Do we have the raw materials to build amino acids and a DNA-like molecule throughout the universe?
Since the mid-20th century, astronomers have used radio and optical telescopes to observe a great variety of the essential chemical elements needed to build amino acids and a DNA-like molecule. The way these raw elements are revealed to us in space is through a special technique known as spectroscopy.
Spectroscopy is defined as the analysis of electromagnetic radiation emitted, absorbed or reflected by atoms and molecules (as revealed by dark or brightly-colored lines at specific frequencies) using a tool that helps to see into the radiation at different frequencies. Each atom of a particular element and each molecule of the same compound emits, absorbs and reflects radiation at its own unique set of frequencies. The way scientists determine what these unique frequencies are to help identify the atoms and molecules is by passing the radiation through a glass prism, which does the job of separating the radiation into its individual frequencies.
Once the unique frequencies have been spread out on a screen to form an absorption or emission spectrum (we call this an electromagnetic fingerprint), it can help scientists not only to determine the chemical composition of distant clouds of gas and dust, and of galactic, stellar, planetary or cometary objects at their very surfaces, but also to help reveal more subtle details such as the temperature, pressure and rate of rotation of these objects, and whether or not they have a magnetic field.
Today, by studying the emission and absorption spectra of nebulae, comets, stars and meteorites, scientists have found a chemical cocktail composed of mostly hydrogen, with smaller amounts of the other chemical elements. Looking at a typical nebula, for every million hydrogen atoms in these clouds, there are about 120,000 helium atoms, a few hundred each of nitrogen, carbon and oxygen, about 100 each of neon and sulphur, and several atoms each of iron, calcium, sodium and potassium.
The existence of these elements tells us that we have the abundance of raw materials to create a DNA-like molecule and any amino acid we care to forge anywhere in the universe, not just here on Earth.
Creating molecules
Do these elements come together to form molecules? Yes, they do. Scientists have uncovered out of the universal "woodwork" no less than seventy-five different molecules. Some of the molecules identified include water, methane, ammonia and the not-so-life-giving hydrogen cyanide. More complex molecules such as formaldehyde, ethanol, formic acid and cyanoacetylene have also been found; and there is certainly no shortage of organic molecules as well. Glycine, the simplest amino acid, is an example of an organic molecule found in space.
|
COMPOUNDS IN A TYPICAL COMET |
|
|
Chemical Name |
% by weight |
|
Water |
57.50 |
|
Carbon dioxide |
14.00 |
|
Carbon monoxide |
6.00 |
|
Formaldehyde |
5.75 |
|
Hydrogen cyanide |
5.75 |
|
Carbon disulfide |
4.60 |
|
Acetylene |
4.00 |
|
Remaining 2.4% in elemental metals and other materials |
|
|
CHEMICALS FOUND IN SPACE |
||
|
Year |
Chemical Name |
Formula |
|
1970 |
Hydrogen |
H2 |
|
1963 |
Hydroxyl |
OH- |
|
1970 |
Carbon monoxide |
CO |
|
1940 |
Cyanogen |
CN |
|
1971 |
Carbon monosulfide |
CS |
|
1978 |
Nitric oxide |
NO |
|
1973 |
Sulfur monoxide |
SO |
|
1975 |
Sulfur nitride |
SN |
|
1937 |
Methylidyne ion |
CH+ |
|
1968 |
Water |
H2O |
|
1968 |
Ammonia |
NH3 |
|
1971 |
Silicon monoxide |
SiO |
|
1975 |
Silicon sulfide |
SiS |
|
1975 |
Sulfur dioxide |
SO2 |
|
1972 |
Hydrogen sulfide |
H2S |
|
1974 |
Hydroinitrogenyl ion |
N2H+ |
|
1976 |
Formyl |
HCO |
|
1970 |
Hydrogen cyanide |
HCN |
|
1978 |
Methane |
CH4 |
|
1969 |
Formaldehyde |
H2CO |
|
1975 |
Cyanamide |
NH2CN |
|
1970 |
Methanol |
CH3OH |
|
1976 |
Cyanodiacetylene |
HC4CN |
|
1977 |
Ketene |
CH2CO |
|
1970 |
Formic acid |
HCOOH |
|
1971 |
Acetylaldehyde |
CH3CHO |
|
1975 |
Ethanol |
CH3CH2OH |
|
- |
Ethyl Cyanide |
CH3CH2CN |
|
- |
Glycine |
C2H5O2N |
As if that wasn't enough, in the 26 September 2014 research journal Science, astronomers at the Atacama Large Millimeter/sub-millimeter Array (ALMA) found evidence of unusual carbon-based molecules in the star-forming gases known as Sagittarius B2. One newly identified molecule is called isopropyl cyanide. The complexity of this molecule suggests that relatively complex amino acids and other important biological chemicals probably originated during star formation.
