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.
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.
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!
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.
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.
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.
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.
