Scientists estimate that planet Earth formed about 4.5 billion years ago as part of the formation of the solar system. It was only about one billion years later that the first unicellular organisms made their debut, and it took about two billion years more for multicellular organisms to appear. Initially these were simple cellular aggregates that later on evolved into more complex life forms with distinct tissues and organs. The most dramatic segment of evolution took place about a half billion years ago when over a "brief” period of 30 million years the foundations were laid for all the groups of animals that still surround us today. The following video describes the basics of how life is believed to have emerged and evolved on Earth, from the formation of DNA to the appearance of human beings. It should be mentioned that the video contains several inaccuracies on which I will elaborate in the following paragraphs.




This video was produced by Cassiopeia Project

First, I would like to point out that the theories described in the video are the ones most widely-accepted by the scientific community and are supported by fossil evidence and genetic analyses.However, some aspects of these theories cannot be utterly proven, especially those concerning events that took place more than a half billion years ago. This is because the geological conditions, as well as, the non-mineralized nature of the organisms typical of this era did not allow for abundant fossil formation. The most controversial issue in this context is the formation of the first living cell – life emerging from stillness. No evidence exists for this momentous event, but lab reconstruction of the conditions that are believed to have prevailed on Earth at that time gave rise to a fluid that consisted of all building blocks necessary for the formation of DNA, RNA, amino acids and lipid membranes.

You may have noticed that the video does not progress linearly with time, but rather skims very quickly through the first few billion years, while later periods are discussed in much greater detail. The reason for this is that during the first 4 billion years the conditions on Earth were very harsh, not allowing for rapid diversification of life forms. With time these conditions became more and more suitable for life, ultimately giving rise to the Cambrian Explosion, during which most major phyla appeared (phylum = the organism classification rank below kingdom and above class, e.g. mollusks, arthropods and chordates). The appearance of human beings marks a small tick on the evolutionary timescale, comparable to one second out of a whole day.

The initial event in the emergence of life was the formation of a self-replicating molecule. At a later stage, the replicating molecule enveloped itself inside a protective membrane that made it possible to sustain the conditions necessary for its replication. In this manner the first living cell emerged.Today it is believed that this molecule was RNA, rather than DNA as suggested in the video. RNA possessed properties of both a replicating, hereditary material and of a catalytic molecule - an enzyme. In fact, there are still several enzymes today that integrate RNA elements in their structure and function, the most widely known being the ribosome (which is a complex comprised of several enzymes). It was only at later stages that DNA replaced RNA as the hereditary material due to its much higher stability. The theory that RNA predated DNA is known as the RNA world hypothesis.

The next milestone in evolution was the appearance eukaryotic cells that contain a nucleus and several additional membrane-enclosed organelles. These specialized subunits enable more efficient assembly and processing of complex proteins as well as compartmentalization of cellular processes. The mitochondria and chloroplasts (in plants) are unique for containing their own DNA and thus manufacturing their own proteins, although they also receive proteins that originate from nuclear DNA. The most widely-accepted theory for this phenomenon is that somewhere along evolution the cell has engulfed a photosynthetic alga (chloroplast) or a "breathing" bacterium (mitochondrion). Instead of being digested, this alga/bacterium managed to survive, and moreover, to develop a symbiotic interaction with the host cell. This is known as the Endosymbiotic theory. The mitochondria and chloroplasts gave their hosts a considerable advantage by providing them with the means to carry out either photosynthesis or cellular respiration, thus boosting their energy balance.

The next important step was the appearance of multicellular organisms. Initially these were very simple beings, but over the next billion years more and more complex life forms appeared and evolved into the forefathers of most species living today. In fact, evolution was a process that accelerated at a tremendous pace: it took a billion years for the first living cell to appear and another two billion years for multicellular organisms to show up, but over the next billion years a huge diversity of complex life forms emerged. The reason behind this accelerated process was the development of life-promoting conditions on Earth. The pre-life atmosphere contained high concentrations of toxic gas, which together with volcanic eruptions, lightning storms and the high acidity of the ocean made life impossible. These conditions, however, did favor the formation of the raw materials necessary for life to emerge. When the first cells appeared they began to change their microenvironment by secreting a variety of chemicals, including oxygen and carbon dioxide. This led over time to a change in the composition of the atmosphere and allowed for the formation of the ozone layer, which filters out lethal ultraviolet radiation. Once these conditions (high oxygen and lack of dangerous radiation) became more stable, a steep rise in the number of new species occurred.

All species formed according to the rules of evolution, the most important of which is survival of the fittest. As a result of the inevitable competition for natural resources, the organisms were divided into predators and prey, and a constant search for habitats commenced. Such a search, together with the forces of evolution, gave rise to the different classes of animals, such as fish, amphibians, reptiles, birds and mammals. The cards of evolution were re-shuffled several times over this time course as a result of major extinction events, which allowed new life forms to appear and prosper. The fifth such event was the extinction of the dinosaurs, and while it is certainly the most well known event it wasn't the biggest one. Some claim that the environmental changes provoked by mankind constitute the sixth major extinction event, overshadowing the extinction of the dinosaurs in its dimensions.

Erez Garty
Department of Biological Chemistry
Weizmann Institute of Science

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