This week: The search for 'weird' life
When we look at life around us, we could certainly use the word tenacious to describe its instinct for survival. Organisms turn up in the most unlikely places: mould that lives on your kitchen walls, lichen that lives in the middle of frozen wastelands, a multitude of spores floating high up in the stratosphere. There is life on the bottom of the ocean floor next to heat vents that could melt lead. And we are not talking about simple bacteria, but complex life forms.
What is amazing about the diversity of life - from the gargantuan blue whale to a scorpion to an intestinal worm living in our bodies - is that we are all carbon-based life forms using DNA or RNA to transmit genes. It would seem that all life on Earth must have originated from the same ancestry to explain such a core similarity.
Recently, the US National Research Council, a leading scientific advisory group, convened a group of scientists to expand the search for extraterrestrial life to encompass so-called 'weird' life - the possibility of life forms that do not use DNA or RNA to develop and propagate. Their report was published this week.
'Weird' life denotes any life that is not the same as us, not made from the basic units of carbon-based life here on Earth. These scientists are trying to improve our odds of recognising life while we are exploring space. They propose that we look for life with a more open mind and a more open definition. They are challenging other scientists to imagine other possible forms of life and report them.
The search for extraterrestrial life has always been limited to the search for water and carbon-organic molecules on other planets, and there is good reason for this. Water has amazing properties that make the perfect solvent for life: it is very stable and able to form strong bonds, and it is able to hold pre-biotic molecules together sufficiently to allow the formation of self-reproducing systems.
Carbon is very common in the universe; it is able to form very complex molecules and long chain polymers; it has properties to form large molecules called enzymes that regulate a huge variety of processes in the body. Not to mention that water and carbon are common on Earth, so we assume it is probably the most likely unit of life everywhere.
But 'weird' life has been seriously considered for a very long time. In 1891, German astrophysicist Julius Scheiner speculated about life based on silicon rather than carbon. As you will recall from high-school chemistry, silicon is the element below carbon on the periodic table. Being in the same group means they share many similar chemical properties.
Like carbon, silicon is also very common in the universe. Life based on silicon would be able to withstand very high environmental temperatures as silicon compounds are stable at high temperatures. But there is a fatal flaw in silicon-based life.
During respiration we exhale carbon dioxide, and presumably a silicon-based life form would likewise, in an oxygen atmosphere, exhale silicon dioxide. The problem is, silicon dioxide is a rather stable solid and not a gas like carbon dioxide. It would be rather difficult to breathe out a solid and would pose a major respiratory challenge for a silicon-based life form.
If this sounds like science fiction to you, you are not alone. Many sci-fi writers have tackled the exact same problem. H.G. Wells once wrote: 'One is startled towards fantastic imaginings by such a suggestion: visions of silicon-aluminium organisms - why not silicon-aluminium men at once? - wandering through an atmosphere of gaseous sulfur, let us say, by the shores of a sea of liquid iron some thousand degrees or so above the temperature of a blast furnace.'
In Stanley Weisbaum's 1934 book A Martian Odyssey, he fictitiously solved the silicon-dioxide problem with an alien species that would move once every 10 minutes and leave behind a solid brick of silicon dioxide.
Many other forms of life have been proposed by real scientists, such as life based on ammonia, boron, nitrogen, phosphorous, even life that is not solid, such as non-corporeal life, energy-based life, bubble life and gravitational life. They are all alive with possibilities, but they all have fatal flaws that can't be explained with our current knowledge. However, what we conceive as commonplace now - such as computers and microwaves - would seem miraculous only 200 years ago.
This search for life elsewhere has also compelled us to look for life in unlikely places on Earth. It is thought there may be life deep under the Earth's crust. As we search for life elsewhere, we are also finding more life around us. This search can only increase our respect for life.