SURELY THE most fundamental problem in biology is to explain how life spontaneously arose on earth approximately 3.8 billion years ago. Although relatively little progress has been made in this field, research occasionally throws up exciting suggestions. Recent research, described by Alexander Bradley in Scientific American, December 2009, suggests a mechanism whereby life could have originated near hot ocean-bed volcanic vents, writes WILLIAM REVILLE
In the 1920s, Russian biochemist Alexander Oparin and British physiologist JB Haldane suggested that the earth’s early atmosphere contained much hydrogen-rich gas, such as ammonia (NH3), methane (CH4), hydrogen (H2) and water (H2O), and the intrinsic chemical energy and reactivity of such a system could have formed the basic organic chemicals necessary for life. In 1953 Stanley Miller and Harold Urey at the University of Chicago bolstered this hypothesis when they heated and electrified a mixture of these gases in a flask for days, thereby creating a range of organic compounds, including amino acids, basic to the biochemistry of life.
In the meantime, geologists have decided that the likely composition of the primitive atmosphere was considerably different to that surmised by Oparin and Haldane and not conducive to the formation of organic compounds by the mechanism demonstrated by Urey and Miller. Yet, as Bradley describes, conditions similar to those envisaged by Oparin and Haldane do exist at recently discovered under-sea hydrothermal vents.
The thin crust of the earth floats on a pliable mantle of magma. Cracks divide the crust into tectonic plates that move on the underlying mantle. In places, adjacent plates pull apart and the underlying magma wells up to the surface. This happens along a great crack under the Atlantic Ocean called the Mid-Atlantic Ridge, as Africa and North America move slowly apart (25mm per year). Volcanoes run along the ridge and water temperature at these hydrothermal vents can reach 400 degrees, heated by molten rock. The water is very acidic and leaches sulphides of iron, copper and zinc from the volcanic rocks. When this water hits the cold sea-water the dissolved sulphides precipitate out of solution, producing a cloud, hence the name for these vents, “black smokers”.
A new hydrothermal vent system called Lost City was recently discovered 15km west of the Mid-Atlantic Ridge, on top of an under-sea mountain called Atlantis – hence the name Lost City. The hottest water temperature recorded here is 90 degrees and the water is alkaline in contrast to the acidic “black smokers”. This water is rich in calcium and, when it mixes with cold sea-water, calcium carbonate (limestone) forms and builds up as giant chimneys that tower 60m above the sea floor.
The water rising from the vents contains no oxygen gas but is rich in hydrogen, methane and sulphide (10 – 100 times richer than the “black smokers”). It has been shown that such highly reduced conditions support chemical reactions, not dependent on a biological source, that make organic compounds from inorganic compounds, eg formate (HCOOH) and acetate (CH3COOH) – a critical step that must have preceded the origin of life. The carbon dioxide needed to make these hydrocarbons comes from naturally occurring CO2 locked in mantle rock. On earth, the rock of the upper mantle is called peridotite. When it is transported to the surface and reacts with water it is converted to serpentinite and this drives the processes described above.
Bacteria live in the Lost City vents. One such group, the methanogens, generate methane and live entirely independently of the energy of the Sun. Methane, as already described, is also produced abiotically at Lost City by ordinary chemical reactions in the rocks and, interestingly, the biochemical pathway used by the methanogen bacteria to produce methane seems to be identical to the sequence of chemical reactions whereby methane is produced geochemically. It seems likely therefore that when primordial life arose it simply co-opted the chemical steps already existing in the rocks and this is how the first biochemical pathways arose.
There is further powerful evidence to support the hypothesis that life arose in such an environment. Genetic analysis of existing life forms allows biochemists to build a family tree showing the relationships of all life on earth. Many of the organisms near the root of the tree inhabit extreme environments, such as the methanogens that live near the Lost City hot vents. This suggests that the last universal ancestor of all life on earth lived in such an environment.
Readers who want details of this fascinating area should consult an article by Nick Lane, New Scientist, October 19th, 2009.
William Reville is University College Cork’s associate professor of biochemistry and its public awareness of science officer– http://understandingscience.ucc.ie