Need more direct evidence for the existence of organic molecules in space? Well, there exists a rare type of stony meteorite known as Carbonaceous Chondrites. A bit of a mouthful to pronounce, but these interesting meteorites contain about one percent by mass of organic matter that resembles the crude oil or tarry substances found on Earth. Although no extraterrestrial organism has been found trying to crawl out of these rocks, let alone dead ones, the total amount of organic matter in the interior of these rocks is too high to be considered entirely due to contamination from the Earth, and must show the ease with which molecules can form within the protective confines of certain porous rocks in space.
|
COMPOSITION OF CARBONACEOUS CHONDRITES |
|
|
Chemical Name |
% by weight |
|
Silicates |
75 – 90 |
|
Water |
1 - 21 |
|
Metals |
0.1 – 3.5 |
|
Carbon |
0.1 – 3.8 |
|
Nitrogen |
0.01 – 0.3 |
As amino acids, the building blocks of life on Earth, are predominantly left-handed structures, an unexpected discovery by planetary scientists at NASA's .Goddard Space Flight Center in Greenbelt, Maryland, found an equal combination of left-handed structure and its "non-existent in life on Earth" mirror image in one gram of a piece of meteorite that broke up in the atmosphere in October 2008, leaving remnants scattered across the Nubian Desert in Sudan. Scientists did not expect to find amino acids because the asteroid that supplied the meteorite was formed at high temperatures many millions of years ago. Yet somehow both left-handed and its mirror structures were found in a piece of the meteorite. As Daniel Clavins said:
"Amino acids are forming in environments that we really didn't think were possible,"
However, the concentrations of these amino acids were not huge. In fact, the scientists were cautious not to claim that life on Earth could have originated on some asteroids. What the study does show is that complex molecules at the level of amino acids can indeed form, under somewhat unusual conditions in the absence of water, inside the protective confines of certain asteroids as long as there is a long enough cooling period to allow atoms to reassemble into molecules on the surfaces of iron and other metals.
Molecules do increase in complexity
With further protection and access to more materials, there is growing evidence that there is no limit to the size and complexity of molecules that can spontaneously be made. Indeed, the relative ease of making organic molecules in space was well demonstrated in the laboratory by a series of famous experiments carried out in 1953 by the American chemist Dr Stanley Lloyd Miller (b. 1930).
While working as a graduate student at the University of Chicago, USA, under the supervision of Dr Harold Clayton Urey (1893-1981), Miller assembled an atmosphere of methane, ammonia, water vapour and hydrogen inside a large reaction vessel to simulate Earth's early atmosphere. Electrical sparks reaching 60,000 volts were passed through the mixture to simulate lightning, and after two weeks the body of water containing the newly-formed molecules—simulating the early primordial pools on Earth—yielded a number of simple amino acids—organic molecules essential for the building of life.
|
THE PRODUCTS OF THE MILLER EXPERIMENT |
|
|
Chemical Name |
% by weight |
|
Tar |
85 |
|
Carboxylic acids |
13.0 |
|
Glycine |
1.05 |
|
Alanine |
0.85 |
|
Glutamic acid |
Trace amounts |
|
Aspartic acid |
Trace amounts |
|
Valine |
Trace amounts |
|
Leucine |
Trace amounts |
|
Serine |
Trace amounts |
|
Proline |
Trace amounts |
|
Treonine |
Trace amounts |
In more recent times, the experiment was repeated using a "chemical concoction consisting of carbon dioxide, water and traces of gaseous nitrogen compounds, to conform with modern theories that such gases were released from the interior of the Earth by the process of volcanism". Despite these modifications to Miller's original experiment and the fact that other forms of energy, such as ultraviolet light and sound waves, were supplied to the reaction vessel, as long as the atmosphere inside the reaction vessel contained no free oxygen, simple amino acids were again produced with relative ease.
