Origin of Life on Planet Water - Index

Spontaneous Generation - The Primeval Soup
Panspermia - Life Hitched a Ride to Earth from Space
Evolution Pathway and Links to the Climate and Atmosphere
Evolution of Planet Water
=>> The First Forms of Life
Evolution of Life as We Know It
How Plants Change the Atmosphere
Is There Life Elsewhere ?

The First Forms of Life on Planet Water

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 micro-organisms. 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, >
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