Life in the Beginning
This is not just one of the most profound scientific questions - but perhaps 'the' most profound question a human can ask!
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What is Life |
How Did Life First Appear
There is no good reason to expect that the principles of evolution are not as ubiquitous in the universe as the principles of, for instance, gravity. Despite the fact that the early earth was bombarded by asteroids, had extreme volcanic activity, was hot, alien and devoid of oxygen, we expect life to happen. And we expect evolution to happen.
In general we can say that with the right ingredients (water, hydrogen, ammonia, methane), in the right quantities, at the right temperature, sufficient amounts of time and an input of energy (lightening or maybe, and increasingly the number one suspect, heat from submerged volcanic vents), random chemical reactions can produce molecules that are capable of making copies of themselves by triggering further chemical reactions. These replicating molecules are the building blocks of proteins and nucleic acids and were able to 'feed' from the rich chemicals emitted from land based or hydrothermal vents.
Once the 'first-replicating' molecule was created from these purely random chemical reactions, subsequent developments were no longer based on chance. Instead, the theory of evolution by natural selection came into effect. As we would expect, molecules that could replicate began to increase in numbers. And the more 'efficient' replicators, those that could compete for the available food and energy in a particular environment, began to flourish, dominate and leave the most descendents. The less efficent died out.
However, modern cells are very complex. They consist of sophisticated molecules. The ones that get most the attention are, DNA, which stores the genetic information, and its close chemical cousin, RNA, which performs the difficult operation of translating this information into protein products. Because of their complexity, its unlikely they could have assembled spontaneously from the chemicals available at that time. Instead life must have started in the simplest possible way.
Okay, so we know that simple chemical elements, Hydrogen and Helium, were created in the Big Bang. And the nuclear fusion process that occurs in stars creates other elements. These elements are in a plasma state when contained within the confines of a star. But when a star dies and explodes in a nova, these elements are ejected and often combine in interstellar clouds. With this life and death process of stars and the subsequent process of chemical evolution in interstellar clouds, complex molecules and compounds can and will develope from these simplier chemical elements.
Recent discoveries confirm that this is a universal process, as we have already discovered complex carbon-containing molecules in interstellar clouds of gas and dust thousands of light years from earth. These are the raw materials of life itself. There presence in environments where temperatures are just a few degrees above absolute zero has changed the belief that large organic molecules, with hundreds of chained carbon atoms, could only develop in hot molecular environments such as on earth.
We know that the early earth was full of simple molecules. And we know that chemical reactions take place when molecules come into contact with each other, in adherence to the laws of chemistry. Molecules can combine to form larger more complex molecules or they can breakdown yielding different molecules. Whether they come into contact with each other or not is based on a randomness in collisions. But the outcome of these collisions is not random but based on the laws of chemistry. So simple atoms and molecules can be transformed into the more complex molecules required to produce life. Complex structures can be built naturally from simplier ones, given enough time.
Carbon is an atom that deserves a special mention as it has properties that causes it to tend to cluster in 'chains' and 'branches' allowing other molecules to cling onto those branches which in turn allows the complexity of the molecular structure to increase.
As some molecules grow in size as a result of attracting other molecules they eventually reach a certain critical size where the bonds holding them together weaken and the molecule splits, in some cases ending up with almost two identical molecules. Each of these two molecules tends to attract the same types of molecules from around them (due to their chemical properties). Some do it successfully, some do not. For the ones that do, as they grow and reach the critical size they also split and end up as almost two identical molecules. and so on. Life could have started in this simple way. A cycle based on a natural chemical reaction that repeated itself, producing by-products that helped sustain and develop the cycle.
What we are seeing here is molecular replication, which is just basically chemicals reacting as chemicals do. We are looking at a primitive form of reproduction. Molecules that are most successful at this tend to increase in number in the population at the expense of molecules that are poor replicators. Another way of looking at this (from a biological instead of chemical point of view) is that the larger molecules that attract the smaller molecules to them, and hence grow, could be regarding as 'feeding' on the smaller molecules. So these replicating molecules appear to have a tendency or a desire to 'feed', grow, replicate and then increase their population - sound familiar? And all this at the expense of non-replicating molecules or less successful replicators. Fundamentally, this is a very similar process to what we see in the plant and animal kingdom - they, or should I say we, too 'feed' (eating is essentially the same as consumimg atoms and molecules and extracting energy and nutrients), grow and replicate. So we have arrived at organic molecules which can self-assemble into microscopic structures, and which increasingly take on the properties of life. That's food for thought!
Replicating is not a black-and-white process - it has different shades - there are varying degrees. Initially the breaking down of molecules after reaching the critical size probably didn't produce anything like a copy of the original molecule. But eventually perhaps the amount of similarities increased. And at some stage the replication process produced 'better' replicators which allowed their population to begin to grow and to dominate, at the expense of less efficient replicators. It was an epic battle between replicators and near-replicators in the first chapter of life.
Why do we not see this replicating process and these replicating molecules being produced today in the environment, if it has happened in the past? Should we not expect to see these very early 'lifeforms' developing today? More than likely, for replicating life to start at that early biological level again it would probably need a lifeless earth. Anything starting out again would become 'food' at this very early stage of biological evolution for life that has already got a foothold throughout the planet, eg bacteria. So the opportunity no longer presents itself, the niche for this early biological replicating process on Earth is probably well and truely taken. Also, the conditions today or in the last few 100 million years are not conducive to that type of life starting again And just as in those times present forms of life would find the environment hostile and even impossible to survive or evolve, so too would the early forms of life find today's and recent environments hostile and impossible to survive or evolve. (Not sure if I agree with that consensus, not an entirely convincing argument, so we will revist this at a later stage!)
So while trying to determine the events that led up to the origin of life on earth is a somewhat daunting task for scientists, much progress has been made at theorising and reconstructing the process that led to these first living and replicating cells. But while we do not have a thorough account of every step involved, our understanding of how it might happen is increasing. Gaps in our knowledge are being filled in. Surely its only a matter of time before all the steps are known and can even be reproduced in our laboratories!
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