Speculations on life’s origins

In my last post I described what it was like on Earth some 3.8 billion years ago when evidence indicates that life first appeared on Earth. The atmosphere was made up of volcanic gases like carbon dioxide, methane, sulfur dioxide, ammonia etc. but no oxygen.  That meant there was no ozone layer in the upper atmosphere to absorb the ultraviolet rays of the sun that cause chemical bonds between atoms to break. That meant that for chemistry to become more advanced and life to originate, it had to have happened in an environment where there was a source of energy and that was away from the light of the sun.

Just such an environment was provided in hydrothermal vents deep in the ocean.  On the map upper left, these vents are indicated by the red dots and are commonly found near places where the Earth’s tectonic plates are moving apart and sea water seeps into cracks in the Earth’s crust. This causes superheated seawater rich in dissolved inorganic compounds, to spew out of the vents and they supply the chemical energy to support up to 100,000 times as many more living organisms as ordinary seawater. These organisms depend on populations of microorganisms that ingest sulfur compounds for their energy to stay alive and reproduce . At first these strange microoranisms, that could withstand such tremendous heat and pressure in the dark depth of the oceans, were thought to be a special type of bacteria but, with the advent of the relatively new science of genetics, astonishingly, an entirely new form of life called Archaea were identified in the early 1970’s – see the ‘Tree of Life’ diagram below on the left.

At present, the most popularly held speculations about how life started on Earth center on thermal vents but, for lack of conclusive evidence, there are still at least 7 plausible theories and there have been lots of suggestions that life might have had a free ride on comets from other star systems. Actually, even if life came from outer space, the question of how life began there would still remain. 
Whatever theory prevails - eventually a loop of chemical reactions occurred in a closed environment that could reproduce itself. Self replication is the fundamental aspect of what we call life because it means there is a whole new set of rules that, once started, takes over  – Darwin’s law of natural selection. The replicas react to their immediate environment and if they possess some change from the original parent, then the offspring can survive normally only if the change is an improvement on the parental design.  Since there have been many
inevitable errors in replication and thus improvements, it is easy to imagine how step-by-step more complex and adaptable offspring proliferated. Evolution was unstoppable. 

Indeed all the evidence points to a single ancestor from which all other life forms on Earth evolved. As a case in point, all the DNA and RNA molecules found in every living cell have a structure such that the information on replication is in a particularly reliable arrangement as is the energy storage system using the Adenosine triphosphate [ATP] molecule. 

A number of scientific teams are using their impressive know-how and ingenuity to try to create life in an artificial environment but I must say I am concerned about their efforts!  They are tampering with the very basis of our existence and - as is illustrated by what has happened here on Earth – once started, life is amazingly persistent.  It also happens to be chiral and in a posting I did on right and left handedness in molecules shows, getting it wrong just in that one sense could be fatal.  Rie 

Chirality

As a break from our harsh Canadian winters we have ‘gone south’ to the Caribbean islands for years. I love beach combing and as a result, I have baskets full of sea shells at the cottage that I have picked up and enjoy handling.

When I read that spiral shells were a good example of the invisible chirality [left or right handedness] that many molecules in living systems possess, I lined up some of my spiral shells with their earliest growing tips down and took the picture above. When you look at the picture, you can’t help but see that the open ‘mouth’ of the shell is always on the left.

Try it out yourself the next time you have any spiral shells at hand, and you may be lucky enough to find a shell that breaks the rule and opens on the right. They are rare, but exceptions do exist, and therefore the shell's outward chirality must be for another reason, because our body’s chiral molecules are always only one-handed without exception! For snails, the reason for the shell coiling happens to be sex. Snails make coiled shells because their sexual organs are usually twisted, and it is difficult with snails of opposite handedness to mate. Picture on the right is the Nautilus shell.

The fact that our body’s DNA, enzyme and sugar molecules, etc., are chiral was not generally known until a widespread medical tragedy occurred. In the late 1950’s, a drug called ‘thalidomide’was developed by a pharmaceutical company as a sedative and a treatment for morning sickness in early pregnancy. When tested on animals it had few or no side effects, and so was used in many countries, including the UK and Canada starting in 1958. It was not until 1961 that doctors fully realized the drug was the cause of severe birth defects – many children developing flippers instead of arms.  It was withdrawn but too late for kids shown on the left .

When thalidomide was investigated, the results knocked organic chemists for a loop. It was discovered that the molecule was chiral, and when synthesized in the lab, equal amounts of two isomers, one left-handed and the other right-handed, were produced. Because molecules involved in our body chemistry are also chiral, the two isomers reacted differently, with one isomer causing the birth defects. Fortunately, under normal circumstances, if we ingest a molecule with the wrong handedness, our bodies recognize it and - unlike the unfortunate thalidomide case where a fetus was in an early stage of development - it is sent to the liver where most toxins are dealt with.

The development of one-handedness in our body chemistry probably happened just by chance when life first formed on our planet. Should we ever travel to another planet many light years away, it is possible that life started there with its molecules having the opposite handedness, and we could eat as much of their food as we wanted, but we would starve to death! Rie