What is Life, Part III

Even armed with NASA’s pragmatic definition of life, it is almost impossible to know what Earth’s very first life form was like. One very real possibility is that planet Earth’s earliest life may have been vastly different from anything we know today. Many experts suspect that the first living entity was not a single isolated cell, because even the simplest modern cells incorporate bewildering chemical complexity.

Most researchers assume that the first life form did not use DNA, given its exceedingly intricate mechanism. It may not even use proteins, which today act as the chemical work horses of cellular life. Naturally, experts propose different ideas regarding Earth’s first life form. Geologists propose that the Earth’s earliest living entity which fits NASA’s definition was an extremely thin molecular coding on a rock. It is easy to imagine the simple behavior of such flat life. It would have just spread across minerals in a layer of only a few billionths of a meter thick. Flat life would have exploited energy-rich mineral surfaces, and slowly spread outwards, from rock to rock.

Whatever that life form looked like, it must have arisen from chemical reactions among the oceans, the atmosphere and rocks.

Our Tendency to Dichotomize

The French anthropologist Claude Lévi-Strauss investigated the mythologies of many cultures. In the process, he recognized deep human tendency to reduce all sorts of complex situations to oversimplified dichotomies. We tend to divide people into friend and enemy. We divide the afterlife into heaven and hell. We divide actions into good and evil. We all know that most situations are much more subtle and complex.

The long history of sciences reveals that scientists are in no way immune to the trap of this kind of oversimplification. In the 18th century, for example, one group of naturalists called the “neptunists”, favored a watery origin for rocks. They fought many battles with the “plutonitsts”, who favored heat to describe the origin of rocks. It turns out that both were right. Rock sometimes form by the action of water, and sometimes by the action of heat, and sometimes even by a combination of both.

A similar contentious and misleading dichotomy raged between 18th century geologists was the one between catastrofists and uniformitarians. Catastrofists espoused the view that brief and cataclysmic events like earthquakes and floods dominated the geological history of Earth. Uniformitarians countered that geological processes are for the most part gradual and ongoing. Again, both groups were correct. Geological changes occur gradually over millions of years, but discrete catastrophic events, like the impact of big asteroids, also influence Earth’s history.

Similarly, there was a time when sharp distinctions were seen between plants and animals, and between single celled and multicellular organisms. Now, those sharp distinctions have become blurred.

I believe that any attempt to formulate an absolute definition of life, one that tries to differentiate between “life” and “non-life”, must represent a similar false dichotomy. Here’s why. It is obvious that the first living cell did not just appear fully formed with all its chemical complexity and genetic machinery. Rather, life must have arisen through a stepwise sequence of emergent events. I see life’s origin as a process of increasing chemical complexity.

What now looks to us as a divide between non-living matter and living cells tends to obscure the fact that the chemical evolution of life occurred in a stepwise sequence. Most of that history is lost, because when modern cells emerged, they quickly consumed all traces of the earlier stages of chemical evolution. They ate the evidence.

Our challenge is to use every available clue to establish a progressive hierarchy of emergent steps, leading from a prebiotic Earth rich in organic molecules to clusters of molecules, to self-replicating molecular systems, to encapsulation and membranes, to cellular life.

This view of life as a stepwise sequence of emergent events also informs the central question “what is life”. Any attempt to define the exact point in which a system of gradually increasing complexity becomes alive is intrinsically arbitrary. Where you or anyone tries to draw such a line is a question more of perceived value than of science. For example, if you value the intrinsic isolation of each living thing, then, for you, life’s origin probably would correspond to the stage when encapsulated cell membranes appeared. Perhaps you most value life’s ability to reproduce. If so, self-replication would be the demarcation point for life.