Simple non-replicating DNA-like molecules and chains of amino acids to form protein can spontaneously appear
The presence of amino acids in an experiment is certainly one thing. But having the amino acids come together to form life is a different story. The presence of amino acids is not enough to state emphatically that life exists, or will arise somewhere, in the Universe. Something else has to direct these building blocks of life towards the formation of a living thing. On Earth, this mysterious organiser of the living world is called DNA.
As DNA is such an important molecule in the formation of life, the question many scientists are trying to answer today is, "Can the constituents of DNA, such as adenine and guanine, be produced spontaneously in the experiment?"
Since the days of Miller's famous experiments, a number of scientists have spent more time with the experiment and added a few more "natural" substances. What the scientists have discovered is that molecules as complex as adenine, guanine, cytosine, ribose, deoxyribose and glucose can be produced using polyphosphates as catalysts.
In a more simpler experiment, Dr Juan Orò of the University of Houston, USA, was able to produce all the nucleic acid bases, including adenine and cytosine, as well as the usual amino acids, by combining a non-oxidizing atmosphere of hydrogen cyanide with an aqueous solution of either ammonia, cyanogen or cyanoacetylene—all considered to be natural substances found in space.
If amino acids and nucleic acid bases are relatively easy to build in nature, given the right catalysts and other readily available chemicals, do they eventually come together to form proteins and simple DNA-like molecules? There is evidence that they do.
In 1977, while working at the National Aeronautics and Space Administration (NASA) Ames Research Center in California, USA, Dr James Lawless and a visiting scientist from Israel, Dr Nissim Levi, found evidence to support the 1947 claim made by British physicist Dr John Desmond Bernal (1901-1971) that ordinary clay could concentrate small chemical molecules in a hot "organic soup" and act as a kind of prebiotic scaffolding for the production of larger, more complex molecules at the very surface of the clay.
Although most clays have a nasty habit of destroying a number of important amino acid structures, Lawless and Levi found that clays containing metals could attract certain amino acids and nucleotides without damage. In particular, Lawless and Levi discovered two types of clays, one containing nickel and the other zinc, which not only attract all the twenty amino acids found in living things on Earth and all the numerous nucleotides comprising DNA, but have been observed to form macromolecules of as many as eight amino acids and multiple chains of nucleotides. Longer, protein-like chains and DNA-like structures seem likely if given sufficient time for their synthesis.
In yet another interesting experiment, researchers at the Center for Chemical Evolution at the Georgia Institute of Technology have discovered a chemical reaction that can occur naturally to help produce the earliest RNA-like molecules.
As scientists have understood, in all animals and plants on Earth there is the all-important DNA molecule, which stores genetic information in a very stable and tough double-helix structure that looks like a spiral ladder design. Then there is another substance called RNA, which is used in all animals to translate the code inscribed in DNA. It is essentially like DNA, except it is single-stranded, like a ladder cut down the middle. There is also another molecule that is simpler than RNA. Known as pre-RNA, it is considered an important molecule on the way to becoming fully fledged RNA, but has just stopped short of becoming one.
Pre-RNA is the easiest of the three molecules — the other two being RNA and DNA — to build. Recognising this fact, researchers focused on this earlier RNA design to see whether it was possible to create it in nature with the right chemical ingredients.
What the researchers have discovered is that not only is it possible to create this RNA-like molecule, but highly probable after witnessing evidence for the spontaneous binding of the phosphate to ribose and having the structure assembled with other bases to form pre-RNA. In effect, the researchers have found a chemical pathway "that we see as important for the formation of the earliest RNA-like molecules," according to the statement by biochemist Nicholas V. Hud, one of the researchers involved in the work (and also director of the Center for Chemical Evolution).
In order for the chemical reaction to actually take place, the researchers used a molecule known as triaminopyrimidine (or TAP for short). Since this molecule can be produced naturally and in reasonable abundance, TAP was mixed with the sugar-like structure of ribose under conditions designed to closely mimic the environment of a drying pond (as can easily happen when the rains of the early Earth stop falling and enough time is allowed for the water on the ground to dry up). This slow removal of water and increasing concentrations of various chemicals, particularly those of interest to the researchers, somehow managed to help assemble a basic RNA-like structure. Observations suggest that up to 80 per cent of TAP reacted with ribose to form the all-important nucleotide structure (i.e., the basic base-sugar-phosphate unit structure for creating a DNA-like molecule).
