Life is pretty easy to recognize. It moves, it grows, it eats, it excretes, it reproduces. Simple. In biology, researchers often use the acronym “MRSGREN” to describe it. It stands for movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.
But Helen Sharman, Britain’s first astronaut and a chemist at Imperial College London, recently said that alien lifeforms that are impossible to spot may be living among us. How could that be possible?
While life may be easy to recognize, it’s actually notoriously difficult to define and has had scientists and philosophers in debate for centuries — if not millennia. For example, a 3D printer can reproduce itself, but we wouldn’t call it alive. On the other hand, a mule is famously sterile, but we would never say it doesn’t live.
As nobody can agree, there are more than 100 definitions of what life is. An alternative (but imperfect) approach is describing life as “a self-sustaining chemical system capable of Darwinian evolution,” which works for many cases we want to describe.
The lack of definition is a huge problem when it comes to searching for life in space. Not being able to define life other than “we’ll know it when we see it” means we are truly limiting ourselves to geocentric, possibly even anthropocentric, ideas of what life looks like. When we think about aliens, we often picture a humanoid creature. But the intelligent life we are searching for doesn’t have to be humanoid.