Exogenesis

“And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.”

Nowadays, there are very few mainstream scientists who regard Genesis as an accurate description of the origins of life on Earth. Not only is there a huge wealth of evidence that the earth is considerably more than 10,000 years old, but Darwin’s Theory of Evolution provides a beautifully simple model, describing how random mutations and natural selection have produced the vast diversity of life that inhabits our planet. However, evolution does not explain how life begun.

The first thing to note is that there is still some debate over the precise meaning of ‘life’. There is no unanimous agreement over whether or not viruses can be described as ‘living’, for example. However, viruses certainly require other living organisms to reproduce.

One theory is that life spontaneously generated from non-living matter (abiogenesis). Aristotle claimed that, for example, aphids spontaneously arise from dew. However, Francesco Redi showed in seventeenth century that maggots do not appear in meat when flies are prevented from laying eggs. Two centuries later, Louis Pasteur showed that bacteria and fungi do not spontaneously generate in nutrient-rich, sterile environments. This is evidence that life does not spontaneously generate from non-living matter. However, although now life generates from other life, there must have been a point in time when life did indeed spontaneously generate from non-living matter, and it probably had to be happening quite regularly. Darwin speculated that a life spontaneously generated in a “warm little pond, with all sorts of ammonia and phosphoric salts, lights, heat, electricity, etc. present, so that a protein compound was chemically formed ready to undergo still more complex changes“. The reason why we do not observe this happening now is because the necessary ingredients would never be able to exist in the necessary conditions, because they would be instantly devoured or absorbed by nearby organisms. Think of planting a small flower. This is easy to do in a small plot of soil, and the flower will thrive. But plant it in a forest, and it is likely that the flower will die, because its light is blocked by the trees above it. Essentially, existing life stops new life from spontaneously generating.

Furthermore, Alexander Oparin suggested in the early twentieth century that atmospheric oxygen prevents the synthesis of crucial organic compounds. Our atmosphere currently contains around 21% oxygen, and oxygen is of course essential to all aerobic organisms. This is an example of the great beauty of evolution: life cannot spontaneously generate in an atmosphere containing oxygen, yet through random mutations and natural selection, some of the properties of oxygen which make it lethal to simple organisms can be exploited by more complex organisms, giving rise structures as complex as the human brain. The earth containing the earliest life forms has been termed an RNA world, with organisms based on RNA instead of DNA and capable of self-replication. It is unknown when this may have happened, but the oceans and atmosphere of the Earth did not develop until around 4.1 billion years ago.

It is estimated that the Earth is around 4.55 billion years old. The oldest evidence of life is 3.5 billion years old, and comes from fossilized stromalites. These are formed by cyanobacteria, a photosynthetic bacteria. This means that photosynthetic bacteria must have evolved no later than 3.5 billion years ago. This means that between 4.1 billion years ago and 3.5 billion years ago, life must have spontaneously generated from various chemicals, and evolved into photosynthetic bacteria. That gives the evolution around 0.6 billion years.

A further complication is that in the period of time between 4.1 billion and 3.8 billion years ago, the Late Heavy Bombardment occurred: a period of time in which the entire Solar System was bombarded with very large asteroids. Evidence for this is provided by craters on the Moon. This bombardment is likely to have obliterated any life that had managed to generate on Earth, which reduces the time between spontaneous generation and evolution to photosynthetic bacteria to a maximum of 0.3 billion years.

In contrast, the universe is around 13.7 billion years old (or 12 billion years old according to Katie Melua – link). At least one cycle of star birth and death is required for the synthesis of carbon, nitrogen and oxygen, all of which are essential to life. This would take several billion years. However, it is very likely that the universe has been theoretically capable of supporting life for upwards of 10 billion years, and that by the time photosynthetic bacteria were present on Earth, life could have existed elsewhere in the universe for over 6.5 billion years. The rather tempting question is obvious: if humans were able to evolve on Earth within 4.55 billion years of its formation, what kind of life could have evolved elsewhere in the universe in over double that time?

There is currently no evidence of any life outside Earth. There are around 400 billion stars in the Milky Way, our galaxy, and there are around 170 billion galaxies in the universe (the size of galaxies ranges from as little as 10 million stars to as many as 1 trillion). If there is nothing particularly special about our Solar System, then any number of the other 70,000,000,000,000,000,000,000 solar systems could contain life. Even if our Solar System is exceptional, perhaps as rare as 1-in-a-billion, there are still 70,000,000,000,000 others like it. These are numbers of homeopathic proportions, which make the possibility of life – intelligent life – on other planets seem not just likely, but almost certain. But yet we have no evidence whatsoever of any extraterrestrial life. The contradiction between the high likelihood of alien life, and the complete lack of evidence for any is known as the Fermi paradox.

There are all sorts of theoretical explanations for this paradox. Perhaps intelligent life always ends up destroying itself before it is able to communicate with other planets; perhaps the Earth is in fact unique and no other planets can support life; perhaps once life becomes intelligent enough to communicate with other planets, it reaches a point in philosophical enlightenment where it decides it doesn’t want to; perhaps the evidence is there, but we can’t see it; perhaps no planet has the resources to support interstellar travel; perhaps interstellar travel is technologically impossible; perhaps Earth is being observed by others who do not wish to interfere; or perhaps they are already amongst us. All we can do is speculate.

So, if life can develop on other planets, can it travel between planets? This concept is known as exogenesis or panspermia.

We know that material can be transferred between planets – we regularly receive meteorites from Mars. It is unlikely that life would be able to survive travel across space: radiation, cosmic rays and stellar winds are all very damaging to life. Although DNA can survive for a few million years in extremely harsh conditions, as found in Antarctic glaciers, this is probably not sufficient time for interstellar movement.

One intriguing theory proposed by Francis Crick, the co-discoverer of the structure of DNA, is that of Directed Panspermia (link). The theory is that planets that may be able to support life may have been chosen by intelligent life on another planet, who then sent life on spaceships containing organisms to these planets. Crick’s conclusion is that Directed Panspermia cannot be completely dismissed, although it is very difficult to estimate its likelihood as many of the necessary parameters are completely unknown.

Of course, even if exogenesis is possible and has happened, life still had to start by abiogenesis somewhere. Occam’s Razor would probably prefer the theory that abiogenesis took place on Earth, and perhaps takes place on other planets, with no transferrals between planets, but again, it is very difficult to estimate the relative probabilities when so many of the variables are unknown.

Finally, if Directed Panspermia is possible, then regardless of whether or not it has happened before or was responsible for the start of life on Earth, one intriguing question is whether or not it will happen in the future. It seems likely that when we finally run out of resources on Earth, we will either have to harness the resources of outside our planet, or become extinct. It could be that in the distant future, we may feel that our only hope of survival is to move to a different planet.

If that ever happens, it’s just possible that it may not be for the first time.

Song (three parts)

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2 Responses to “Exogenesis”

  1. That’s actually fascinating, I’m not sure I get quite a lot of it, but it’s an intriguing idea, especially the last sentence. And in fairness to Katie Melua, she did qualify that estimation with “that’s a guess”.

  2. That was part of Simon Singh’s problem – it’s not a guess, it’s a very good estimate!

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