Pondering unanswered questions of biology

Many huge questions remain to be answered in biology. These include: How is the process of development from embryo to adult programmed and controlled? What is the molecular basis of brain function? What is the molecular basis of consciousness? How did the first living cell arise on Earth?

I will discuss aspects of the last question in this article, and I will return soon to the question of consciousness. (We deal with no rubbish in this column!).

The basic unit of all living organisms is the cell. Chemically the cell is composed mainly of nucleic acids, proteins, fats and carbohydrates.

The smallest parts of these various compounds that can exist are giant molecules - macromolecules. The macromolecules are mostly polymers, i.e. are made of many repeating units (monomers) of a simple type of chemical compound.

Thus, protein macromolecules are made of repeating units called amino acids, carbohydrate macromolecules are made of repeating units called sugars, and nucleic acids are made of repeating units called nucleotides.

Life is based on the element carbon, i.e. the various monomers and macromolecules in the cell all have carbon backbones. Simple carbon-containing molecules are distributed on a widespread basis throughout the universe and, where conditions are suitable, these molecules combine and con dense into the simple monomer biomolecules on which life on Earth is based.

For example, the Murchison meteorite that fell in Australia in 1969 contains 10 of the 20 amino acids found in the proteins of living organisms.

Conventional scientific wisdom now predicts that conditions on the early Earth were such that the essential biomolecules present in life today spontaneously formed and dissolved in the oceans. These compounds then interacted with each other over millions of years in a process of chemical evolution which produced the full range of macromolecules found in the living cell today.

Fats formed fatty membrane bubbles which enclosed various combinations of these macromolecules and biomolecules, until eventually the first living, self-replicating cell was born. This cell contained a genetic programme which specified key elements of the cell's activities and this programme was copied and transferred to each new generation of the cell that arose.

All the diverse variety of life on Earth today is descended from that original cell.

There are two parts to this overall scenario. Firstly, we have a living cell spontaneously arising from a chemical soup after a long period of chemical evolution. Secondly, we have the original life-form evolving into many different species of life over billions of years.

Our present level of scientific knowledge gives us every reason to expect the second development (evolution of diversity from the first simple form), but little reason to expect the spontaneous rise of life from the chemical soup.

Charles Darwin and Alfred Russel Wallace provided the answer to the second part of the scenario - how diversity arises out of simplicity - with the theory of evolution by natural selection.

They noted that natural variety exists amongst the members of any species, and that only a fraction of the young of any species survives to reproduce.

They reasoned that those members of a species with characteristics particularly suited to the environment would survive longer and have a better chance of reproducing than other members of the species with less-well-suited characteristics.

These better-suited characteristics would therefore spread through the population at the expense of more ill-suited characteristics and would produce a species "designed" to fit the environment.

So, start out with a simple form of life, add enough time and it is inevitable that alligators will wallow in swamps, elephants will roam plains and humans will write articles.

BUT THE first part of the scenario remains largely a mystery. It is true that ingenious experiments have shown that if you simulate the conditions thought to exist on the early Earth, many or most of the basic monomer biomolecules are spontaneously formed. But we have not gone much beyond that.

And it is a staggering leap in complexity to go from simple biomolecules to the immense complexity of the simplest conceivable living cell. Many scientists have considered the probability that life would arise in this manner only slightly over a billion years after the formation of the Earth.

Fred Hoyle, the famous British astronomer, declared that the likelihood that life arose spontaneously on Earth in this timeframe is about as great as the likelihood that a tornado rushing through a junk-yard would spontaneously assemble a working jumbo jet.

The spontaneous rise of life on Earth from a chemical soup does not violate any physical principles. If it did, science could not advocate the idea.

The second law of thermodynamics says that disorder inevitably increases with time in a closed system. Life is the opposite to disorder, so how could it rise up in time?

Well, the early ocean was not a closed system. It was an open system exchanging both matter and energy with the outside. Much external energy must be supplied in order to form and maintain life, and life on Earth is entirely dependent on the energy of the Sun.

When energy is supplied from the outside it can be used to do work and cause entropy to decrease locally - this is what happens when you tidy your office. However, the energy expended in tidying the office is used purposefully by you. In your absence the Sun could shine forever through the window and no tidying would occur.

Because something can happen does not mean that it will happen. With our present level of scientific understanding the improbability of life spontaneously arising from a chemical soup seems staggeringly large.

Nevertheless, science does assume that this is what did happen. Something akin to an act of faith is involved here.

But science is full of examples where what appeared to be extremely complex matters turned out to have simple explanations, e.g., the mechanism of inheritance.

It may well be that science has yet to discover a new law of thermodynamics that will demonstrate how complexity inevitably increases spontaneously under certain circumstances in some open non-living systems fuelled by an external source of energy.

William Reville is a senior lecturer in biochemistry and director of microscopy at UCC