The Definition of Life

Inquiry into Life (13th ed. by Sylvia Mader, published by McGraw-Hill) is the textbook I use to teach Principles of Biology. Since it is a biology textbook, one of the first things it does is define a living thing. It does so by listing seven characteristics of living things:

1) Living things are organized
2) Living things acquire materials and energy from their surroundings
3) Living things reproduce
4) Living things respond to stimuli (react to their environment)
5) Living things are homeostatic (that is, they maintain nearly constant chemical and physical conditions inside their body)
6) Livings things grow and develop
7) Living things have the capacity to adapt to their environment

Pretty simple, right? It is easy to see how common, living things fit these characteristics. The human body is organized, humans acquires materials and energy by eating, reproduce, grow and develop, react to their environment, have constant internal conditions (such as constant body temperature or blood pH), and we can adapt to our environment.

But what about tricky things, like a quartz crystal? Let’s go through each of the characteristics in turn and see how ell a quartz crystal fits.

1) A quartz crystal is organized. It has a regular arrangement of silica and oxygen molecules.
2) A quartz crystal can acquire materials and energy. For example, silicate can come out of solution and join a quartz crystal, thus the quartz crystal has acquired the silicate.
3) A quartz crystal can grow. When silicate is added to a quartz crystal, the new material is used to build up the crystal. In other words, the crystal grows.
4) A quartz crystal can reproduce. A new crystal can grow off of an existing crystal.
5) A quartz crystal can respond to stimuli. When pressure (a stimulus) is applied to a quartz crystal, the crystal responds by creating an electrical current. This is called piezoelectricity.
6) A quartz crystal is homeostatic. Its molecular structure remains constant.

About the only things a quartz crystal can not do is adapt to its environment. When the environment changes, a group of quartz crystals can not change in response: they will all remain exactly the same.

So, am I seriously suggesting that a quartz crystal might be alive? No, but this is a good illustration of how these seven characteristics of life are only barely able to define a quartz crystal as a non-living object. After all, there was only one criterion that the quartz crystal did not meet: they can’t adapt to their environment.

True, many of the “living” characteristics of quartz are a stretch. They acquire materials by pulling silicate out of solution from around them? That’s just a natural process. But isn’t an animal eating a natural process as well? But they are still different. So what is this difference?

Information. Laws of physics drive the movement of silicate so that it bonds with a quartz crystal. They crystal itself is not doing or directing anything. In contrast, an animal eating is not a passive process. Instinct drives an animal to eat, the structure and chemistry of the digestive tract and the various controls on the digestive tract (both hormonal and neural) cause food to be broken down and absorbed, and molecular machines at the cellular level use this food to build new cells to cause the organism to grow or to maintain its structure. Ultimately, the acquisition of materials and energy is driven by information in DNA which provides instructions for the molecular machines, the hormonal and neural controls, and the structure and chemistry of the digestive tract.

Matter of fact, most of the characteristics of life can be reduced to information. We have already discussed acquiring materials and energy, so that is 1). The others are:

2) The structure of an organism is directed by 3) growth and development which is controlled by information in DNA. In contrast, the structure of a quartz crystal is controlled entirely by physical laws.
4) The process of reproduction requires division of cells, which is controlled by information in DNA. The growth of a new quartz crystal is entirely controlled by physical laws.
5) Organisms respond to stimuli in a controlled manner. Signals are received, the information from the signals is processed, and an appropriate response is given. This process requires existing receptors, processing units, and the necessary information to guide them, which once again ultimately comes from the information in DNA. In contrast, piezoelectricity is a natural phenomena dictated by natural laws.
6) Homeostasis is actively controlled by an organism. Once again, this requires receptors, processing units, and a response, all of which requires information in DNA. A quartz crystal maintains its structure only because it is a very stable mineral.

Now, don’t think that it is DNA that causes life. DNA is merely a medium that stores genetic information. The real control in all of these characteristics is the information itself.

Seeing how almost everything an organism does is ultimately dictated by information, wouldn’t it be easier to define a living thing in terms of its informational content? But, my textbook can not do that. Why not? Because it must focus on adaption, not information.

Unlike the other six characteristics, adaption is not necessarily bound to information. The important factors in adaption are survival and inheritance. Whatever feature allowed for survival must be passed on to the survivor’s offspring or the adaptation will be lost in the first generation. In living organisms, information codes for these adaptations. However, information is irrelevant to the identification of adaption, since adaption is only defined in terms of survival and inheritance. The fact that information codes for adaptations actually poses a problem because information must have been created by something, and the only thing known to create information is an intelligent being.

In order to prevent any acknowledgement of a Creator, it is necessary to ignore the importance of information to life. Thus, the only thing that could possibly distinguish life from non-living matter is physical descriptions of living things. That only works so far, as illustrated by the quartz crystal, since both living and non-living objects partake in natural processes. So, adaption must be added as part of the definition of a living thing. After all, if there is no Creator, organisms must have gained their organization and methods for acquiring materials and energy, reproduction, growth, development, and homeostasis from something, and adaption is the best explanation around.

Or there really is a Creator.