What Is Life

What Is Life?

This question is seemingly so simple, and yet it has vexed some of the most knowledgeable scientists and philosophers for decades. While hardly the first writing on the subject, physicist Erwin Shrödinger's (of Shrödinger's cat fame) 1944 book What Is Life? Is one such example. It is an interesting early attempt to use the ideas of modern physics to address the question. Both James Watson and Francis Crick, codiscoverers of DNA, credited this book as being an inspiration for their subsequent research.

The definition of life is not settled even today. Modern scientists have managed to list a series of critical features that seems to identify life. A living being should have most, if not all, of the following features:

• It must be able to regulate the internal environment of the organism;
• It must be able to metabolize or convert energy in order accomplish the tasks necessary for the organism's existence;
• It must grow by converting energy into body components;
• It must be able to adapt in response to changes in the environment;
• It must be able to respond to stimuli;
• It must be able to reproduce.

These features distinguish it from inanimate matter.

Life is able to respond to stimuli. Photo by Elena.

While these properties can help one identify life when one encounters it, they don't really give us a sense of the limitations imposed by the universe on what life might be like. We can ask ourselves if a would-be science fiction writer is being ludicrous when he or she bases a story around an Alien with bones made of gold and liquid sodium for blood. So what does our current best understanding tell us that life requires? A combination of theory and experimentation suggests that there are four crucial requirements for life. They are (in decreasing order of certainty):

• A thermodynamic disequilibrium;
• An environment capable of maintaining covalent interatomic bonds over long periods of time;
• A liquid environment;
• A structured system that can support Darwinian evolution.

The first is essentially mandatory. Energy doesn't drive change, rather energy differences are the source of change. “Thermodynamic disequilibrium” simply means that there are places of higher energy and lower energy. This difference sets up an energy flow, which organisms can exploit for their needs. It's not fundamentally different from how a hydroelectric power plant works: there is a place where the water is deep (high energy) and a place where the water is shallow (low energy). Just as the flow of water from one side of the dam to the other can turn a turbine to create electricity or a mill to grind grain, an organism will exploit an energy difference to make those changes it needs to survive.

The second requirement is essentially nothing more than saying that life is made of atoms, bound together into more complex molecules. These molecules must be bound together tightly enough to be stable. If the molecules are constantly falling apart, it is hard to imagine this resulting in a sustainable life-form. It is this requirement that sets some constraints on which atoms play an important role in the makeup of any life. Hopefully after this discussion, you'll understand the reason for the oft-repeated phrase in science fiction “carbon-based life-form.”

Requirement number three is less crucial; however it's hard to imagine life evolving in an environment that isn't liquid. Atoms do not move easily in a solid environment and a gaseous environment involves much lower densities and can carry a far smaller amount of the atoms needs for building blocks and nutrition. Liquids can both dissolve substances and move them around easily.

Finally, the fourth requirement might not be necessary for alien life, but it is crucial for Aliens. Certainly multicellular life of the equivalent not be the first form of life that develops. The first form that develops will be of a form analogous to Earth's single-celled organisms (actually, most likely simpler... after all, modern single cell organisms  are already quite complex). In order to form species with increasing complexity, small changes in the organism will be necessary. Darwinian evolution is the process whereby a creature is created with differences from its parents. The first thing that is necessary is that the organism survives the change. After all, if the change kills it, it's the end of the road for that individual. Once there are changes that both allow the daughter organism to survive and possibly confer different properties, selection processes become important. Creatures who subsequently reproduce more effectively will gradually grow in population until they dominate their ecological niche.

Many forms of life exist. Photo by Elena.