Many scientists today place special value on information as the key to life. They argue that life began with a genetic mechanism to pass information from one generation to the next. In this context, the question “what is life” becomes fundamentally a semantic question. It’s a subjective matter of taxonomy, rather than any absolute divide. Nature supports a rich variety of complex emergent chemical systems. Scientists are learning to craft a wide variety of those systems in the laboratory as well, but no matter how curious or noble the behavior of these systems may be, none of them comes with a label “life” or “non-life”.

Don’t get me wrong, labels are extremely important. They are vital for effective communication. However, I think that defining life is not helpful because there is so much we don’t know. Early attempts to classify animals purely by their color or shape ultimately failed. Similarly, early efforts to classify chemical elements according to their physical state (solid, liquid or gas) were unhelpful in elaborating a chemical theory.

We are in no position to define life. We don’t know if life’s biochemistry is highly constrained, or if there are many chemical solutions to life. It is much better at this point to keep an open mind and just describe the chemical characteristics of whatever we find.

What is Life, Part II

So, here we continue with the “what is life” issue. A general definition that’s able to distinguish all imaginable living objects from the diversity of non-living objects remains elusive. Even today, we know relatively little about the cellular life on Earth. It’s been said that a shovel full of soil would contain hundreds of microbial species that are unknown to science. That’s not to mention the vast range of plausible non-cellular life forms that might be discovered elsewhere in the universe.

I have to conclude that endorsing any sweeping definition of life based on so little knowledge is like trying to define music after listening to a single Elvis Presley song.

Top-Down and Bottom-Up

So, what do we do? As you can imagine, scientists crave a definition of life. Such a definition remains elusive, but they adopted two complimentary approaches in their efforts to distinguish that which is alive from that which is not. On the one hand, many scientists have adopted the top-down approach. Top-down refers to the effort to scrutinize all modern living organisms and fossil entities to identify the most primitive forms that are or were alive. It turns out that primitive microbes and ancient fossils have the potential to provide relevant clues about life’s early chemistry.

I must say that I find this top-down strategy inherently limited. At least so far, all known life forms are based on biochemical sophisticated cells containing complex molecules, including DNA and proteins. Any definition of life based on top-down research is limited to what appears to be modern biochemistry.

By contrast, a small but determinate army of investigators adopt the so-called bottom-up approach. The principal objective of bottom-up researchers is to device laboratory experiments to mimic the emergent chemical process of environments in the ancient Earth. Ultimately, the bottom-up goal is to synthesize a self-reproducing chemical system in the laboratory. That’s an effort that might help clarify the ancient transition from non-life to life.

You might think that all bottom-up researchers hold a common view of what would constitute the first synthetic life form, but research actually leads to an amusing range of diverging opinions regarding what’s alive. Each scientist has a tendency to define life primarily in terms of his or her own chosen chemical or biological specialty. One notable group focuses on the origin of cell membranes. To them, life began when the first encapsulating membrane appeared.

Other well respected research teams study the emergence of metabolic cycles. Those are the process by which cells gather and use atoms and energy. Naturally, for them, the origin of life coincided with the origin of metabolism. Lots of other groups investigate the origin of primordial RNA, which many experts consider to be the first genetic material. For them, the origin of RNA is equivalent to the origin of life.

There are many other workers who focus on viruses, minerals or even artificial intelligence; and each researcher advocates his own definition of what constitutes life.

NASA’a Definition

Into this mix, quite a few philosophers, theologians and science fiction writers have injected a variety of more abstract views and speculations on the possible phenomena that might said to be alive. The possibilities seem endless: counscious clouds in space, high temperature silicate minerals, a self aware internet. Such proposals sound at times farfetched, but there is so much we don’t know.

Consequently, the scientific community, with the support of NASA and other governmental agencies, holds regular meetings to explore the definition of life. After all, if one of NASA’s primary missions is to look for life on other worlds, then we’d better have a clear definition for planning future missions. It’s amazing how the “what is life” question sparks passionate arguments.

Gerald Joyce, a biologist working at the Scripps Research Institute, developed a widely accepted definition for life, at least in the context of NASA’s space exploration. He concluded that “life is a self-sustained chemical system capable of undergoing Darwinian evolution”.

