So far we have covered five characteristics of life: organization, nutrition, excretion, response and reproduction. However, these characteristics do not go far enough into explaining what life actually is.
The levels of biological organization do not stop at the cell or population. We can go smaller than the cell in terms of organelles, molecules, and atoms and we can go larger than populations in communities, ecosystems and biospheres. Biochemistry concerns itself with the smaller end of the spectrum.
There are other characteristics of life as well. Movement and Growth and Development are important distinctions between living and not living. A rock does not move by itself nor does it grow and develop, it cannot make itself bigger. A plant or animal, on the other hand, can. Homeostasis, or the ability to maintain state despite changes in the environment, is another important characteristic.
So, in reality, there are 8 base level requirements of life:
- Growth and Development
These characteristics describe the difference between living and dead or inorganic matter. However, now that we are living in the age of technology and we are witnessing the birth of artificial intelligence, we need to consider a few more things. For example, if someone were to build a highly intelligent robot that could build itself new pieces so it could get bigger, develop new circuits for itself and was capable of making a new version of itself from its old pieces, it would pretty much satisfy the 8 requirements above. Naturally, we all know that a robot is not alive, but how can we prove it? We need more requirements of course.
Life has two things that a robot can never have: Order and Complexity and Evolution.
To explain Order, let’s investigate photosynthesis. Photosynthesis occurs in the chloroplast. It does not happen in a test-tube with a mixture of chlorophyll and other chloroplast molecules. This means that there must be a certain way in which the reaction occurs, it requires order. In terms of complexity, let’s take a look at DNA and the genome. Any given cell contains all the information required to make any cell in the body. For example, a muscle cell has all the genetic information to make a neuron. Complexity arises in the cell’s ability to ‘switch’ on and off different areas of the genome and the sheer volume of information involved. Could one robot circuit board encode a perfectly ordered system while still containing all the information required for every other process that the robot could ever possibly do? I think not. To design such a robot is much to difficult and complex.
Evolution has enabled the survival of life and the diversity of life. Evolution is the idea that all living organisms on earth are modified descendants of a common ancestor. Everything from humans to amoeba have come from the same common ancestor. This theory has been used to explain the unity and diversity of organisms as well as their ability to adapt to different environments. It would be far too difficult to design a robot that would give rise to as many different species as evolution has in living organisms, without human input.
In reality, there are 10 characteristics of life (take that robots and rocks!). These characteristics are the things that make living organisms unique. It is a biologist’s job to understand living organisms and explain life.