school energy&environment

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. These are unicellular organisms, similar to bacteria, extremely thin filaments whose shape is very similar to the present-day organisms known as cyanobacteria or blue-green algae (these are prokaryote organisms whose cells do not have a nucleus nor other internal organules). The study of the sediments in which these were found, enables us to establish that they lived in a marine-environment with shallow, warm waters, perhaps a lagoon. The entire Archean Eon, the first and most ancient geological period, was dominated by bacteria : for a billion years, no other type of fossils has been found.
The following Eon, the Proterozoic Eon, stretched over a period of about 2 billion years. The study of fossils and rocks of this Eon shows the appearance of organisms, the stromatolites, colonies of cyanobacteria, capable of carrying out photosynthesis, that modified the composition of the Earth’s atmosphere, enriching it with oxygen and preparing the subsequent step in the evolution of life. Approximately 1.4 billion years ago, a remarkable step was made in life, with the appearance of eukaryote cells, which were characterized by a nucleus and internal organules, similar to the cells that form all the superior living organisms, including man – however, we must wait for 300 million years more to see multiple-cell organisms appear on the Earth. Life continued to evolve very slowly, characterized by simple organisms, with soft bodies, without shells, teeth, skeletons or carapaces – structures that appeared starting from the next Eon, the Phanerozoic Eon.
The great revolution took place at the beginning of the Paleozoic Era, that opens with the Cambrian period, 540 million years ago : it was here that what the paleontologists call the “Cambrian explosion” took place. Almost suddenly the evolution of life accelerated enormously with the appearance of over 100 phyla (a phylum is the larger systemic subdivision of the animal kingdom): to properly understand the real explosion of new forms of life, just think that today there are about 30. Most of the present living organisms descend from the organisms of the Cambrian Period . However many organisms became extinct without leaving any present descendents. Many were strange bizarre animals from our point of view, that do not have an equivalent in the present animal kingdom, and for this reason it is difficult to understand their way of moving, the environment and the living conditions, and in some cases even to identify the different parts of the body. The Burgess site, in the Rocky Mountains, in British Columbia, is famous for having disclosed the most vast and bizarre samples of the strange creatures of the Cambrian Period, so strange that some experts have hypothesized that these may the results of a period of evolutive “experiments”, in which only those that were most “successful” had descendents. The most characteristic and well known fossils of the Cambrian Period, of creatures that are extinct today, are the Trilobites (the present organisms that are most similar to these strange creatures are the horseshoe crabs). During the Paleozoic Era, which lasted approximately 300 million years, fish (approximately 440 million years ago), insects (approximately 380 million years ago), amphibians (400 million years ago) and reptiles (little over 350 million years ago) appeared, and towards the end also the precursors of mammals appeared. Towards the end of this era, during the Carboniferous Period, luxuriant forests covered vast areas of our planet and gave origin to the principal carbon deposits that are used today. The end of this Era, which was so full of life, however, was marked by the greatest mass extinction of all times, in which 80-90% of all the species disappeared, the causes of which are still unknown.