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Evolution and the brain
Our Evolutionary Inheritance

HelpLink : The cocktail of lifeLink : L'apparition de la vieLink : Exobiologie, de l'origine de la vie à la vie extraterrestre
Link : Once upon a time 3,8 billion years agoLink : Images de génétique, biotech, embryologie Link : Où se cache la vie dans l’Univers ?
Research : Cyrille Barrette
Original modules
Tool Module: What Is Evolution?What Is Evolution?

The Infinitely Large, Infinitely Small, and Infinitely Complex

The following list tells you approximately how far back in time certain major events in the history of the evolution of the universe and of living things occurred.

- 14 billion years ago: the Big Bang.

- 4.6 billion years ago: Earth formed.

- 4.45 billion years ago: Moon formed when Earth collides with a mini-planet.

- 3.9 billion years ago: first rock formations on Earth appear.

- 3.5 billion years ago: first living things on Earth appear.

- 3 billion years ago:first blue-green algae and eukaryotes appear.

- 2 billion years ago: blue-green algae dominate the Earth as atmospheric oxygen level increases to 1%; first eukaryotic cell appears.

- 1.5 billion years ago: eukaryotic cells with mitochondria appear.

- 1 billion years ago: sexual differentiation occurs.

- 650 million years ago: first multi-celled life forms appear.

- 400 million years ago: first terrestrial plants appear.

- 300 million years ago: carbon dioxide fixed by plants; atmospheric oxygen increases to 21%.

- 200 million years ago: major orders of the animal kingdom (reptiles, mammals and birds) appear.

- 40 million years ago: first primates appear.

Lien : The family tree of lifeLien : L'origine de la vieLien : Tool Module: The Connection between Ontogeny and Phylogeny


Scientists can say fairly precisely when life on Earth began (see sidebar). They also know that the very first sputterings of life did not come out of nowhere, but were rather part of a continuum of cosmic evolution that links us to the stars, which gave birth to the atoms now present in the molecules that our bodies are made of.

But how do we define a living being? In other words, what properties must a system have for us to characterize it as living? Most typically, a living organism exchanges matter and energy with its environment while retaining its independence. As French scientist and philosopher Henri Laborit put it so nicely, “a being's only reason for being is to be.”

The structure that enables organisms to be independent is the cell. Its stability is ensured by negative feedback mechanisms that give it this relative independence with respect to its environment.

  Tool Module: Cybernetics

The problem is that something as inanimate as a factory built by human hands might be regarded as satisfying these criteria for a living cell. Hence another, essential requirement must be added: to be regarded as living, a system must reproduce and evolve by natural selection. In the language of evolutionary biologists, living things are entities that replicate and that are subjected to selective pressures in their environment.

Life thus also necessarily involves another ingredient: chance, because random chance is the ultimate source of the variations on which natural selection operates.

Life also implies the capacity to take information acquired over generations and store it in memory... in other words, to retain the useful outcomes of chance. That is what DNA does. DNA is a long molecule found in all cell nuclei in the human body. The sequences of nucleotides in DNA contain the information used to manufacture proteins, which are basic building blocks of our body's cells.

The steps involved in manufacturing a protein are fairly well known. First, DNA is transcribed into messenger RNA (mRNA). Next, this mRNA is translated into proteins by ribosomes and transfer RNA (tRNA) molecules outside the cell nucleus, in the cytoplasm (see diagram).

Now, certain proteins are essential for the replication of DNA. So the question arises, if DNA cannot replicate without these proteins, what DNA was use to make these proteins themselves? It's the molecular version of the question of the chicken and the egg, this time applied to the origin of life itself!


Linked Module : DNA HomeLinked Module : The DNA molecule is shaped like a twisted ladderLinked Module : DNA from the beginning

To resolve this paradox, we must consider another molecule very closely related to DNA: RNA.

A definition of living things as entities that replicate and that undergo selective pressure in their environment matches the minimum definition of life proposed by many biologists, such as Richard Dawkins.

Note in passing that this definition does not assume that organic matter is the only possible substrate for the development of life forms. Given the exponential rate at which computer systems are currently developing, it would be very ill-advised to state too categorically that a process of reproduction and differential selection will never occur on a silicon base.

Dawkins even asserts that a new kind of non-carbon-based life form already exists, in the form of what he calls "memes". A meme is the mental equivalent of a gene: any idea, concept, image, skill, etc. that can be transmitted from one human brain to another, and of which some variants spread more readily than others.

Tool Module: Darwin's Natural Selection


Lien : L'ADNLien : L'ARNLien : La transcription : de l'ADN à l'ARNLien : PROTÉINE
Lien : EnzymesLien : PROTEIN SYNTHESISLien : Synthèse des protéinesLien :  ADN
Lien : Les Acides RiboNucléiquesLien : Biology Hypertextbook ChaptersLien: Présentation de la biologie moléculaire
Histoire : DES ENZYMES PAR MILLIERSHistoire: La génération spontanée : Histoire d'une hypothèse persistante

In the 1920s, two researchers, Oparine and Haldane, independently hypothesized that the first organic compounds were formed in a primordial soup where chemical reactions occurred while there was still no oxygen in the atmosphere.

In 1953, Stanley Miller and Harold Urey confirmed this hypothesis with a famous laboratory experiment showing that the basic compounds of organic life (amino acids, sugars, and nitrogen bases) form spontaneously under conditions similar to those on primitive Earth.

Though scientists now believe that the conditions recreated by Miller and Urey in their experiment did not precisely match our more recent understanding of conditions on Earth nearly 4 billion years ago, this experiment remains an important milestone in the history of our quest for our origins.

Expérience : L'origine des briques élémentairesExpérience : La fin d'un mythe : l'expérience de MillerChercheur : L’origine de la vie  Stanley L. Miller 1930 -

The cell is the basic unit of life. The lipid membrane and sugars are essential components of the cell. In addition, proteins and nucleic acids (DNA and RNA) play fundamental roles in the life of the cell.

Proteins play many roles within the cell. Structural proteins constitute the cell's framework, while defensive proteins (antibodies) neutralize microbes that try to invade it and transport proteins carry oxygen to it, and so on. Lastly, enzymes are an especially important class of proteins that promote certain chemical reactions within the cell.

Lien : DNA HomeLien : The DNA molecule is shaped like a twisted ladder.Lien : DNA from the beginning

All proteins, whether they are structural proteins or enzymes, are manufactured according to plans imposed by DNA and carried out of the cell nucleus by messenger RNA.

These two nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are long chains composed of four types of molecules called nucleotides. Every nucleotide consists of a base, a sugar, and one or more phosphate groups.

In DNA, the bases are adenine (A), guanine (G), cytosine (C) and thymine (T). In RNA, uracile (U) replaces the thymine. Also, the sugar found in DNA is a deoxyribose, while the sugar in RNA is a ribose.

For a long time, RNA received little attention compared with DNA, of which our genes are made. But since the early 1980s, RNA has been the focus of much research, following the discover of its enzymatic properties, which place the origins of life in a new light.

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