THE idea that life sprang on a continual basis from non living matter (spontaneous generation) was widely believed for over 2,000 years from the time of Aristotle, until it was finally disproved by Louis Pasteur. The idea of spontaneous generation was smoothly succeeded by the theory of panspermia, i.e., that pre formed life was seeded on earth from elsewhere in the universe.
This theory was again superseded in mainstream science by the theory of chemical evolution. This is now the generally respected scientific hypothesis to explain the mechanism whereby life spontaneously arose on Earth about 3 1/2 billion years ago. Nevertheless, the theory of panspermia persists as a respectable minority scientific opinion.
In the 19th century, when Pasteur conclusively showed that, under present conditions, life can only arise from pre existing life, there was little point in pondering how life might have first arisen on Earth. At the time, far too little was known about the histories of life, biology, and the Earth. Nevertheless, Pasteur's dictum had such a powerful resonance that some scientists proposed the idea that life had no origin, but always existed just like matter and energy.
According to this idea, the origin of life on a planet, previously lifeless, could only be effected by seeding preformed life on to the planet from elsewhere.
Several well known 19th century physicists approved of the idea of seeding of life, e.g., William Thompson, who later became Lord Kelvin (1824-1907). The idea was elaborated in 1908 by Svante Arrhenius (1859-1927), a Swedish chemist. He christened the theory panspermia (from the Greek, meaning "seeds all").
Arrhenius proposed a mechanism whereby life could hop from planet to planet. He proposed that biological organisms such as bacterial spores and viruses are ejected from planets by electrical forces and propelled through space by the weak pressure of starlight.
Such organisms become attached to dust particles in space, increasing their mass, and allowing them to fall under gravity towards the nearest star, perhaps to be captured by a planet of that star. A direct consequence of this theory is that all living things in the universe are biochemically related to each other.
One aspect of the theory can be ruled out straight away. Life is not as fundamental as matter and energy, and could not have existed from the birth of the universe. Cosmologists tell us that, in the beginning, only hydrogen existed.
Hydrogen is still the most abundant element in the cosmos, making up over 60 per cent of the mass of the visible universe. The various other elements that exist have been produced in stars, and are being produced, by nuclear fusion processes. Since living matter is composed of chemical combinations of many different elements, life cannot be as old as the cosmos.
There is also reason to doubt that spores of life can successfully journey long distances through space. Many scientists believe that any biological organism would be killed in space by lethal radiation - ultraviolet, X rays and cosmic radiation. These radiations would have a long time to interact with the bacterial spores since interstellar distances, and therefore travel times, are so immense.
On the other hand, experiments have been carried out in which bacterial spores were exposed to intense ultraviolet rays, extreme cold and vacuum conditions in order to mimic the environment of interstellar space. Some of these studies claim that, if the spores were carried within the molecular clouds which roam around space, a fraction of the spores could remain viable for many millions of years, long enough to travel from one solar system to another.
But, on the other hand there is evidence that biological spores neither leave our earth nor enter into our vicinity from elsewhere. One would expect the moon to intercept many particles leaving the Earth, and also biological particles arriving from elsewhere.
However, moon samples brought to Earth by the Apollo missions contained no organisms. Also, tests carried out by the Viking mission to Mars found no detectable biological activity in Martian soil.
Many people will have heard of the proposition put forward by the astrophysicist Fred Hoyle. He claims that over 80 per cent of all interstellar dust grains are bacterial and algal cells. This conclusion is based on the light absorbing properties of the dust grains. It is estimated that the mass of these grains in our Milky Way galaxy is equivalent to five million suns. In other words, space is teeming with life! But why is this life not falling on to the moon?
Another recent version of the panspermia theory is called directed panspermia.
This theory holds that life was not seeded on Earth by accident, but was sent in interstellar spacecraft by other intelligent beings. The best known directed panspermia hypothesis has been proposed by Francis Crick and Leslie Orgel. Crick and Orgel believe that enough time existed between the origin of the universe and four billion years ago for an advanced technological race to have developed elsewhere, capable of this directed panspermia. Crick and Orgel believe that life originally began elsewhere in the universe in a spontaneous process similar to the conventional scientific explanation for the origin of life on Earth.
Hoyle, Crick and Orgel are all brilliant scientists. Their ideas on panspermia are based on serious considerations and deserve our serious attention.
ONE consideration that gives pause for doubt about the conventional scientific explanation for the origin and development of life on Earth is the absolute uniformity of the basic biochemical life plan in all living organisms. Almost every conceivable environmental niche on Earth is inhabited by its own form of life - fissures in the deep ocean floor, dry rocks in the deep freeze of Antarctica, scalding hot springs, etc.
The overall shapes and forms assumed by life are dramatically different - bacteria, jelly-fish, whale, oak tree, mouse, elephant, etc., etc. But the basic biochemical plan of the cells of every living organism is pretty much identical.
The conventional scientific explanation for this is that life arose in a unique event and in a particular biochemical configuration. All species of life that now inhabit the Earth are descended from that original ancestor.
However, is it not somewhat surprising, in view of the spectacular variations shown by life at other levels, that there is essentially no variation in the basic biochemical plan of life? One possible explanation for this is that life never had any choice in its basic biochemical plan because the basic seed stock was planted here from elsewhere in the universe.