According to this opinion, life incorporates three distinctive characteristics. First, all life forms must be chemical systems. That means that computer programs or robots are not alive. The second characteristic is that life grows and sustains itself by gathering energy and atoms from its surroundings. That’s the essence of metabolism. Finally, all living entities must display some sort of variation. According to the concepts of Darwinian evolution, natural selection of more fit individuals inevitably leads to evolution and the emergence of more complex entities. A system that does not have the potential to evolve does not fit this definition of life.

There’s still so much we don’t know, but this NASA inspired definition is probably as general, useful and concise as anyone is likely to come up with, at least until we discover more about what’s actually out there.

What is Life

We usually think that life is easy to recognize, that it would be obvious if something is alive or not. It turns out that is not that easy. The question “what is life” is asked in very different contexts by different groups of people. For centuries, theologians have hotly debated life’s definition and relation to the beginning of human life. Does life start at the moment of conception? Or does it begin when the brain’s first response, or with the heart’s first beat? In some theological doctrines, life commences not with a physical process, but rather at the unknowable supposed instant known as “ensoulment”.

At the other end of our human journey, doctors, lawyers and politicians require a definition of life in order to deal ethically with patients with brain death. As we saw with the contentious case of Terri Schiavo (the woman who spent more than a decade in comma), lots of people have intense and emotional views on this issue.

In sharp contrast with these ethically difficult and emotionally charged issues, are the more abstract ongoing scientific efforts to define life. A must read book on the origin of life is Noam Lahav’s “Biogenesis”, which was published in 1999. Lahav’s works in the Hebrew University at Jerusalem, and he has been involved in origins research for almost 40 years. His book is filled with insights, as well as countless technical details. As part of his text, he prepared an appendix with lots of different scientific definitions of life, which are written by over 48 different authorities.

These definitions span 150 years period, from the mid 18th century to the late 20th century. It’s worthwhile thinking about a few of those:

- Alexander Oparin: he reflects the view of many authorities. Life can be defined by a combination of traits. He says: “Life may be recognized only in bodies which have particularly special characteristics. These characteristics are peculiar to living things, and are not seen in the world of the dead.” What are these characteristics? In the first place, there is a definite structure or organization. Then there is the ability of organisms to metabolize, reproduce others like themselves and the response to stimulation. The problem is that Oparin’s characteristics are not unique to life. Many non-living systems have definite structure and organization (think about your car or your PC). Oparin says that organisms obtain energy from their surroundings to grow and reproduce, but fire does that also. Many natural non-living systems, such as flowing water or drifting clouds, respond to stimulation.

- John Desmond Bernal: an influential 20th century biological theorist, who provides a longer list of characteristics. He says: “Life is a partial, continuous, progressive, multi-form, and conditionally interactive self-realization of the potentialities of atomic electron states”. I don’t know about you, but that definition seems to be hopelessly fussy and unhelpful in distinguishing life from non-life.

- Stuart Kauffman offered a more promising definition of life in 1993. He claimed: “Life is an expected collectively self-organized property of catalytic polymers”. Embedded in this statement are a couple of key ideas. Kauffman said that life is self-organized. That is, life is a collective emergent phenomenon. He also states that life relies on chemicals to promote the production of more copies of themselves. In Kauffman’s view, life might be a relatively simple collection of self-replicating chemicals. That includes much more primitive entities than modern cellular life.

- John Maynard Smith proposed a short and persuasive definition of life in 1975. He describes life as “any population of entities which have the properties of multiplication, heredity and variation”. Here Smith introduces two key ideas and thus comes closer to a useful set of criteria. First, all life possesses information that’s passed from one generation to the next. That key idea of heredity may not be unique to life, but it is certainly one of life’s most important characteristics. Second, life displays variations. In life, heredity isn’t perfect like a Xerox copy. Variation, in turn, leads to evolution by natural selection.

