The Rise of Life
on Earth
Nusrat
Khandaker
It
is estimated that the earth was created about 4.5 billion years ago.
How long life has inhabited it, no one knows, although it is assumed
that the simplest form of life came into being more than 3.8 billion
years ago. For a long time researches and studies have been carried
out to determine what the first forms of life were, their nature and
behaviour and during which time period they came into existence. Scientists
came up with many different theories regarding the first forms of
life but still today they are not sure of the very first species of
life on Earth.
Some scientists
think that the first form of life originated during the primordial
era. The question is, what kind of place was the primordial Earth
actually like? Was it fiery or was it moderate? Could it even have
been chilly? One school of thought suggests that the Earth was covered
with ice when life started to emerge. Due to such temperature and
atmospheric conditions, compounds like formaldehyde, cyanide and ammonia
were formed and these reacted with water to form glycine
(a type of amino acid). These were one of the basic molecules in a
living cell. Others accept the theory that the Earth was a temperate
region when life originated. Molecules of aldehyde phosphate,
which were found in ponds and lakes and other water bodies, united
to give rise to the progenitors of RNA (another very important constituent
of a living cell). According to some scientists, since the Earth was
a very hot place at the time of its birth, it could have been a blazing
orb of hot magma when life was born and the first life forms must
have been heat-loving organisms called "thermophiles".
These were able to adjust themselves to the scalding atmospheric conditions.
Gases released from this magma could react to form such important
compounds as acetic acid and others, which are vital components required
in the synthesis of specific organic compounds.
The "thermophiles"
were said to have existed during a time period known as the Precambrian.
This period started from the time of the creation of the Earth and
lasted for about three and a half billion years. The organisms said
to be existing in this time period were all minuscule and microscopic.
These were the ones, which facilitated and aided the birth and development
of the later organisms. They developed the DNA, the proteins and all
those basic molecules essential for making the first living cells.
They were even said to have devised and invented the methods of photosynthesis,
respiration and reproduction. A certain rock, found in an island near
Greenland, bears some evidence of life about 3.86 billion years ago.
This rock, though having lost nearly all of its anatomical features,
had had biological features similar to those of all the forms of life
that have risen ever since.
A certain scientist
named Stanley Miller argues that the most credible and accepted theory
about the evolution of life on Earth was that, the first type of living
beings was born when the climate of the Earth was very hot. He assumed
that the first forms of life were actually the end products of some
reactants in the atmosphere during the primordial time, which had
undergone a series of reactions at that time under those atmospheric
conditions. He supposed that if he could create an atmosphere similar
to that of the primordial age, and carry out some similar reactions,
he would be able to show if these reactions really did cause any forms
of life or any basic component to evolve. He demonstrated an experiment
to prove this hypothesis. A glass flask was filled with methane, hydrogen
and ammonia, made to resemble the primordial atmosphere. Another flask
positioned below the first one was filled with water representing
the ocean. The electric current acted as the lightning sparks. By
passing the electricity through the gases in his so-called "atmosphere"
Miller wanted to generate chemicals which would fall into to the pool
of water just like rain. This resulted in a thick yellow mixture which,
when tested, turned out to be a rich brew of amino acids.
Miller's conception
of the primordial atmosphere was contradicted by other scientists
who claimed that the atmosphere rather consisted of carbon dioxide
and nitrogen, instead of hydrogen, methane and ammonia. In an attempt
to react carbon dioxide and nitrogen however, they obtained a trivial
amount of organic material which, when mixed with water, formed a
dilute solution. Scientists found it hard to conceive how such complex
an aspect as life could have evolved from such a feeble, dilute solution.
So they came to the conclusion that some other objects had aided the
reaction and thus made the conditions feasible for life to start developing.
They guessed that asteroids, meteorites, comets and even interplanetary
dust particles, brought with them the ingredients required to produce
amino acids, which along with some vital chemicals, reacted to form
living cells.
It is suggested
that three branches of organisms, the bacteria, archaea and eukaryotes,
arose from the main branch of thermophiles. The oldest manifestation
of the eukaryotes was about 2 billion years ago. These eukaryotes
had a nucleus a complete cell structure. Then, approximately 1 billion
years after that, mixing of genes among the eukaryotes led to eruption
of life among them. Before long sponges, worms, multi-celled organisms
and aquatic invertebrates like clams and trilobites began to emerge.
This was during the late Precambrian and the Cambrian era. Around
0.2 billion years ago, dinosaurs came into existence but they became
extinct within 65 million years of their reign. Human beings have
been living on this Earth for about 4 million years now. It is amazing
how life began on this Earth through microscopic unicellular creatures.
Still today scientists are trying to find the exact source from which
such a marvelous phenomenon as life has emerged.
