I cannot think of a more fascinating question than the riddle of how life began. One long-standing proposal, never supported by more than a handful of modern scientists, is pan spermia, the notion that preformed life was seeded on to Earth from elsewhere in the cosmos.
Fred Hoyle, the British astronomer, is an ardent advocate of a form of panspermia and, in an article co-written by Chandra Wickramasinghe (The Biochemist, December 1999), he summarised his hypothesis.
The traditional scientific explanation is that life spontaneously arose in the early oceans around 3.5 billion years ago. Conditions then favoured the formation of simple organic molecules that accumulated in the oceans. These chemicals reacted and interacted with each other for a long time to form a rich "prebiotic soup", eventually containing all the biomolecules necessary for life. Then the first living cell arose.
Several weaknesses in the above hypothesis are mercilessly attacked by Hoyle and Wickramasinghe. First of all, recent evidence indicates that life existed on Earth as long ago as 4.1 billion years. At that time the Earth was under vicious bombardment by asteroids and comets, producing massive craters.
Subsequent weathering has largely erased the evidence of this bombardment, but we can see the violent effects on the pock-marked surface of the moon, where no weathering has occurred. How could life have established itself on a planet under such intense bombardment?
Let's say that life didn't begin until after the bombardment had stopped 3.8 billion years ago. The first clear fossil evidence of life is 3.5 billion years old. This would allow a quiet development period of 300 million years in preparation for the first living cell. Hoyle claims that such a period is far too short to allow the slightest chance for life to begin.
Enzymes are essential catalysts in living cells. Enzymes are proteins, and proteins are long strings of units called amino acids. Amino acids come in 20 different varieties. Each chemical reaction in the cell is catalysed by a unique enzyme. The sequence of amino acids in an enzyme determines the nature of the enzyme.
Estimates have been made of the minimum number (256) of the simplest enzymes necessary to sustain life. Hoyle calculates that the probability of the various amino acids combining together by chance to produce these enzymes is one in 10 to the power of 6,900 i.e. 10 multiplied by itself 6,899 times.
This is a mind-numbingly large number. By comparison, the number of atoms in all the galaxies visible through the largest telescopes in the observable universe is only 10 to the power of 79.
Hoyle concludes that, considering these probabilities, life could only have arisen given the longest possible time-frame (from the birth of the first stars to the formation of our Earth, 10 to 15 billion years) and the volume of the entire universe in which to happen.
Hoyle proposes that life began as a simple bacterium long before the formation of our solar system. Once formed, life spread rapidly throughout the universe, and was present in the cosmic material that coalesced to form our solar system five billion years ago.
Life spread rapidly through the power of geometrical growth. A bacterial cell under favourable conditions will easily grow and divide into two daughter cells in two hours. These will become four in another two hours, and eight after six hours, and so on.
In about four days multiplication proceeds to two to the power of 40 cells, the size of a sugar cube. After 20 days the bacteria, growing optimally, reach the mass of a cluster of galaxies.
The spaces between stars contain elongated dust grains, each about 0.5 millionths of a metre long. They are mostly spread out thinly but in places are concentrated with greater density to form interstellar clouds. Hoyle postulates, based on their light-absorbing properties, that these grains are desiccated bacteria.
We now know that bacteria can live in extremely inhospitable conditions, e.g. several kilometres deep in the Earth's crust and in deep-ocean hot vents where the temperature exceeds the boiling point of surface water.
Certain types of bacteria live in the intensely radioactive environment of nuclear power plant cooling systems. It might therefore be possible for bacteria to arise and to survive in the harsh conditions of outer space.
Hoyle and Wickramasinghe suggest a vital role for comets. Comets have been described as "frozen dirty snowballs", but there is evidence that the cores of large forming comets are warm and liquid. Hoyle proposes that cosmic bacteria replicated into large numbers in these cometary cores.
Comets are probably associated with all solar systems in the cosmos. Far beyond the orbit of Pluto, at the edge of our solar system, is a huge storage cloud of comets called the Oort Cloud. The early Earth was intensely bombarded by comets, and this is how Hoyle proposes that life arrived here. The core contents of a large comet would be released relatively gently on collision with Earth.
Fred Hoyle is a brilliant scientist, and his proposals deserve serious consideration. But one thing puzzles me. As I understand it, interstellar dust commonly falls on to planets. Therefore the dust should be present in the soils of planets in our solar system.
Let us say that, for various reasons (e.g. biological activity), this dust would be very difficult to spot in Earth soils. However, soil on the moon and on Mars is relatively inert. But no such organisms have been seen in soil recovered from either body.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC