Life on Earth: Is our planet special?
For Greek philosophers like Aristotle, Earth lay at the centre of a small universe and the idea of alien life was unthinkable.
Since then the tide of opinion has turned. Astronomers have shown that Earth may be just one of myriad habitable worlds.
Meanwhile biologists have shed light on how life might have originated here, and therefore on other planets too.
Far from being unique, many now regard Earth as an ordinary lump of space rock and believe that life "out there" is almost inevitable. But could the truth be somewhat more complex?
On Friday, top scientists are meeting at the Geological Society in London to debate this very issue, posing the question: "Is the Earth special?". What emerges is that aspects of our planet and its evolution are remarkably strange.
End Quote Dr Richard Ghail Imperial College London
There was this one amazing chance event that made the Earth the way it is”
Prof Monica Grady is a meteorite expert at the Open University. She explained in what sense the Earth could be considered special.
"Well, there are several unusual aspects of our planet," she said. "First is our strong magnetic field. No one is exactly sure how it works, but it's something to do with the turbulent motion that occurs in the Earth's liquid outer core. Without it, we would be bombarded by harmful radiation from the Sun."
"The next thing is our big Moon," continued Prof Grady. "As the Earth rotates, it wobbles on its axis like a child's spinning top. What the Moon does is dampen down that wobble… and that helps to prevent extreme climate fluctuations" - which would be detrimental to life.
"Finally, there's plate tectonics," she added. "We live on a planet that is constantly recycling its crust. That's another way that the Earth stabilises its climate." This works because plate tectonics limits the amount of carbon dioxide escaping into the atmosphere - a natural way of controlling the greenhouse effect.Earth that never was
If these factors were important for life flourishing on Earth, an obvious question is what went wrong for our moribund neighbours, Venus and Mars?
One popular explanation is the Goldilocks Effect. This states that Venus was simply too close to the Sun and overheated while Mars was too far away and froze. Between these extremes - like the baby bear's porridge - Earth was "just right" for life.
Indeed, just this week astronomers confirmed the discovery of an Earth-like planet in this "habitable zone" around a star not unlike our own.
Dr Richard Ghail, an expert on Venus at Imperial College London, is highly sceptical of this Goldilocks theory, however.
"For me, the key thing is that Venus has a lower density than the Earth," he told the BBC. "That difference was fixed early on in the formation of the Solar System when there were lots of planetary collisions." In the case of Venus, collisions led to accretion into a single planet, but with Earth, the lighter material was flung off to form the Moon.
One effect of Venus's lower density is that its interior melts more easily. So, whereas the Earth has a swirling core that is part solid and part liquid, the core of Venus is entirely liquid - and strangely calm.
In Dr Ghail's opinion, this has led to a spiral of doom for Venus. Without a turbulent core, no magnetic field was generated. And no magnetic field meant that Venus was mercilessly battered by solar radiation, causing it to lose all its water.
Because water is needed to "lubricate" plate tectonics, the crust stopped recycling. Consequently, carbon dioxide built up in the atmosphere and the greenhouse effect ran out of control. As a result, today, Venus is a lifeless inferno whose surface is hot enough to melt zinc.
Dr Ghail said: "When you think about it, there was this one amazing chance event [a collision that flung off the Moon] that made the Earth the way it is." If that had not happened, life on Earth might not have evolved at all.
Given that Earth's history was shaped by a single improbable event, one might be tempted to assume that life elsewhere must be extremely rare.Cosmic rarity?
Wrong, argues Dr Nick Lane, a geneticist at University College London. He believes that the emergence of life is probable on any wet, rocky planet.
Dr Lane explained the reasons for his confidence, saying: "One of the most common minerals in the Universe is olivine; interstellar dust is full of it. When olivine and water mix on the seafloor, the reaction is exothermic." That is, it gives off heat.
The environment produced by this reaction "provides analogues for all six essential processes of living organisms," continued Dr Lane. But the especially important thing is that it releases "a rich source of chemical energy that is much easier for an organism to tap than, for example, the Sun's energy".
Thus, wherever olivine and water mix in large quantities, conditions are favourable for the emergence of life.
Consequently, life is not limited to planets that orbit a star; conceivably it could also exist on asteroids drifting through deep space. Simply put, "The Earth is not special," concluded Dr Lane.
Prof Simon Conway Morris, a renowned palaeontologist at the University of Cambridge, is not entirely convinced by these arguments, however.
"I would tend to raise one cautious eyebrow to such arguments," he said. After all, there is a horrible gulf between elementary chemical systems and the creation of fully functioning cells. It is a gap that we have been remarkably unable to bridge experimentally."
Prof Conway Morris concluded: "One important jigsaw piece that is rarely mentioned in these discussions is Fermi's Paradox." This is the concept of the Great Silence; in other words, if life is common in the Universe, why have we not managed to contact it?
And that surely is the key. For in the absence of verifiable alien contact, scientific opinion will forever remain split as to whether the Universe teems with life or we are alone in the inky blackness.