THE ORIGIN OF LIFE. Thrusday meeting. 17-I-2008. WHAT IS LIFE?. Autonomy : this seems the most enigmatic property of life. Life organisms seems to decide what they want to do because their behavior can not be predicted by any known law.
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Thrusday meeting. 17-I-2008
Autonomy: this seems the most enigmatic property of life. Life organisms seems to decide what they want to do because their behavior can not be predicted by any known law.
Reproduction that includes a replication of the replication apparatus.
Metabolism: every organism processes chemicals through complicated sequences of reactions obtaining energy to carry out tasks, such as movement and reproduction.
Nutrition: crucial to life is a continuous throughput of matter and energy. But also the Great Red Spot of Jupiter is a fluid vortex sustained by a flow of matter and energy and nobody suggest that it is alive.
Complexity and Organization: All known forms of life are amazingly complex and part of its unpredictability comes from this property. The most interesting thing of life complexity is that it is organized. Components of an organism cooperate so that it continues living as a coherent unity.
Growth and development: Individual organisms grow but a more interesting thing of living organisms, treated as a class, is development. In fact, is variation combined with replication what leads to Darwinian evolution. And this evolution is essential so life could adapt to the changes in the environment.
Information content: In recent years scientists have stressed the analogy between living organisms and computers. Life is information technology writ small. What is significant in organisms information is that it must mean something. In other words, information must be specified.
Hardware-software entanglement: Nucleic acids store life software but proteins are the real workers and constitute the hardware. There is a highly specific and refined communication channel between them mediated by the genetic code. The entanglement of the hardware and the software in living organisms is quite amazing and is an advanced product of evolution.
Permanence and change: Another paradox about life concerns the strange conjunction of permanence and change. The job of genes is to conserve the genetic message, but without change adaptation is impossible. This conflicting demands lies at the heart of biology.
Manyofthese properties are somehow a paradox: random and specification, software and hardware, permanence and change… There is a tension between conflicting properties in living organisms.
A cell that can be considered as an elaborate self-sustaining community of molecules, each dependent on the others. Let us simplify the question just considering DNA and proteins.
Both are really complex molecules and it is highly improbable that have appeared at the same time (1 over more than 10130) and started to work together within a cell.
But DNA requires proteins to be made and proteins needs DNA to be produced. And here were are not talking about RNA that makes the communication between them.
So the first living organism must not be constituted by these complex and interconnected molecules.
But before discussing that it is interesting to point out that all the work done in the research of the origin of live supposes the three domains of live (archaea, bacteria and eucarya) comes from a common ancestor. Some of the reasons for believing that are the following:
* Every known organism shares a common physical and chemical system. It is quite significant that genetic instructions are implemented using an universal code. An this code is much specific to believe that could come from different ancestors.
* Molecular chirality is another evidence for a common ancestor. For example DNA is coiled like a right-handed helix, but no law of nature forbids a left-handed DNA. The same chirality, whether left or right depending on the molecule considered, is common to all living things.
It is important not to confuse the last common ancestor with the first living organism. As we have seen the interconnection and complexity of the molecules that constitute a cell can not have been appeared together but them should have been present somehow in the last common ancestor to explain the similarity of all known forms of life today.
So nobody expects that nucleic acids and proteins came into being ready-made, with their mutually beneficial properties already inscribed. A crude association of chemicals must have arisen first and, somehow along the way, a separation occurred into hardware and software, chicken and egg.
Some scientists believe that RNA came first (because it can store genetic information and can mimic certain enzymes but with less power).
Other scientists think that proteins came first. Although here the problem of how proteins could self-replicate is difficult to solve he have the example of the BSE (mad-cow disease): a fragment of protein that can replicate and spread.
So the paradox remain: scientist have not reached an agreement of what appeared first, the chicken or the egg.
One interesting thing about life is his capacity of adapt to the most extreme conditions. The microbes that live in extreme conditions are called superbugs:
Some bacteria refuse to grow unless they were immersed in sulfuric acid. Sometimes the concentration of the acid is astonishing, enough to dissolve strips of metal!
Places with high salt content, like the Dead Sea.
Intense cold: bacteria have been found in water trapped beneath the Antarctic ice sheet. Some can tolerate being cooled to liquid-nitrogen temperatures or even lower.
