(ORDO NEWS) — Life is pretty easy to recognize. It moves, grows, feeds, excretes, multiplies. Everything is simple. In biology, researchers often use the acronym “MRSGREN” to describe it. It stands for movement, breathing, sensitivity, growth, reproduction, excretion and nutrition.
But Helen Sharman, the first British astronaut and chemist at Imperial College London, recently claimed that undetectable alien life forms could be living among us. How can this be possible?
While life may be easy to recognize, it’s actually hard to define, and scientists and philosophers have been arguing about it for centuries, if not millennia. For example, a 3D printer can reproduce itself, but we wouldn’t call it alive. On the other hand, the mule is known to be sterile, but we will never say that it does not live.
Because no one can agree, there are over 100 definitions of what life is. An alternative (but imperfect) approach is to describe life as “a self-sustaining chemical system capable of Darwinian evolution”, which fits many of the cases we wish to describe.
Lack of definition is a huge problem when it comes to the search for life in space. The inability to define life other than “we will know it when we see it” means that we do limit ourselves to geocentric, perhaps even anthropocentric, ideas about what life looks like.
When we think of aliens, we often think of a humanoid being. But the intelligent life we’re looking for doesn’t have to be humanoid.
Life, but not as we know it
Sharman says he believes in the existence of aliens, and “there can be no two options.” Moreover, she wonders: “Will they be the same as you and me, consisting of carbon and nitrogen? Maybe not. Maybe they are already here and now, but we just do not see them.”
Such life will exist in the “shadow biosphere”. By this I don’t mean the realm of ghosts, but undiscovered beings, perhaps with different biochemistry. This means that we cannot study or even notice them because they are beyond our understanding. Assuming that it exists, such a shadow biosphere is likely to be microscopic.
Why haven’t we found it yet? We have limited opportunities to study the microscopic world, since only a small percentage of microbes can be cultured in the laboratory.
This may mean that there are many life forms that we have not noticed yet. We now have the ability to sequence the DNA of uncultivated strains of microbes, but this only detects life forms that we know contain DNA.
However, if we find such a biosphere, it is not clear whether it should be called alien. It depends on what we mean: “extraterrestrial” or simply “stranger”.
Silicon based life
A popular proposal for alternative biochemistry is silicon-based biochemistry rather than carbon-based biochemistry. This makes sense even from a geocentric point of view. About 90% of the Earth is made up of silicon, iron, magnesium, and oxygen, which means that there is everything you need to create potential life.
Silicon is similar to carbon, it has four electrons available to create bonds with other atoms. But silicon is heavier, with 14 protons (protons make up an atomic nucleus along with neutrons) compared to six in carbon.
While carbon can form strong double and triple bonds to form long chains that are useful for many functions, such as building cell walls, the situation is much more complicated with silicon. It has difficulty creating strong bonds, so long-chain molecules are much less stable.
Moreover, conventional silicon compounds such as silicon dioxide (or silica) are generally solid at Earth temperatures and insoluble in water. Compare this to highly soluble carbon dioxide, for example, and we will see that carbon is more flexible and offers many more molecular possibilities.
Life on Earth is fundamentally different from the basic composition of the Earth. Another argument against the silicon-based shadow biosphere is that too much silicon is locked up in rocks.
In fact, the chemistry of life on Earth roughly matches that of the sun: 98% of atoms in biology are made up of hydrogen, oxygen, and carbon. Therefore, if viable silicon life forms existed here, they could have evolved elsewhere.
However, there are arguments for the existence of silicon-based life on Earth. Nature is capable of adaptation.
A few years ago, scientists at the California Institute of Technology succeeded in deriving a bacterial protein that created bonds with silicon essentially bringing the silicon to life. So even though silicon is less flexible than carbon, it can find ways to assemble into living organisms, potentially including carbon.
And speaking of other places in space, such as Saturn’s moon Titan or planets orbiting other stars, we certainly can’t rule out the possibility of silicon-based life.
To find it, we must somehow go beyond terrestrial biology and find ways to recognize life forms that are fundamentally different from carbon. There are many experiments testing these alternative biochemistries, such as the experiment done at Caltech.
Regardless of what many believe that life exists elsewhere in the universe, we don’t have any evidence for it. Therefore, it is important to consider all life as a value, regardless of its size, quantity or location.
Earth supports the only life known to us in the universe. So no matter what form life may take elsewhere in the solar system or the universe, we must be sure that we protect it from harmful pollution – whether it be terrestrial life or alien life forms.
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