January 29, 2013 by Pyers Symon
The origin of life is one of the few remaining big questions left in science, something that the creationist loons frequently remind us of, but progress is being made on all fronts and, most importantly, attempts are being made to put life’s origins on a sound theoretical basis – something that has been to a large extent missing.
Firstly a very difficult question: what is life ? In the 1940’s Erwin Schrodinger who was at the time a refugee in Dublin (initially he had fled to Oxford but his perceived unorthodox private life – he lived in a threesome – wasn’t appreciated by the locals!), wrote a book “What is life?” [i] in which he stated that life is a physical event by which I mean that the same guiding laws of physics (even though that Schrodinger said that some of these physical laws had yet to be discovered!) are applicable to biological entities as much as anything else. More recently Addy Prost, an Israeli professor of chemistry, has written an update to Schrodinger called “What is Life?: How chemistry becomes biology” [ii] (a book I will be referring to again) where he argues that a new branch of chemistry – systems chemistry – provides deep insights into both life and its origins, especially his contention that life follows a path which he terms “Dynamic Kinetic Stability” or DKS.
So where do we start? Firstly by looking at the conditions that prevailed in the prebiotic Earth. Popular imagery has a fiery landscape, full of volcanoes and molten lava. This is almost certainly wrong and in fact it is probable that liquid water existed on the planet within a few hundred millions of years after its formation because the visible and heat radiation from the Sun was 25% lower than it is now (in fact a lot of research is going on as why liquid not frozen water was present [iii] but that is another story!) This happened after what is known as the “Late Heavy Bombardment” when the Earth has hammered by rocks, some vastly bigger that the one which punched a hole in the Yucatan 65 million years ago. Water seems to have appeared from comets (which might have been part of the LHB) and volcanoes forming primitive oceans .
Over 50 years ago - almost as a bit of fun, Stanley Miller, a PhD student at the Uni of Chicago, asked his supervisor, the Nobel Prize winner Harold Urey, did one of the most famous experiments in our story. As is well known, he took chemicals that he considered might be present in the atmosphere of the early Earth, and rather like Macbeth’s witches, boiled, sparked and UVed them and stood back and watched what happened. Within a few days, a brown brew appeared which he then analysed. Various biological chemicals were found – glycine ( the simplest amino acid ) plus one of two other chemicals of interest. In fact, later analysis (50+ years later using much more sensitive equipment) showed that ALL 20 of the amino acids found in proteins – plus a few other.
In fact the Miller-Urey experiment is an early example of what is known as “systems chemistry” which is effectively throwing all the ingredients of a reaction together and seeing what happened……
However famous the Miller-Urey experiment is, its fame rests mainly on being the first attempt at such an experiment – some of its assumptions are now considered wrong. So we need to look at some basic biology.
What is Life?
We all know what life is – don’t we? But it is actually rather difficult to pin down. I remember an acronym used in schools (repeated in Adam Rutherford’s dual book Creation Origin of Life/Creation the Future of Life) called MRS GREN
M Movement - All living things move, even plants
R Respiration - Getting energy from food
S Sensitivity - Detecting changes in the surroundings
G Growth - All living things grow
R Reproduction - Making more living things of the same type
E Excretion - Getting rid of waste
N Nutrition - Taking in and using food
Actually, all of these refer to “Life as we know it, Jim” and it is rather simplistic when we are dealing with abiogenesis since somewhere we need to split non-living (or a better phrase might be “proto-living”) from living. This is where the tricky, slippery concept of entropy comes in as mentioned earlier. Entropy is a measure of randomness. The way the universe works is simple: entropy (randomness) in the universe is increasing. Everything runs downhill, chaos – like my house – is increasing, and will always happen until the end of the universe. But – and it’s a BIG but – the 2nd Law has a couple of rather big get out clauses: it’s a statistical law - local variations can occur, entropy can be reduced locally as long as there is a corresponding increase elsewhere. In our case, we reduce energy by taking energy from the sun – and it is the sun’s entropy that is increased. Now since the sun is just so big, it can absorb a lot of entropy increase over a very long time! (as an aside : it’s not the energy itself that is important; it is the energy flow. If the earth had the same hemisphere pointing at the sun, this would become, apart from bloody hot, a steady state. It’s the fact that the planet absorbs energy during the day and releases it at night. We just grab a bit during that flow. Think of a waterfall, flowing between the top [day] and the bottom [night]. If the water stayed at the top all the time, it would be useless, but when it flows over the falls we can capture some of its energy with something like a turbine).
The second important cop-out is that the 2nd law applies only to closed systems. The Earth isn’t a closed system – its an open one and allows, from life’s point of view, energy flow between systems (oceans and atmosphere; earth and moon [tides] etc )…..
In fact one definition of life is “an entity that actively causes a local decrease in entropy which allows an increase in complexity” (Other definitions exist: one nice one is “a self-sustained chemical system that is subject to evolutionary pressures and changes.” The problem with this definition, from NASA, is that it seems to refer to individual life forms, whereas Darwinian evolution exerts its influence upon populations.)
All the above does is to say that life doesn’t break any physical laws – is bends them, twists them, uses them, blindly, to its advantage but doesn’t actually break them.
So where do we start? Firstly we need to look at some of the issues:
· When did chemistry become biology?
· The chicken and egg: DNA codes for proteins but proteins are needed for DNA control. So how is that circle broken?
· Why is life chiral? Most ( and pretty well all beyond the simplest ) organic molecule exist in mirror images.
· Origin of the genetic code
When did Chemistry Become Biology?
The Late Heavy Bombardment stopped around 3.8 billion years ago and yet, within a very short space of time (say 500 MY ) well formed life had appeared on Earth since the oldest fossils, stromatolites, date from that period.
“But if (and oh what a big if) we could conceive in some warm little pond with all sorts of ammonia and phosphoric salts,—light, heat, electricity &c. present, that a protein compound was chemically formed, ready to undergo still more complex changes, at the present day such matter wd be instantly devoured, or absorbed, which would not have been the case before living creatures were formed.”
Charles Darwin, Letter to Joseph Hooker, 1 Feb 1871
Our first problem with abiogenesis is simple: what is life? We all know a living thing when we see it but to actually pin it down is rather tricky. Biologists tend to discuss things like the physiological functions (reproduction, metabolism), whether the subject is susceptible to evolutionary pressures and the like. Physicists refer to that property of life that it is organised and seems to go against the Second Law of Thermodynamic’s (it doesn’t of course – despite what some creationists still say – since the 2nd Law refers to closed systems whereas we exist in an open system). In fact one definition of life is an entity that is able to organise itself (ie decrease its internal entropy) by using free energy that is provided by the sun. What is noticeable is the relative lack of input, until recently, from chemists!
These definitions are important when dealing with the origins because there seemed to be a smooth transition from non-living chemical systems to living ones. We know this because of the remarkable similarity between all current life forms at the molecular level: the universal use of DNA as the genetic material (I am forgetting viruses before anyone asks!); the fact that the genetic code (the mapping of DNA triplets to amino acids) is the same throughout modern living systems; the chirality (handedness) of life; use of glucose as a fuel etc etc.