Radiation: microbes have been found inhabiting the waste tanks of nuclear reactors, ingesting uranium, plutonium, and other radioactive elements.
Pressure: some bacteria can be subjected to several hundred atmospheres whereas other can survive in almost a perfect vacuum. An example were the retrievement of some microbes from the surface of the Moon were they have been for two years while attached to a camera housing on the Surveyor III spacecraft.
Hot: Some microbes, called thermophiles, can live at 113 degrees Celsius. An other interesting thing about these microbes is that they have been found in the deep sea where no sunlight penetrates.
But microbes not only live on and near the seabed, they also inhabit the sedimentary rock strata beneath the ocean floor. Some of them have been found nearly a kilometer into the seabed.
There are reasons to think that the last common ancestor was similar to those bacteria that life near the deep-sea hydrothermal vents.
When the life appeared there were on the Earth many cosmic impacts that would have sterilized the Earth’s surface repeatedly. With vaporized rock boiling the oceans and melting the land, conditions would have been lethal to a depth of tens of meters at least.
Other things like ultraviolet radiation, volcanic eruptions, climatic variations due to aerosols and changes on the atmospheric pressure due to the cosmic bombardment made the surface of the Earth a difficult place to live on.
Another advantage of a deep location is that the raw materials needed for life were readily available. Hydrogen, methane, ammonia, hydrogen sulfide and other reducing gases. These are just the sorts of chemicals needed to synthesize biomolecules efficiently (Miller-Urey).
Energy is another factor to consider. Near deep-sea hydrothermal vents there is an enormous thermodynamic drive to form organic compounds.
The most compelling evidence that life began hot and deep comes not from chemistry but from genetics. The genes of extant organisms enfold a record of the past. The technique of gene sequencing let us infer which group of organisms has evolved least over time.
The record of the genes suggest that the universal ancestor lived deep beneath the Earth’s surface, at a temperature well above a hundred degrees Celsius, and probably ate sulfur.
We are not talking about the first living organism but about the last common ancestor that was a quite complex organism.
Life may have started in a completely different environment during calm periods and invaded the hot subsurface region at a later date.
Then, during to periods of massive cosmic impacts only thermophiles survived. Or even could come from space as we shall see later on.
Genetic research only give us information of the last common ancestor, but as it was showed, some much more primitive form of life should have existed first.
When conditions are not good enough for bacteria to survive they become an spore. Encased in a thick protective coat, dehydrated and dormant, its molecules almost cease to move. They can remain in this state hundreds of millions of years just to start again to function and to reproduce when the environment is fine for them.
Experiments have shown that spores can survive to space vacuum with its low temperatures.
The thing most dangerous for them are the solar ultraviolet, although that are much more resistant than any other organism to ultraviolet.
So it is possible for bacterial spores to remain into a meteorite in the space for long periods of time.
In this case live could come from Mars that resembled Earth in the time when life were originated.
One question remains: how a rock from Mars through to the space due to an impact could have inside life? The high temperatures reached in the impact seem to be high enough to kill even the most resistant thermophiles.
But there is a model that explain how it is possible:
The incoming asteroid or comet punches a hole in the ground. The energy released is so great that most of the impactor itself is vaporized.
Directly beneath ground zero, the explosive release of energy squashes the rock, vaporizing or melting much of it as a result.
The compression wave then propagates laterally, spreading out through the surrounding terrain and deep into the ground.
The elastic energy stored in the subsurface rocks as a result of this compression is then liberated again and this delivers an enormous vertical force to the overlying layers.
The surface rock cannot be compressed because it is free to move upwards and has also the advantage that the incoming object has evacuated a huge tunnel in the atmosphere.
Moreover recent computer calculation predicted that 7.5 per cent of rocks ejected from Mars will arrive to Earth and a third of them in the first ten million years.
In conclusion: provided that there are superbugs that can resist really extreme conditions and that can travel in a meteorite it is not impossible to think that life comes from Mars. Or even that microbes from Earth went to Mars and there is still organisms under the ground.
Although here we have not explained all, many advances have been done in the research of the origin of life in biology, chemistry, genetics…But more answers are found more questions appear.
Perhaps the most fundamental question is:
It is life an strange accident that requires many random processes so that the most probably think is that we are alone in the universe?
Or there are laws to be discovered so the correct vision of the universe is a self-complexifying one governed by ingenious laws that encourage matter to evolve towards life?