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Evolution and the brain

Help Les Origines de la vie, Nouveaux Concepts, Nouvelles Questions Le monde à ARN Origine de la vie
Origin of life on Earth Early earth D'où vient la vie? Link :  What is Life?:How chemistry becomes biology

To explain how life originated in the primordial soup that prevailed on Earth shortly after it formed, many alternative hypotheses have been proposed.

One is that the first molecules were brought to Earth by meteorites or comets.

Another is that they may have appeared on inorganic surfaces in the form of molecular proto-organisms that were immediately capable of manufacturing their own components.

Still another is that the first forms of life developed from viruses. Viruses are very simple, essential, cell parasites. They have atypical molecular mechanisms that might have originated very long ago and been tested during the early stages of evolution but not retained in the prokaryotes or eukaryotes we know today.


Proteins are synthesized using information contained in DNA. But DNA itself must be synthesized when a cell reproduces, and this synthesis requires proteins, in particular enzymatic proteins such as DNA polymerase.

For scientists who study the origins of life, this poses a serious problem: DNA cannot have come first, because proteins are needed to make it. But proteins cannot have come first either, because information from DNA is needed to make them. To resolve this paradox, scientists had to determine how DNA might have been synthesized without enzymatic proteins, from the basic building blocks of life.

In the early 1980s, Thomas Cech and Sydney Altman discovered that certain forms of RNA could act as catalysts, just like proteins. These special forms of RNA are now called ribozymes.

Ribozyme RNA molecules can play the roles of both DNA and proteins. They can thus resolve the chicken-and-egg-like paradox that scientists faced. On the basis of certain molecular markers, several researchers have concluded that RNA probably appeared on Earth before DNA. The problem of the origins of life has thus become in part the problem of the origins of RNA, which is still far from being solved.

In fact, the very notion of the first molecule may not have any real meaning. Some mechanisms of protein synthesis simpler than those we now know may have existed in the past and allowed the appearance of primitive but effective forms of molecules with catalytic or autocatalytic properties.

Structure of the 5S ribosomal RNA subunit
of Haloarcula marismortui.
Source: 5S Ribosomal RNA Database

Likewise, any attempt at a precise definition of life itself may well be unrealistic. Life in fact might be regarded as appearing gradually in systems of increasing complexity. Thus, a pebble is inert, but crystals grow, and clays may serve as matrices that facilitate biochemical reactions. Proteins do the work of cells, and viruses can reproduce. Protozoans are autonomous, and the cells of metazoans are specialized and organized into tissues.

When we look only at the extremes on this continuum, or set excessively arbitrary criteria for what constitutes life, we often find ourselves confused and frustrated by contradictions of language. Perhaps the best understanding of life that we can hope to achieve may be limited to observing how matter behaves and how this behaviour gradually reaches higher and higher levels of complexity.

In the late 1970s, a microbiologist named Carl Woese discovered a third group of organisms living on Earth: archaebacteria. Though archaebacteria are prokaryotes, they are no closer to conventional bacteria than to eukaryotes on the universal genealogical tree. The archaebacteria group comprises a very large number of organisms that live under extreme temperature conditions. Some archaebacteria are found in hot springs whose temperatures are close to the boiling point of water. Many researchers have drawn a connection between this observation and the fact that temperatures were probably far higher on primitive Earth than they are on Earth today.

For Woese, the discovery of archaebacteria meant that Darwin's cherished doctrine of a common ancestor might need to be questioned. According to Woese, there are at least three forms of primitive cells from which all current life forms originated: primitive prokaryotes, primitive eukaryotes and primitive archaebacteria, all of which could trade genes with one another.

Link : Origins: What was life? Experience : Acides aminés extraterrestres Link : New cellular evolution theory rejects single cell beginning Link : New Theory of Cell Evolution Rejects Single-Ancestor Doctrine Tool Module : What Is Evolution?


Ribozymes Ribozymes Come Ready for Action Ribosomes are ribozymes L'ARN dans l'arène
La découverte des « interrupteurs de commande » de gènes a été annoncé comme étant la plus grande avancée scientifique de l'année 2002 Link : First life: The search for the first replicator
Thomas R. Cech, Ph.D.
The Protein that Wasn't There: The Discovery of Ribozymes


Living matter is composed of a few large families of complex molecules that are themselves made up of simpler molecules. Proteins, the most complex molecules, are thus manufactured from smaller molecules, the amino acids. The sequences of amino acids in proteins are strictly determined by the building plans contained in molecules of RNA (ribonucleic acids). These RNA molecules themselves are only temporary copies of the plans for genes, which are contained in the cell nuclei in the form of DNA (deoxyribonucleic acid).

Structure of a catalytic RNA molecule (ribozyme)
Source : Thomas Cech Laboratory
  Until the early 1980s, all of the known enzymes were proteins. Then Thomas Cech and Sydney Altman made a discovery that propelled the RNA molecule to the centre of all the debates about the origins of life (and won Cech and Altman the 1989 Nobel Prize for Chemistry). Messenger RNA molecules often contain a sequence called an intron that is not used in translating the RNA into a protein and is eliminated shortly after the RNA is synthesized. What Cech and Altman were surprised to find was that no enzyme was required to eliminate the intron, because the intron itself served as the catalyst.

This type of RNA, called a ribozyme, can thus act as an enzyme by catalyzing changes in covalent chemical bonds in the structure of a substrate (which itself is often RNA). It even seems possible that it is this kind of RNA (the kind contained in a cell's ribosomes) that manufactures the proteins for our cells by catalyzing the chemical reaction that assembles their basic building blocks, amino acids.

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