Where did we come from?
The main science-based ideas propose life on Earth began in shallow, sunlit water, or around hydrothermal vents in the deep ocean.
The latter idea is intriguing because it means that we can look for life in the oceans under the ice on Europa and other moons in the outer Solar System.
Another idea that has been discussed for years is "panspermia", which proposes space is filled with the seeds of life, and these thrive, multiply and diversify in any environment to which they can adapt. If the "basic stuff of life" came from out there in space, how did it arrive here safely?
Every night we see meteors (shooting stars), which are little pieces of grit coming into our atmosphere at many kilometres a second. The short-lived, glowing streak we see in the sky is that piece of grit being heated to thousands of degrees by friction, and then vaporized. While being burned away it is decelerating at tens or hundreds of times the force of gravity.
This does not sound like a good way for prebiotic materials—the building blocks of life—to safely arrive on a new world. It does not take much heat to break down the carbon-based molecules on which earthly life is based. However, scientists looking at meteorites, chunks of cosmic material that reach the ground without being completely burned away, see a more optimistic situation than we might expect.
A typical meteorite is usually a lump of rock or iron that has been heated and melted by its passage from space to the ground. However, when something is moving tens of kilometres a second, it does not take long to get down to the ground. Basically, although the heat of its passage through the atmosphere might be intense, that heat may not have time to penetrate deeply into the meteorite. If the lump of cosmic material is big enough, the heat might not reach its centre at all.
The deceleration stresses would certainly kill any animal, but things of molecular sizes embedded in the body of the meteorite would be quite happy. This is why a lot of effort is going into searching for prebiotic materials inside meteorites.
DNA, a fundamental ingredient in life as we know it is a double spiral comprising an enormous sequence of combinations of four chemicals known as bases, rather like a long story written with an alphabet of four letters.
Ten years ago, scientists found two of those bases in meteorites. Now, meteorites have yielded the remaining bases. Moreover, there is evidence a meteorite that hit our planet several billion years ago contained at least some of these bases. This was while the Solar System was still forming and the Sun had not yet been born.
If this is the case, it seems that there is a supply of prebiotic material available whenever new stars and planets are forming. It looks as though the seeds of life could have come from outside. If this happened to Earth, then those same ingredients must have found their way to almost all other worlds, orbiting other stars as well as ours. It is true that conditions on many worlds could have been too hostile for life to take root. However, it is hard to imagine that our world is the only one on which it succeeded.
There have been suggestions chemicals that can act as the seeds of life can be blasted off a planet by an impact, and subsequently find their way to new worlds. However, at the moment it seems likely they are formed in space, and then incorporated into rocks that end up as meteorites.
There is a very big distance between having the four ingredients of DNA and having that complicated molecule itself. However, it happened here, so it is likely to have happened elsewhere. Interestingly, it does seem as though the universal standard life form is likely to be carbon-based, just like life on Earth.
That does not however mean it has to look anything like us, or like any of the diverse life forms sharing our world.
• Venus, Jupiter, Mars and Saturn are lined up in the dawn glow, in order of increasing brightness.
• The Moon will reach it last quarter on May 22.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.