Lahav goes on and on citing definitions of life, and remarkably, no two definitions are the same. I think you can see this lack of agreement might represent a problem for those of us who search for signs of living organisms in other worlds, as well as for anyone interested in the origin of life. After all, how can you be sure that you discovered life, or that you figured out the process of life origin, when you can’t come close to defining what exactly life is? In spite of generations of work by hundreds of thousands of biologists, we still have no universally accepted definition.

To be continued…

In the Beginning

In the beginning... there was a singularity. Physicists tell us that the universe, as we know it, began between 10 and 20 billion years ago, at a moment in time they call the Big Bang. Our own star is comparatively young. Estimates are that it formed about 5 billion years ago. As our solar system was forming, cosmic dust gradually got swept up and began to form planets. Scientists estimate that our own planet reached its present size at about 4.6 billion years ago. That is generally taken as the age of the planet Earth.

In the beginning, planet Earth was a really miserable place. The way that the planet was formed, with ever larger and larger chunks of material slamming into it, created an enormous amount of heat. When the planet first formed it was melted. It was no place where one could ever conceive of life existing. Less than a billion years later, however, the fossil record clearly shows that life was there. This life was in the form of simple cells that resembled the bacteria we see around us today.

This is pretty fast work, especially when you consider that it took about a half a billion years just for the Earth to cool enough to actually have rocks and an atmosphere. In fact, some scientists now argue, based on fossil evidence, that life might have been present even earlier, as earlier as four billion years ago.

What we can take from this is that life appeared on the planet almost as soon as it was possible to do so. As soon as there were rocks to record the existence of life, we find evidence that life is there.

Where do these organisms come from?

In the beggining, life originated on the early Earth from non-living materials. All of the diverse forms of life we see around us today have arisen from some common, primitive, single original living entity. This is pretty deep stuff, a very cool idea.

There are alternatives to this account, of course. Many religious faiths hold that, in the beginning, life was bestowed on the planet by the work of a deity, but this is a pretty boring idea. Another alternative, one that has been suggested repeatedly over the years by a number of scientists, is the panspermia hypothesis. It suggests that the first life on Earth came from somewhere else in space.

Both of these alternatives, however, beg the question of how living matter could arise from non-living matter. That brings an important question into the table.

What’s the minimal difference between living and non-living materials? This is basically the same as asking “what is life?”. This question has been around for a long time. For me, however, with all the knowledge we have today at our disposal, to address this question is pretty simple.

So, what is life?

Life is defined by what is called organic chemistry. The most fundamental difference between living and non-living matter has to do with chemistry. Living things all have in common the fact that they are made of a particular class of chemical compounds. These are compounds that are built around the unique chemical properties of the element carbon. These kinds of compounds are called organic compounds. They are called that way because they are uniquely associated with living organic things.

There are only four kinds of organic compounds, broadly speaking. The first kind are amino-acids. These are the things that make up protein. The second kind of organic compound are the nucleic acids. These nucleic acids are DNA and RNA. The third class are the carbohydrates. These are what we commonly call sugars. The last general class of organic compounds are the lipids. Lipids are what we commonly call fats in many cases, but lipids can actually take a number of different forms.

These organic compounds have particular and quite sophisticated chemical properties that are unique to them. There is one property that is particularly remarkable, that is that the complex organic compounds that we find on the planet today, the stuff we are made of, is generally only produced through the action of living things. Another way to put this is that the creation of new organic matters depends on the existence of organic matter. You can’t make more organic compounds unless you got compounds to make them.

We can be quite confident given what we know about how the planet was formed the early Earth was entirely inorganic. Then, we have to ask: Where did the organic compounds that life depends on came from in the beginning? At this point, you might think that I’m going to throw Intelligent Design and Creationist arguments at you. I won’t, don’t worry. I’ll create some tension and leave that question unanswered, until next time, when we talk about the exciting, random and unintelligent origin of life.

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