Where did life come from, and when did it start? Current evolutionary evidence is used to explain how the great variety of present-day animals and plants came into existence; it supposes that life on Earth began with relatively simple forms which, over hundreds of millions of years, underwent a series of small but significant changes which combined to produce a succession of living organisms that have become more varied and more complex.
Working backwards, the study of evolution should lead us to the very first life-form, from which all others are descended. However, there are great gaps in the available evidence, and the best that researchers can do is make intelligent guesses. Various religions offer their own account of the origin of life. For example, the Christian Bible describes how God created the land, animals and man in six days. In 1650 Archbishop James Usher studied the book of Genesis and estimated the date of Creation as 4004 BC. More recent calculations have narrowed it down to around 9.00 am on 23 October of that year. Other cultures and religions have their stories of the creation of life. All have two features in common. First, life originated before mankind had any knowledge of the physical, chemical and biological principles that form the basis of life as we now understand it; and second, life is masterminded by a divine external force and falls outside the scope of science. The ‘Cosmic’ or ‘Space’ theory of the origin of life explains its presence on Earth by assuming it was brought here from elsewhere in the Universe, for example in a comet or meteorite. Even if life-forms could withstand the rigours of interplanetary space travel this does not answer the basic question of how life evolved in the first place – it simply removes the problem to another place.
Until about 130 years ago it was commonly believed that life could arise spontaneously from non-living matter – the theory of ‘spontaneous generation’. However, as more was learned about the nature of life, scientists began to question this theory. Maggots in meat, for example, previously thought of as being generated spontaneously, were shown to hatch from flies’ eggs laid on the meat. But although it was gradually proved that larger organisms always evolved from eggs or other, there was still uncertainty about the origin of micro-organisms, which seemes to be generated from non-living material. In the 1850s the French biochemist and microbiologist Louis Pasteur (1822-1895) finally provided evidence against this theory. He showed that boiling broth destroyed all the living things in it, and if the broth was kept free from the micro-organisms floating about in ‘clean’ air it would remain uninfected.
Although the spontaneous generation of life is not known to occur today, it has been proposed that there was a time early in the Earth’s history when conditions were favourable for such a series of events to occur. The ocean of the primeval Earth contained a rich supply of organic molecules and the atmosphere, although devoid of oxygen, was rich in methane and ammonia. Acting on these simple molecules were the combined effects of radioactivity, ultraviolet radiation from the Sun, heat from volcanoes and the electrical discharges in lightning. In such conditions the synthesis of the building blocks of life – amino acids for proteins, and nitrogenous bases for nucleic acids -may have occurred.
The American biochemist Stanley Miller (1930-) in the 1950s recreated Earth’s primeval conditions and succeeded in synthesizing some simple amino acids and nitrogenous bases. Over the vast span of time such molecules may have joined to each other in temporary complexes, and one such complex may have developed a sort of membrane so that its internal composition became stabilized. Such a complex, floating in the ‘soup’ of the primitive sea, was possibly the first cell and the first living thing on Earth.