"It is amazing that these nucleosides and bases actually assemble on their own, as life today requires complex enzymes to bring together RNA building blocks and to spatially order them prior to polymerization," said Brian Cafferty, a graduate student at Georgia Tech and co-author of the study.
The study was published on 14 December 2013in Journal of the American Chemical Society under the title "Spontaneous Prebiotic Formation of a β-Ribofuranoside That Self-Assembles with a Complementary Heterocycle".
Despite all this interesting work, scientists have yet to see a self-replicating macromolecule like DNA spontaneously come to fruition before their very eyes, leading ultimately to the formation of a living organism. The process leading to this last important step is apparently too slow (perhaps taking hundreds of millions of years) and there seems to be little the scientists can do to speed up the process in the laboratory.
First extraterrestrial protein discovered
While the study and the published scientific article are yet to be peer-reviewed (as of 4 March 2020), it strongly looks like scientists have discovered the world's first extraterrestrial protein embedded in a 30-year-old meteorite left preserved in a museum. After DNA, protein is considered the next most important chemical molecule, as without it, life of any sort in the universe cannot exist.
It was discovered almost by accident after an instrument used to analyse the amino acids in the meteorite was modified to make it more sensitive and capable of modelling the structures found inside on a computer. Not only did the scientists find higher concentrations of the amino acid glycine, but it was bound to some metals (such as iron and lithium) and another structure that was later identified as a protein. In other words, glycine was not isolated but part of a protein structure.
The protein structure and its glycine attachment is not a known protein here on Earth but has some similarities to known proteins. One point of difference was the presence of deuterium isotopes (which do not occur naturally here on Earth, but are common enough in meteorites, and hence the reason for the extraterrestrial status given to this structure). As a result, scientists have decided to call it hemolithin.
The theory that life began on Earth
Assuming that life did arise naturally here on Earth, one important piece of scientific work being done to try and speed up the process is that of Professor David Deamer at the University of California, Santa Cruz.
Deamer is currently working on the idea that perhaps bubbles (or vesicles as he calls them) may have played a vital role in the formation of the first single-celled organism here on Earth. If these bubbles could be formed on, say, the surface of the clays talked about by Lawless and Levi, it might be possible to string together enough amino acids to build a spherical protein-like membrane that would help to maintain the bubble for longer. Of course, this does not explain how a self-replicating DNA is created. Deamer is trying to speed up the process in the laboratory by inserting a ready-made self-replicating DNA into a vesicle containing organic matter. He hopes that one day it will be possible to create artificial life in a test tube. As Deamer said:
"So we think that vesicles were very much part of the first forms of cellular life. All life today is cellular, it means a membrane surrounding a compartment. And what we are doing here is stepping through the evolutionary process that must have occurred on the Earth about 3.5 billion years ago when the first forms of cellular life appeared."
After inserting a piece of DNA into his vesicle, has he succeeded in creating life? Deamer said:
"Is it alive? Not yet. But it's a shared goal of a number of laboratories now to actually make artificial life."
Can making bubbles help to make new life? Perhaps in a typical domestic bathtub where people can sit inside and start blowing bubbles with a bit of soap and some baked beans. But in the early days of the Earth? We will have to wait and see.
The theory that life began in outer space
As with any good scientific argument, there should always be an opposing view. Indeed, if the time provided by the Earth to create a self-replicating DNA is anything to go by, there is another way.
First proposed by Professor Chandra Wickramasinghe of Cardiff University in the UK with the support of his scientific colleague Sir Fred Hoyle, the idea is that the Earth could have been seeded by an alien bacterium with its own alien DNA, which hitched a ride on an icy comet and crashed on to the planet over 4.6 billion years ago. It is not unlike the way a sperm with its own self-replicating DNA arrives on the surface of an ovum to kickstart life.
Could we be the aliens?
