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.

The Origin of Life, Part III: Primitive Cells

The experiments of Miller, Fox, Ferris and others had shown that complex polymers could arise spontaneously on the early Earth. We know, however, that the organic molecules that make us up are not just a jumble of things floating around in a primordial soup, they are highly ordered. They come in highly ordered packages. There are many such packages in living systems, but the most fundamental one is what we call the cell. All living things are made of units called cells. Minimally, for something to be living, requires a barrier between the living part and the non-living part. That barrier is what would define the cell.

Is it possible that some cell-like structure could arise spontaneously on the early Earth? Here, too, laboratory experiments suggest that the answer is yes. A number of experiments have been done that demonstrate, under conditions that are not too rigorous, that you can get aggregations of molecules that would spontaneously form cell-like structures.

This kind of spontaneously made cells are called protobionts. You can actually make protobionts, it is not difficult to do. You can make them under a number of different kinds of conditions. For example, if you have the right kind of lipids, you can almost literally put them in water and they spontaneously form a package where there is a membrane of lipids that encloses some central space.

The most remarkable kind of protobiont, called coacervate, is one that has been made to self-assemble out of a solution that includes polypeptides, nucleic acids and polysaccharides. If you have the right conditions, you can make these to self-assemble into a cell-like object. What is really interesting about coacervates is that if you then throw into the mix some real biological molecules, a protein enzyme that you’ve taken from a real living cell, for example, the coacervates can take up those enzymes. They would bring them inside of themselves.

Those enzymes would start working inside the coacervates. What enzymes do is to process some kind of biological molecule into another. Once these enzymes have been taken up by these coacervates, it would also start doing the reactions and putting out the products. This is really getting remarkably close to something that we might want to call living.

I don’t say that we can make primitive cells. Nobody has actually made a cell that any biologist would look and say “oh, that’s a cell you just made”. People are trying to do that now, but it hasn’t been done yet. We can, however, make cell-like things and it doesn’t seem to be any big trick. These things spontaneously form, we know that for sure.

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