The Origin of Life research project
Michael J. Russell & Allan J. Hall
This is an explanation of how life might have originated. It is written for non-specialists. A detailed account was published in 1997 in the Journal of the Geological Society of London, an appropriate journal because we consider that a major geological process, the cooling by seawater of rocks under the floor of the ocean, played an important role in the origin of life. Such a process might seem remote from our everyday knowledge of life but it has now been known for more than twenty years that genetically primitive micro-organisms are to be found living at warm springs on the ocean floor.
The challenge of this research is to explain how a relatively simple 'living' organism similar to a single bacterial cell could form, function and reproduce.
A 'living' cell assimilates nutrients, uses energy and generates waste. It consists mainly of carbon-based (i.e. 'organic') molecules that also contain hydrogen and other elements.
The defining structural feature is a mainly waterproof container, the cell membrane. Inside is a watery solution with a high concentration of organic molecules as well as some inorganic salts.
A simple living cell need not have a nucleus of concentrated genetic material, DNA, but it does require the presence of this large molecule.
DNA consists of two molecular chains, long sequences of simpler molecules, that can detach from each other like an unzipping action, each chain then becoming a template for the assembly of a new chain.
DNA can therefore reproduce itself. The molecular sequence of DNA controls the systematic construction of all the organic components of the cell most of which are 'renewable', that is, they degrade and the molecular building blocks are recycled.
A living cell therefore recycles organic molecules as well as producing and accumulating them. It is therefore rejuvenating, a remarkable property that gives single cells longevity. Cells can grow in size and reproduce, for example by splitting in half, each daughter cell carrying a copy of the original cell's DNA
There have been many changes in the structure of cells through time as life has evolved within the constraints of DNA control, but the changes are probably small in comparison to the first step from naturally produced chemicals to the first living cell. We suggest that the earliest cells functioned in a similar way to present day cells so all the main components and mechanisms had to come together at the same time in the same place.
This leads to the question of what life 'does' rather than what life 'is', and to tackle this question we need to understand natural sources of energy and what forms of energy are involved in life processes.
Consideration of other natural phenomena in relation to energy can help us understand life and its initial requirements:
- What does a waterfall do?
It drains water from a higher to a lower height, giving the water a lower gravitational energy.
- What does a warm spring do?
It is a plumbing system that drains warmth (thermal energy) from underground and dissipates it on the surface.
- What does life do?
It is a chemical system that drains and dissipates chemical energy. For example, animals can gain chemical energy from sugar in food and inhaled oxygen, a process known as respiration.
Present day life depends to a large extent on solar energy that drives the chemical systems of green vegetation. Plants use water and carbon dioxide from the air to produce organic molecules with oxygen as a waste.
We consider that the first living cells formed on the floor of an ocean on the earth thousands of millions of years ago. Life 'emerged' at the sites of warm submarine springs where chemical energy was focused and the mixing of spring water with seawater could lead to the precipitation of chemicals.
It is now well established that this process can lead to metal-rich mineral deposits, often containing iron sulfides but with very variable chemistries.
How could the first cell have assembled itself in such a setting on the early Earth or on any similar stony, wet and sunny planet?
The precipitation of chemicals on mixing of solutions can form a barrier preventing further mixing and precipitation.
At the warm spring we envisage the formation of a special precipitate that not only formed a boundary that inhibited mixing but also provided a template for the assembly of chains of organic molecules, and acted as a catalyst for electrochemical reactions.
The initial membranous precipitate consisted mainly of small groups of iron and sulfur atoms. Iron-sulfur groups still play an essential electrochemical catalytic role in all living cells. Our research has focused to a large extent on the origin, nature and role of iron sulfides.
As a boundary, the precipitate concentrated organic molecules such as amino acids. These formed at depth below the spring where water and its dissolved chemicals reacted with rocks containing iron and iron-rich minerals.
The boundary also concentrated other chemicals that could participate in chemical reactions. But eventually the boundary evolved by a process of 'organic take-over' into a cell membrane consisting of organic molecules.
