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.

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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.

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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. 

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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.

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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.

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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.

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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.

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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.

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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.

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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…

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