What exactly is life? How did it arise, and what are the limits to its tolerance? Looking beyond our world has helped us find some answers.
By Wilson da Silva
WHAT IS LIFE? We all think we know the answer. Living things do stuff: they breathe, take in nutrients or energy, grow and age, reproduce, and they respond to stimuli.
You can get more technical by saying a living thing “has DNA” — deoxyribonucleic acid — the blueprint of life at the core of every cell. They also have an inherent form of internal self-organisation: in other words, they display ‘negentropy’: temporarily and locally counteracting the universal trend of increasing entropy in the cosmos.
But even when all such criteria are met, it may be difficult to determine if something is alive. Take a virus. It contains a strand of DNA (or RNA), but cannot move by itself. It neither breathes nor grows. Yet, when a receptive host is nearby, it attaches itself to the unsuspecting victim; in effect, responding to stimuli. It inserts its genetic material into the cell and forces the host to make more copies of the virus — it is, without doubt, reproducing.
It’s true that a virus hijacks a host’s cellular facilities in order to reproduce; but one could also argue that the commandeering of the cell by a virus is the equivalent of a predator eating a creature and then producing offspring: that is, taking one creature’s molecules, digesting them, and then refashioning the molecules into a copy of itself.
A virus is even more efficient: it sidesteps the need to refashion molecules, and uses a host cell’s internal organs and chemistry in situ to survive and multiply. In some cases, external factors can cause a virus to merge into host DNA, in order to hide until conditions are more favourable for reproduction (which also shows a response to stimuli). If a virus is not alive, how about the merged organism?
Even bacteria were once not considered alive, since they were “too small to have parents”, until it became clear that bacteria multiplied via binary fission, or cell division, as well as genetic recombination.
For a long time, the debate was largely philosophical. Then the field of astrobiology began in earnest in the 1990s: scientists across a range of disciplines sought to understand how life might arise on other worlds beyond our own.
It quickly became obvious that our knowledge of life on Earth — what it is, how it came to be, and its limits of tolerance — were largely unknown. We live with life all around us, and this familiarity has limited our scientific analysis. In a sense, we have taken life for granted.
This prompted one of the most exciting programs in biology: a hunt for ‘extreme life’. What are the limits for life? How far up into the atmosphere or deep into the soil does it stretch? How deep into ice, how much heat and sulphur, or how much salt, can life tolerate?
The results have been staggering. Life has been found everywhere: from the super-heated waters of seafloor volcanic vents, to the waterless bitter cold of Antarctica’s Dry Valleys; in caves dripping with sulphuric acid and, conversely, in highly alkaline solutions. Living organisms happily survive in saturated (and normally deadly) salts; endure massive doses of ionising radiation; or derive food and energy from such unpromising inorganic materials as manganese, iron and sulphur compounds.
Life is incredible. And it is far more diverse, ubiquitous and tenacious than we ever imagined. It’s clear that wherever it can gain even a tenuous foothold, life thrives.
We didn’t know this about life on our own world until we tried to understand how it might arise elsewhere. Which is why, these days, scientists are a lot more confident of the chances for life existing beyond Earth.
In searching for life elsewhere in the universe, we are already understanding more about ourselves — and about life itself.