The definition of life has always been a point of debate among biologists and scientists. Despite inhabiting the planet for billions of years, what life really is still remains a mystery for human beings. While theories abound this phenomenon, the latest definition of life by some biologists and researchers has once again put it under the microscope.

Generally speaking, the prerequisite for something to be called ‘alive’ demands a long list of attributes—the ability to reproduce, to grow, to react, and to defend itself and so on. But the requirements aren’t always met in certain species. Mules cannot reproduce, neither can bees, except for the queen, and of course, some humans lack the ability as well. However, they are very much alive. Furthermore, frozen bacteria are inactive, but alive.

Recently, Gerard Jagers op Akkeruis of the University of Wagening, Netherlands defined life in an entirely new perspective. In an interview with Astrobiology magazine, Akkeruis says, “People have focused on facultative properties like breathing or moving, and then say if we combine a few of those properties we are close to defining life. But there are always exceptions. What my idea does is, it turns the whole thing completely upside down. I focus on the minimal, absolutely necessary properties and I don’t care about any facultative properties”.

Akkeruis refers to life as an ‘operator’ and the term applies to organisms, atoms and molecules. These so-called operators are entities that work as specific and self-organised processors. These do not only include human beings and all other living organisms, but also some non-living things such as molecules. Their level of complexity then ranks these operators. These operators work step-by-step in a network of interactions—without the ability of interacting neurons, which gives the brain sensors to touch, smell, taste, act, etc. Without these abilities to interact with the living world, the brain would be useless.

In 1944, Erwin Shrodinger defined life as something that avoids equilibrium. While we are alive, our body keeps its balance in structure but once we die, it breaks down through chemical and bacterial processes, resulting in it becoming a single entity with the earth—similar to a glass of chilled water that warms up until it reaches room temperature and becomes one with the air surrounding it. According to this definition, life takes in energy and releases by-products—which is what scientists looked for in the samples from Martian soil. The results were inconclusive.

A group of scientists also believe that water is the main ingredient for life. But astrobiologist Benton Clark, from the University of Colorado and Lockheed Martin, differs. He says that water is only the prerequisite for life, as we know it on our planet and according to our observations. Since life adjusts to the most difficult of situations even on our planet, it may survive on sulphuric acid in other worlds.

Yet another definition of life cropped up recently, this time by Gerald Joyce of the Scripps Research Institute. Joyce calls life “a self-sustaining system capable of Darwinian evolution”. But Darwin’s theory itself is an enigma since the time taken is so long that one cannot be accurate as to how life evolved. Moreover, Clark believes that life reproduces and uses energy and these instructions are “embedded” in the DNA and RNA that we have.

But, at the end of the day, all these theorists agree on one thing: the definition of life is as elusive now as it was centuries ago.