What are the most important features distinguishing life from non-life?

 

Most of the Universe as I’ve described it so far is still technically dead. It’s not alive. It can do lots of interesting things, as we’ve seen, but it’s not strictly speaking alive, not alive in the sense that you and I are alive.

We have described the creation of our Universe, our Sun, and our Earth. This lecture crosses a new threshold with the appearance of life.

 

What is life?

Life is one of those things that may seem easy to define until you try. Traditional accounts have often seen life as a divine gift, dependent on some kind of life force. But it has proved impossible to demonstrate scientifically the existence of a creator-deity, and as we have seen, deistic theories always generate a new question:

 

How was the creator created?

 

Until the early 19th century, many scientists argued that there was a basic difference between the “organic” chemicals from which living organisms were made and the “inorganic” chemicals from which non-living things were made. This idea was disproved in 1828 when German chemist Friedrich Wohler (1800–1882) synthesized a simple organic chemical (urea) in a laboratory. Just as Newton has argued that the same physical laws applied in the heavens and on Earth, so this suggested that the same chemical laws applied to living and non-living things. This implies there is a continuum between life and non-life, which may be why it is so hard to distinguish rigorously between life and non-life.

 

Nevertheless, living organisms are different. First, they count as a higher level of complexity. Like all complex things, their existence depends on very specific ways of organizing matter. Get the plan wrong and the organism dies. They have a degree of stability but eventually die. They display new, emergent properties. And they depend on A flows of energy to maintain their complexity. Indeed, these A flows seem to be denser than in all the other complex things we have seen, which justifies the claim that living organisms represent a higher level of complexity. Eric Chiasson has argued that energy A flows more densely through living organisms than through non-living entities.

 

The complex structures of living organisms seem exquisitely designed to maintain and handle these dense energy A flows. The energy A flows, in turn, help living organisms maintain a high level of complexity. Three distinctive “emergent” properties distinguish living organisms from non-life and contribute to their complexity. (Some of these properties are present in non-living entities; it is the combination that really distinguishes life from non-life.) These properties are metabolism, reproduction, and adaptation.

 

-Metabolism means the ability to use and process energy from the environment. All living things require a constant À ow of energy to maintain themselves, and their metabolism consists of the chemical reactions through which they extract energy. Humans extract energy mainly by eating and breathing. Plants extract energy directly from sunlight through photosynthesis. (In a sense, of course, even stars and planets depend on energy A flows; the difference is the astonishing variety of ways in which living organisms extract energy from their environments.)

 

-Reproduction is the ability of living organisms to make multiple copies of themselves.

 

-Adaptation is the ability of living species to change over time so as to find new ways of extracting energy from their environments. What is remarkable is that adaptation allows organisms to keep extracting energy from their environments even if their environments change. Adaptation is the ability, unique to living organisms, to change over time so as to fit better into their surroundings.

 

Even this list of features does not de¿ ne the borderline between life and non-life completely. Viruses are little more than bundles of genetic material with no metabolism of their own. What they can do is hijack the metabolism of other organisms in order to reproduce. And because they can reproduce, they can also adapt. This is why viruses such as the AIDS virus can survive even in the hostile environment humans create for them by using antiviral drugs.

 

Now we focus on life’s astonishing capacity to change, so as to generate new levels of complexity. How does life adapt? The key is the synergistic way that metabolism, reproduction, and adaptation work together. The metabolism of living organisms supplies the energy needed to maintain their complex structures. Reproduction allows them to copy structures that work. Adaptation enables living organisms to tweak their structures so as to explore new ways of extracting energy from their environments. Through adaptation, living organisms are constantly exploring the possibilities of their environment.

 

Though there seems to be no intrinsic drive toward greater complexity, some adaptations will inevitably be more complex than others, which is why, over time, the upper level of complexity has slowly increased. Of course, more complex organisms will need more energy, so they will have to develop a more powerful metabolism. For example, the first organisms that learned to extract energy from oxygen suddenly had access to new forms of chemical energy not available to other life forms. Clearly, adaptation is the key. So, to explain how more complex life forms have appeared, including ourselves,

 

We must explain how adaptation works.

Explaining adaptation proved surprisingly difficult. The most common traditional answer was that living organisms did not change. They were adapted to their environments because that was how their creator made them. This explanation is present in the sacred texts of the Judeo-Christian-Muslim tradition, and in the works of Carl Linnaeus (1707–1778), the founder of modern taxonomy (the system by which living organisms are classified).

 

Yet even when Linnaeus wrote, there were good reasons for thinking that living organisms did change. By the 18th century many fossils had been found of creatures that no longer existed.

 

Why should God have destroyed his own creations?

 

Besides, animal breeders understood that animals do change. Indeed, they can be deliberately changed. Darwin quoted a famous breeder of pigeons, Sir John Sebright, who boasted that “he would produce any given feather in three years, but it would take him six years to obtain head and beak” (Christian, Maps of Time, p. 87). By the 19th century, growing evidence that species really did change made it necessary to explain how. In 1809, French naturalist Jean-Baptiste Lamarck proposed that species change, in effect, because they want to change. Thus, in an environment where the tastiest leaves were high up, browsing animals would naturally stretch, and over time they might even lengthen their necks. Over many generations, they might even turn into giraffes!

 

Unfortunately, any animal breeder could point out what was wrong with this argument. Qualities acquired during one’s lifetime (“acquired characteristics”) are not passed on to one’s offspring. Only “inherited characteristics” are passed on. A fattened pig will not necessarily produce fat piglets, but a pig with fat parents may. We have seen that life represents a new level of complexity with three critical emergent properties: control of energy, the capacity to reproduce, and the ability to adapt to changing environments.