As a template, the iron sulfide precipitate that consisted of small crystals of only a hundred atoms or so, could bond chemically to, and assemble a sequence of, the molecular components of RNA, a chain molecule which is very similar to DNA and which plays a supporting role in genetic evolution.
Once organised on the iron sulfide, the RNA could influence the assembly of amino acids into proteins, the assembly of further chains of RNA, and the assembly of DNA. Eventually, these new large organic molecules could reproduce themselves through the interaction of DNA, RNA and proteins, without the requirement of an iron sulfide template.
As a catalyst the groups of iron (and nickel) sulfides could activate molecular hydrogen (and probably methane which consists of carbon and hydrogen) which also formed at depth in the spring. The hydrogen is essential for the synthesis of organic molecules.
Electrons produced as a by-product (and representing the dissipation of energy) were transferred to a type of iron, known as ferric iron, dissolved in seawater The ferric iron was produced from dissolved ferrous iron (richer in electrons) at the ocean's surface by sunlight (solar energy).
The same processes cause the reddening of the surface of Mars as iron-bearing minerals have 'rusted'. This is one of the reasons we believe life would have emerged on Mars too.
In summary, we see life as having resulted from the interaction of warm sulfurous springs on the ocean floor of a young Earth.
The springwater contained hydrogen and key organic molecules produced at depth by reaction of water, carbon-containing gases such as carbon monoxide, and iron-rich minerals.
To confirm the generation of the organic molecules in this environment is perhaps the most difficult aspect of the research. Experimental demonstration of organic synthesis in such special conditions is extremely difficult.
The seawater was acidic due to a high concentration of carbon dioxide and it contained ferrous and ferric iron, the former mainly from hot springs and the latter because of the oxidation of ferrous iron by sunlight.
An important chemical precipitate that formed on mixing of the spring water and the seawater was iron sulfide.
This provided the focal point for the development of the complex organic molecules that interacted to generate independently existing and reproducing cells.
The first living cells could function and grow using carbon from carbon monoxide, methane and/or carbon dioxide dissolved in seawater. They gained chemical energy from linking molecular hydrogen being emitted by the warm spring and ferric iron dissolved in seawater.
Other nutrients such as phosphate, nitrogen in the form of ammonia, and trace elements were available in the same environment.
Genetic evolution eventually permitted life to escape from a dependence on ocean floor chemical energy and a later major step was the use of solar energy, a process known as photosynthesis.
The surface of the early Earth was very different to that of today. The atmosphere, ocean and ocean floor were different chemically, as were the few land-masses. These have changed chemically through time, especially as a result of oxygen in the atmosphere which was only produced once the mechanism of oxygen-producing photosynthesis had evolved.
It is therefore unlikely that life could still be emerging at springs on the ocean floor.
We are using a knowledge of geological processes that lead to ore deposits of metals such as lead, iron, zinc and gold to understand what warm metalliferous springs would have been like under the ocean of the early Earth.
We also use theoretical chemical calculations using sophisticated software packages of the type used to understand fluids in oil reservoirs and the behaviour of toxic wastes in groundwater.
And we also use our understanding of how microbial organisms have evolved and interacted with natural chemical processes during the four thousand million years or so that life has existed on the Earth.
Further information and a detailed up-dated explanation are given in our illustrated web pages at: www.gla.ac.uk/projects/originoflife
Russell, M.J. and Hall, A.J. (1997) The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front. J. Geol. Soc. London. 154, pt 3, 377-402.
Russell, M. J. and Hall, A. J. (1999) On the inevitable emergence of life on Mars. In: J.A. Hiscox (ed), The Search for Life on Mars. The British Interplanetary Society. 26-36.
Russell, M.J., Ingham, J.K., Zedef, V., Maktav, D., Sunar, F., Hall, A.J. and Fallick, A.E. (1999) Search for signs of ancient life on Mars: expectations from hydromagnesite microbialites, Salda Lake, Turkey. J. Geol. Soc. London. 156. 869-888.
Michael J. Russell,
Allan J. Hall,