The theory known as panspermia was once ridiculed by other scientists when it was first proposed in 1996. As Wickramasinghe said:
"[We were viewed as] a pair of crazy lunatics who had jumped onto the wrong bandwagon and there were no arguments presented to say that you are wrong for such a such a reason." (1)
But now a kind of silence of the clowns from those who thought the two scientists were crazy has taken place as more scientists take the idea seriously. The interest is particularly well-founded now that we know that microorganisms on Earth can and will survive in the heart of, say, a nuclear reactor. These extremophilic creatures have been found to survive in highly alkaline water in the presence of spent fuel rods, eating holes in the stainless steel cylinders. Similarly, bacteria frozen in the deep ice of Antarctica for tens of thousands of years can now be revived at room temperature in the laboratory without any trouble. So why not millions or hundreds of millions of years? This means that bacteria can survive in space. In fact, Earthly bacteria are said to be thriving on the Moon thanks to the successful Apollo mission to land on the lunar surface in 1969. Perhaps this explains why the U.S. government is not so keen to return to the Moon in a hurry.
At any rate, if bacteria can survive in space, why not in another part of the universe?
NASA has looked into this issue with more than a glancing interest. After carrying out some tests, the explanation that life could have started outside of the Earth has convinced enough scientists to the point where they are looking more closely for the evidence in space. The only problem is where. Since the Moon is currently contaminated with Earthly bugs, and Mars is likely to be too after several probes have landed on its surface, NASA wants to send another probe to a more pristine environment in space.
NASA's first attempt to gather the evidence began in 2003 and was repeated in 2006 when it sent into space a US$158 million (A$300 million) spacecraft called Contour (short for COmet Nucleus TOUR) into space to analyse the icy material in the nucleus of two comets - Comet Encke in 2003 and Comet Schwassmann-Wachmann 3 in 2006. The results were inconclusive. Perhaps the quantities of alien bacteria were too small and/or hidden deep inside the comets? Undeterred by the results, NASA has realized that it may be enough for one of these comets to land in a place where, in the right environment, an alien bacterium could multiply into quantities that could be detected by scientists. Therefore, a plan is underway to send a probe to Europa in 2018. Why? Because the moon is believed to have liquid water beneath its icy surface and should have collected enough icy comets over billions of years to determine whether or not the panspermia theory is a reality.
The idea that life could exist on Europa is credited to astronomer Guy Consolmagno SJ, now curator of the Vatican's own astronomical observatory outside Rome, Italy. In his early work at MIT, using computer modelling to determine the internal structure of the outer moons of the solar system, he jokingly thought and wrote in his Masters thesis how it might be possible for little bugs to swim in the liquid oceans of water below the icy crust of Europa. NASA is not quite laughing. In fact, NASA is so serious about the thought that it is determined to find out if this theory is true.
The project is currently being headed by engineer Dr Bill Stone of Stone Aerospace under contract with NASA. As Stone said:
"The reason that it [Europa] is of such great interest is the fact that, for sure, it has water. Because high resolution photographs showed that the surface indicated tidal cracking of ice in such a fashion that it proves pressure ridges similar to what we see in the Arctic ocean on Earth. You can only have that with an ocean of water underneath. You have water, you have a high possibility for the presence of life. So that's why Europa is such a direct hit in terms of the astrobiology interest." (2)
Until scientists have thoroughly explored this moon, we can only speculate about the possibilities. And hope at the same time that we do not contaminate Europa with our own bacteria and spoil the results.
Or would scientists be better off looking for more advanced alien life that we can observe with our eyes to avoid this contamination possibility?
Life ought to be sufficiently complex enough to discern the difference between Earthly and alien
This contamination issue discussed above raises an important point. What if our technology cannot physically determine whether a bacterium is earthly or not? If the panspermia theory of the origin of life on Earth ends up being true, perhaps an alien DNA-like molecule that started life on Earth is no different from our own DNA. The potential is there for all the DNA molecules frozen in an icy comet to have the same nucleotide structure. In which case, can we be sure that we have found the ETs?
The only way to be sure is if life on another world evolves to a level of complexity that our own eyes can observe without the use of instruments. While scientists may say that an alien bacterium is all the proof they need, the reality to proving it is alien may be much harder than they think. The risk of terrestrial contamination of distant worlds by man-made probes would only complicate the task, leaving more questions than answers. But if the alien life is already highly developed and visible to the naked eye, it makes it impossible to claim contamination so soon after the discovery. Scientists would have to conclude that ETs do exist. Isn't it time for scientists to start solving the problem of interstellar travel to get the conclusive proof?