5 to 3.8 billion years ago, the bases were set for the formation of the “ingredients” that led to the birth of the first cells. This remote world can now be found in one of the most inhospitable areas of the Earth: in the sources of warm water and in the volcanic fumaroles in the oceanic ridges. However, there are no “records” of the very first phases of life in the geological layers. The most ancient fossils, dating back 3.5 billion years, were found in sedimentary rocks in North-Western Australia.

How are they reconstructed?

Paleontologists carry out a patient investigating task, in which even the smallest element may be fundamental for the reconstruction of the types and habits of the life of a creature of the past, at times it can be a very small bone fragment. Generally, even in paleontology, as in geology, the principle of actualism is used, in which it is hypothesized that similar organs (a paw, a skull, a backbone) in organisms of the present day and of the past, had the same purpose and worked in the same way.

Paleoecology

Having reconstructed the physiognomy of living beings in the past, it is also important to reconstruct their environment and lifestyle. The sediments in which fossil remains are found often provide important indications with regard to the geography of the environment in which the organisms lived, in particular for those organisms that are found in their “living position”, as they died and became fossilized, and therefore in their natural environment. Also, the association with other fossil species can help us to understand in what type of environment they lived.

The dinosaurs’ adapting capacity

In Alaska and in South Australia (which at the time was joined with Antarctica) fossils of Cretaceous dinosaurs that lived in territories situated beyond the Antarctic Circle were found. At the time, the climate was not as severe as it is at present, but due to the long Polar nights, the temperatures must have been quite low in the season of low insolation, probably only a few degrees. Some researchers, when faced with these exceptional findings of animals that were traditionally believed to be cold blooded in an environment with low temperatures, hypothesized that they ventured in long migrations during the colder periods, or that they went into lethargy, somewhat like tortoises and amphibians in our times.

Stories of daily living

The study of fossils, organisms that died millions of years ago, sometimes offers incredible surprises with the discovery of organisms struck by death during their daily activities. These findings are extremely precious for the reconstruction of the way of life, they offer us the possibility to observe, in a surprisingly vivid manner, some scenes of daily living, at times cruel, at times gentle and moving. In a cave in Mount Generoso, near the frontier between Italy and Switzerland, near Chiasso, a cave was discovered in which a group of cave-bears had hibernated…