Life on Earth began with simple chemicals formed when it was quite young, but what started the chain of events that led to the immense variety of living beings and to the almost unbelievable complexity of their individual structure?
The problem of the origin of life on our planet begins with the question of how proteins and nucleic acids, the two chemicals basic to all living organisms, could have originated naturally on the surface of the Earth. Without much difficulty a good organic chemist today can synthesize all the substances vital to life found in a living cell. But how could these have arisen spontaneously, in a process known as abiogenesis (the production of life from non-living matter)?
A brilliant idea about how this could have happened was expressed by the distinguished American scientist Harold Urey in the 1950's. Urey's idea is based on the nebular theory of the origin of the Solar System, according to which protoplanets originally possessed extensive atmospheres of hydrogen and hydrogen compounds, such as methane, ammonia and water vapor. Chemical elements in these compounds (hydrogen, carbon, nitrogen and oxygen) are exactly those that form amino acids-the basic "building blocks" of long protein molecules. Urey theorized that, when subjected to ultraviolet radiation from the Sun and electric discharges from thunderstorms in the atmosphere, the molecules of these simple compounds could unite to form more complex amino-acid molecules.
To confirm his ideas, Urey asked one of his students, Stanley L. Miller, to carry out an experiment by putting a mixture of hydrogen, methane, ammonia and water vapor into a test tube and subjecting it for several days to an electric discharge. When the contents of the test tube were analyzed, they revealed the presence of several amino acids normally found in proteins, thus constituting a brilliant confirmation of Urey's hypothesis. Presumably, during the early existence of our planet, when its atmosphere still consisted of hydrogen and hydrogen compounds, amino acids were continuously produced in that atmosphere. These substances were slowly precipitated to the surface, forming concentrated solutions on the fringes of the ocean waters. This process, then, provided one chemical component essential for life.
Much less is known about the origin of nucleic acid, the other component fundamental to life as we know it. The molecular chains of nucleic acid contain atoms of phosphorus, which are not likely to be found in the atmosphere. Also, the synthesis of nucleic acids requires high temperatures rather than ultraviolet radiation or electric discharges. One bold hypothesis suggests that nucleic acids were produced as the result of the activity of rain-washed volcanoes, but experimental evidence for this hypothesis is not yet conclusive.
The next problem, of course, is how such solutions of proteins and nucleic acids in the ocean waters could have evolved to form the first living organisms capable of reproduction. There is hardly any doubt that a Darwinian "struggle for existence" principle operated very early in the evolution of life on the Earth. In fact, we can trace Darwin's evolutionary principle back past the vague borderlines of life to simple inorganic reactions. If a mixture of powdered iron and silver is exposed to oxygen, more iron oxides than silver oxides will be produced because the oxidation of iron proceeds at a faster rate than that of silver. Similar, if more complicated, evolutionary chemical processes must have been occurring among protein molecules dissolved in the waters of primordial oceans. Those molecules whose reactions were intrinsically faster had an obvious edge on the slower ones.
This early development of organic matter remains hidden from us behind a heavy curtain of mystery because early biochemical reactions could not leave fossil traces in the rocks of those periods. Neither do we have a record of when and how organic molecules acquired the ability to produce other molecules with the same chemical properties. All we can surmise is that such developments must have occurred during the geological eras that preceded the formation of our oldest known fossil-a primitive bacterium some 3.2 billion years old found near Barberton, South Africa.
There is not much chance, either, of finding extensive evidence of the organisms that preceded the first fossils, since earlier forms of life must have been miniature, soft-bodied organisms not much different from organic molecules. In fact, if through some miraculous device we were able to go back in time to a point about three billion years ago, the primordial pools of water and rocky slopes of the land masses would appear to be lifeless. Only through minute examination would we discover that life was already present on the surface of the planet, and that numerous micro-organisms of many different kinds were even then hard at work in their fight for existence.
At this early stage in the evolution of our planet its surface was still warm, and a large part of the water that now fills the ocean basins existed in the atmosphere, forming a thick layer of heavy clouds. No direct sunlight could penetrate this heavy atmosphere to reach the surface of the Earth, and any life able to arise in such damp darkness must necessarily have been limited to micro-organisms that could survive entirely without sunlight.
Some of these primitive organisms must have fed on organic substances dissolved in the water around them. But others grew accustomed to purely inorganic food. This second class of "mineral-eating" organisms can still be found in the "sulfur and iron bacteria," which obtain their vital energy through the oxidation of inorganic compounds of sulfur and iron. The activity of such bacteria has played quite an important role in the development of the Earth's surface. For instance, the iron bacteria may possibly be responsible for many thick deposits of bog iron ore, the main commercial source of iron in the world.
As time went on, the surface of the Earth grew cooler and cooler. More and more water accumulated on the surface, while the heavy clouds blocking the Sun gradually thinned out. Under the action of the Sun's rays, now readily piercing the atmosphere to reach our planet, some primitive micro-organisms were slowly developing the substance chlorophyll. This let them use the energy of solar radiation to convert carbon dioxide in the air into simple compounds necessary for their growth. Thus the possibility of "feeding on the air" opened up new horizons for the development of organic life-eventually culminating in the present highly developed and complex forms of the modern plant kingdom.
But some of the primitive organisms chose another way of development. Instead of synthesizing their food from carbon dioxide in the air, they obtained carbon compounds in "ready-to-use" form by feeding parasitically on plants. Some of these parasitic organisms soon developed the ability to move-a great advantage in the competition for food.
Not being satisfied with a purely vegetable diet, some members of the parasitic branch of living things then began to eat one another, and the need to catch prey-or to flee when pursued- led to even better means of moving around, culminating, after hundreds of millions of years, in the advanced locomotive adaptations of the animal world.
Rapid motion through the water could not be achieved by soft-bodied, easily deformed animals. Such swiftness requires rigid streamlined shapes, and muscles working in tandem with rigid "moving parts." Rigid body parts also afford protection from attack, while at the same time providing better weaponry for attacking others.
In the Earth's primeval oceans-where life was a struggle in which the fittest survived-these advantages resulted in the evolution of the soft, jelly like forms of the animal world into heavily armored creatures with powerful claws.
The development of rigid parts proved of great value to animals, and incidentally to modern paleontologists too. Whereas information about the soft-bodied organisms of the past can be gleaned only from their occasional imprints in soft sand-imprints through mere chance preserved to the present time-animals that possessed rigid shells or skeletons can be studied from their fossils almost as well as if they were living today.
The historical record of life on the Earth begins, properly speaking, at the time when animals began to develop rigid parts and bodies. While modern museums are full of shells and skeletons that permit us to visualize the life forms of the more recent past, the very first living organisms are a secret lost in time.
Viruses – Where Did They Come From?
Viruses are tiny entities that reproduce inside the cells of other organisms.
They are made up of DNA or RNA, which hold their genes, and a protein coating. Some viruses are surrounded by an envelope of fat when they are not inside a cell.
All types of living organisms on Earth can be infected by viruses.
Viruses don’t leave fossils, so nobody knows exactly when or how viruses first appeared on Earth.
The three most common hypotheses regarding how viruses came to exists are:
1. Viruses were once small cells that became parasites of large cells and lost genetic information that they no longer needed.
2. Pieces of DNA and RNA escaped from other cells and became viruses.
3. When the first self-replicating molecules arose from nucleic acids and proteins, some became cells and others became viruses. Cells and viruses evolved together.