under the ground, under the sea or from above

Under the Ground, Under the Sea

The synthesis of complex hydrocarbons as well as the origins of life have tended to focus on the power of the sun to shine its UV radiation on a pond but with the discovery of an ecosystem completely independent of the suns rays have shifted the scientific view slightly in the last half of the twentieth century. The sea floor, beyond the continental shelf, tends to be a boring grey drab place with a few randomly scattered worms or brittle stars but over all its pretty lifeless, so imagine the surprise of the crew of the sub-marine Alvin in February 1977 when they discovered the chemotropic ecosystem at the volcanic vents in the middle of the Abyssal plain!

The shrimps and tube worms of this community both heavily rely on the bacteria, which brake down the hydrogen sulphide for their metabolic pathways. This food source comes from the mineral laden waters coming from the surrounding hydrothermal vents called black or sometimes white smokers (the minerals that precipitate out look like clouds of smoke!). The water is over 300 °C but the pressure of the over lying water keeps it liquid.

This meant that life could have originated at thermally active sites especially with the discover of microorganisms living deep under the ground in mines and even around geysers (this is covered in ORGANIC VS INORGANIC). However, there is a problem with the origin of complex hydrocarbons or indeed any complex large molecules at mid-ocean vents, and that is the destructive nature of the vents themselves.

Seawater is at the present day reasonably rich in amino acids but though the vent communities contain amino acids the water sampled from the area shows no amino acids or anything of significant complexity! The extreme temperatures and pressures denature the molecules as soon as the organism is no longer actively maintaining them i.e. the worms died!

One of the first islands to form on the Earth. The picture shows a bright sunny day, but the atmosphere is likely to have been dark and smoggy. (Painting by Bill Hartmann.)

This would seem to suggest then that the life existed first and then over took the niche rather than originating in it. However, this does not neccessarily mean that these processes did not place a part in the prebiotic chemistry, especially if the first volcanic islands would have been appearing with similar mineral outgassings. The deep cracks and fissures that would have occurred on the first island would have had mineral rich sea water percolating through them but at the same time would have been offering sheltered environments for delicate chemical environments to occur.

In fact even our own cells tell of a periods in our earliest evolution where Fe and S where abundant to be used for in catalysing reactions or as other component parts of metabolism.

From Above

From the Gods

A volcanic eruption on the early Earth, showing the dark smoggy atmosphere and tremendous lightning displays that accompanied it. (Painting by Bill Hartmann.)

The ancient Greeks believed that the god Zeus would throw down thunder bolts to the Earth which Hepheistus / Vulcan had moulded for him. One theory for the origin of complex hydrocarbons is that the early sea full of phosphate group and other salts would have been bombarded by lightning allow the random synthesis of amino acids and other organic compounds. This was show in Millers Life’s origin experiment where he synthesised amino acids and other organic compounds from passing an electric spark through as reducing atmosphere similar to what is thought to have existed on the early Earth (this is discussed in more detail PREBIOTIC VS THE BIOTIC WORLD).

The chances of this occurring seem remote until it is realised that the early Earth was almost continuously being bombarded by electromagnetic storms so they would have been quiet a lot of lightning!

From Beyond

Within the Oort cloud that still exists at the edge of our solar system today molecules ranging from water and amorphous carbon to glycine (the simplest of the amino acids) to hetrocyclic organic polymers. These have been detected using various absorption spectra techniques. These molecules, though shielded from the majority of the UV radiating from the sun are never the less still bombarded and this is thought to be the origin of the more complex molecules. Organic molecules actually tend to be relatively chemically inert and only react where high activation energies are available, such as shock synthesis. Living cells manage these same syntheses by lowering the activation energies using enzymatic catalysts. It is possible that the UV which destroys any organic molecules of appreciable complexity within 24 hours of them leaving Earth's protective atmosphere, are at low enough levels at the very edge of the solar system, to actually cause polymerisation to occur.

However these molecules would disperse and not polymerise further with each other to create the complex hydrocarbons. However the transparent glass-like quality of compacted ice would allow the UV to enter into vesicles full of a gaseous mix of the simpler molecules such as NH4 and CH4 along with H2O and H2. This confinement would have allowed the generation of more complex molecules to occur especially as differentiation of the H2 into free radicals would have allowed redox reactions to occur easier.

Another potential synthesis pathway for the production of complex hydrocarbons in the Oort cloud are the contraction waves generated by the early sun or indeed by other stars in their death throes i.e. shock waves from red giants. These momentarily give the molecules the high-energy input they need to polymerise.

This however does not explain how these molecules then got to the surface of the Earth without being destroyed by the high levels of UV that would have bombarded it the closer they got to the centre region of the solar system. One of the current theories is that they become embroiled in a cosmic dance involving the lumps of ice they are embedded in being hit by others and being slung into an eccentric orbit where they may have subsequently impacted upon the earth.

Rocks from Space

Another potential source of hydrocarbons are some of the rocky blocks that either some how failed to be accreted into a bigger structure or have since been blasted into a thousand pieces. These are meteorites and occur in several forms, the main one that interests us in respect to hydrocarbons and early life are the carbonaceous chondrites.

Shock synthesis could occur producing amino acids from the impact of these extra terrestrial bodies into carbonate rich sediments (abiogenic in orgin). However it is thought that it is more likely that volatiles within meteoric bodies may react in a similar way due to them being heated when the body of rock passes to close to the sun. These would then have impacted on the Earth, releasing the building blokes for life, that is if the molecules were not destroyed on impact!

So the building blocks of even the complex hydrocarbons themselves could have been imparted to the Earth by bombardment from above, comets and meteorites that fell from the sky. But there is one more way that material from above could have caused the chemical evolution of complex hydrocarbons and ultimately life and that is one of the oldest theories called panspermia. This is the theory that life can arrive on a planet from a spore, seed, or cell derived from life elsewhere in the universe, though this theory was abandoned due to UV levels it has recently had a rejuvenation though some aspects have been modified.

Instead of life being directly seeded it is thought by some scientists to be possible instead for either the amino and nucleic acids that are synthesised in the oort clouds of solar systems could be carried far and wide across interstellar space to land on planet with a sufficient solvent to allow polymerisation into complex replicating structures to occur, and that solvent would not nessacceraly have to be water!

Or by the continuing accretion of the fine carbon rich dust to the Earth, though this would not have substansally altered the concentrations in the early Earth's oceans it may have done so around the coastal regions where excessive tides would have create little pools that where concentrate but periodically replenished with water. In fact the concentration levels of these primary building blocks happens to be the main obstacle to these theories. In order for polymerisation to occur with or without the aid of catalyst they have to be in relatively high concentrations, which could not be achieved by these methods. However there are several ways in which these concentrations could have been reach but this is covered in SOAP BUBBLES FROM OUTER SPACE.