The first forms of life were probably prokaryotes (cells lacking a nuclei).
Recent studies of fossils and ancient rocks has suggested that the earliest forms
may have been cells that reduce sulphate to sulphide. There is some debate about how
early this occurred.
In 3.47 billion year old rocks in Western Australia, researchers Yanan Shen of Harvard
University, Donald Canfield of Odense University in Denmark, and Roger Buick of the
University of Washington claim to have found indications of a microorganism that consumes
sulfate and produces sulfide as a waste product. (Both sulfates and sulfides are
compounds containing the element sulfur)
These findings were deducted from sulphur radioisotope levels rather than imprints of the
organisms themselves.
Sulfate-reducing bacteria had been known to exist at least 2.72 billion years ago, but
this finding pushes the date of their existence back an additional 750 million years.
This would mean that bacteria that reduce sulfate to sulfide are among the oldest known
life forms on the planet.
Even these primitive bacteria are probably too complex to be the very first forms.
The chemical evolution leading to cellular life on earth almost four billion years
ago probably passed through a stage where RNA alone performed all of the functions of the
modern macromolecules RNA, DNA and protein - the so-called 'RNA WORLD'.
Only RNA can perform all these functions.
RNA World
RNA is the most likely component of the simplest organism. It is everywhere, highly
adaptable, and stores information within the cell and within the organelles of the
chloroplast and mitochondria - thought to have arisen from primitive prokaryotic cells.
For a while, the only thing RNA did not seem capable of doing was catalyzing chemical
reactions. However view changed when in the late 1970s, Sydney Altman at Yale University
and Thomas Cech at the University of Colorado at Boulder independently discovered
RNA molecules that in fact could catalytically excise portions of themselves or of
other RNA molecules.
In 1986, Harvard chemist Walter Gilbert coined the term "RNA world" to designate a
hypothetical stage in the development of life in which "RNA molecules and cofactors
[were] a sufficient set of enzymes to carry out all the chemical reactions necessary
for the first cellular structures."
Today it is almost a matter of dogma that the evolution of life did include a phase
where RNA was the predominant biological macromolecule.
But was this the first stage? Some form of abiotic chemistry must have existed before
RNA came on the scene.
There must have been a set of unknown chemical reactions that generated the RNA world
and sustained it throughout its existence (as opposed to metabolism--the set of reactions,
catalyzed by protein enzymes, that support all living organisms today).
By definition, protometabolism (which could have developed with time) was in charge
until metabolism took over.
For more on this see The Beginnings of Life on Earth - by Christian de Duve Scientific
American Sept-Oct 1995.
However even this RNA WORLD was itself too complex to evolve directly from organic molecules
found on the prebiotic earth.
More likely, the RNA world emerged from, and was supported by a primitive sort of metabolism
perhaps fueled by the bonds in sulfur-containing compounds called thioesters.
Advanced forms of life existed on earth at least 3.55 billion years ago. In rocks of that
age, fossilized imprints have been found of bacteria that look like cyanobacteria,
photosynthetic organisms present in the world today.
Carbon deposits enriched in the lighter carbon-12 isotope over the heavier carbon-13
isotope--a sign of biological carbon assimilation--attest to an even older age.
On the other hand, it is believed that our young planet, still pounded by
volcanic eruptions and battered by falling comets and asteroids, remained inhospitable to
life for about half a billion years after its birth. This leaves a relatively short
time window of perhaps 200-300 million years for the appearance of life on earth.
How life formed is still highly conjectural, though there are many clues from the earth,
from outer space, from laboratory experiments, and, especially, from life itself.
The history of life on earth is written in the cells and molecules of existing organisms.
Thanks to the advances of cell biology, biochemistry and molecular biology, scientists
are becoming increasingly adept at reading the text.
A Short History of Life
At the end of the RNA world, the earliest type of cells resembling modern forms
were presumed to be prokaryotes, cells without a special centre or nucleus and which
are free-living independent organisms.
They can make proteins and reproduce without the need of a plant or animal host and
they represent the simplest known biological systems capable of independent life. Bacteria
are prokaryotes.
The first prokaryotes would probably have used the raw materials that surrounded them
perhaps feasting on the 'Primeval Soup'?
As their number grew, and as their source of food may have declined with the increase in
organisms and the rise in oxygen levels in the atmosphere, the prokaryotes adapted to
their new environment by developing photosynthesis and other means of using the sun's
energy to build structure (negative entropy) upon which to feed.
Simple blue-green algae still form an efficient part of the biosphere today. These bacteria
may therefore represent the first and most successful adaptation of life to its environment.
The transition to oxygen-rich atmosphere occurred about 2.0 billion years ago and eukaryotes,
cells that contain a nucleus, appeared between two and one billion years ago. There is a
potential problem here as most eukaryotes are aerobic and yet there is evidence of them
occurring about 2 billion years ago when oxygen level had just began to rise in the
atmosphere. The probable answer is that the earliest eukaryotes were probably anaerobic
types such as the modern Gardia. These types live without oxygen and without
mitochondria, which are presumably were originally independent prokaryotes that were acquired
by eukaryotes by endosymbiosis.
Eukaryotes keep their DNA structures within the nucleus and have between 10 to 1000 times
more than is found in prokaryotes
Evolution has led to the increasing specialization of cells in their function and structure,
and to the development of larger, more complex cells, such as organs and the large living
creatures that incorporate them.
Each human being consists of about ten billion cells, which appear in about a thousand
different varieties, the complexity of the whole being achieved through the process of
natural selection.
Although life has been in existence on Earth for at least 3.5 billion years, during most
of that time it consisted mainly of microorganisms. For 500 million years there were only
prokaryotes and for another 1000 million years there were only unicellular types
(prokaryotes and eukaryotes). This represents about half the time life has been on earth.
Quite recently in geological time, about 600 million years ago, there was a sudden
increase in the distribution, number and variety of organisms - due probably to the
build-up of significant amounts of oxygen to a threshold level, and the availability
of new habitable environments (wetlands and land surfaces).
This was the beginning of the Cambrian period and is noted for the appearance of organisms
with hard parts - shells, carapaces and skeletons - that were well preserved in
sedimentary rocks to form the fossils that we find today.
During the Cambrian era an enormous proliferation of life forms emerged and new
adaptations followed one another at a breathtaking speed.
In rapid succession,
the first fish and the first vertebrates appeared;
plants, previously restricted to the oceans, began the colonization of the land;
the first insect evolved, and its descendants became the pioneers in the
colonization of the land by animals;
winged insects arose together with the amphibians;
the first trees and reptiles appeared;
the dinosaurs evolved;
the mammals emerged, and then the first birds;
the first flowers appeared;
the dinosaurs became extinct;
the earliest cetaceans (ancestors to the dolphins and whales) arose and
in the same period the primates (the ancestors of monkeys, apes and humans).
Less than 10 million years ago, the first creatures to resemble human beings evolved,
accompanied by a spectacular increase in brain size.
And then just a few million years ago, the first true humans emerged. Humans have occupies
Planet Water for only 0.03% of the time life has been here. This is only 2.5 hours in the
span of one calendar year.
Modern humans have only been here for less than 10 minutes !!!