September 28, 1969, the inhabitants of the village of Murchison in the Australian state of Victoria woke up around 11 am. Woke up famous. Fame fell on them literally from the sky: Murchison’s neighborhood covered a stone rain from the wreckage of a fairly large meteorite.
Witnesses managed to observe how the flaming ball fell to pieces in the air, leaving behind a thick smoky trace. Some of the debris damaged the building. Soon, with an area of 13 thousand square meters. Km, almost 110 kg of “celestial stones” were collected, fragments of 680 g to 7 kg in weight. This was a great success, because in the hands of scientists caught samples that had not had time to change and become polluted under the influence of terrestrial conditions. Studies of the fragments of the Murchison meteorite began already in the 1970s and do not cease to this day, bringing one sensation after another.
The first surprise was the estimate of the age of the meteorite – dating pointed to its exceptional antiquity. Apparently, the object is no less than 4.65 billion years, which makes Murchison a body more ancient than the Earth and even the Sun (the age of the Sun, according to modern data, is 4.57 billion years). Perhaps it appeared at the first stages of the formation of the solar system, and perhaps – flew to us from far more distant regions of the universe. So this or otherwise, but the chemical analysis of the meteorite brought a sensation even greater.
Chemistry and Life
In its composition, Murchison refers to a fairly rare group of carbonaceous CM-chondrites containing a large amount of iron in the form of hydrosilicates and magnetite. In these hydrosilicates there are many water molecules – about 10-15% of its total mass. Such meteorites are less than 5% of all celestial bodies, which astronomers regularly collect from the surface of the Earth. There is also a lot of carbon in it in the form of pure graphite, soot, and also … organic compounds.
L-and D-isomers of alanine, one of the simplest amino acids. They rotate the plane of polarized light passing through them (each in their own direction), but under the action of circularly polarized radiation, they are gradually destroyed.
The debris of the meteorite, for the most part, remained in Australia, but many of them diverged in the scientific laboratories of the world. And soon the researchers discovered amino acids in them, the “building blocks” of proteins common to living organisms on earth: glycine, alanine, and glutamic acid. There were also rare amino acids that are not part of the proteins: pseudoleucine, isovalin, and also diamino acids. There was also a luxurious set of hydrocarbons.
This composition was reminded to many by the mixture used by Miller and Jüri in their famous experiments, in which they modeled possible mechanisms of the appearance of biomolecules on the young Earth. Recall that, testing the hypothesis of Alexander Oparin, these scientists tried to reproduce the conditions that existed on a primitive planet. A mixture of the corresponding gases (ammonia, hydrogen, methane and carbon dioxide) in the presence of water was exposed to ultraviolet radiation, heating and electric discharges of “lightning”, as a result, in the flask, indeed, the most diverse organics, including simple amino acids, accumulated.
Approximately the same composition had a Merchison meteorite. Is it billions of years old, like a unique space capsule, to shore the “imprint” of that distant and exciting era when the Earth was young? Not really: later it showed a small amount of nitrogenous bases, links of DNA and RNA chains. These products were also observed in the experiments of Miller-Urey, but in Murchison they turned out to be rather unusual.
Coccoid and filamentous cyanobacteria and fungi from the Murchison meteorite
In 2008, American and British scientists analyzed the isotopic composition of carbon in the nitrogen found in the meteorite. The radiocarbon method allowed them to estimate the amount of heavy carbon isotope 13C, in comparison with the “usual” 12C. It was shown that the nitrogenous bases of uracil and xanthine from the Murchison meteorite contain, respectively, as much as 44.5 and 37.7% of 13C. For terrestrial conditions this is an absolutely impossible quantity. Analogous conclusions made it possible to make an analysis of the content of nitrogen isotopes in Murchison’s amino acids: heavy 15N in them is much larger than that characteristic for amino acids of terrestrial origin. So, does the meteorite organics really have extraterrestrial nature? ..
Studies of the Murchison meteorite are still ongoing, and in 2010, German chemists led by Philippe Schmitt-Kopplin, using ultra-sensitive mass spectrometry, showed that its organic composition is exceptionally rich and diverse, far from being exhausted by several amino acids And a pair of nitrogenous bases. Scientists have not been able to identify all compounds, but the number of different molecules is estimated to be at least 14 thousand, and most likely – is close to 50 thousand, including 70 different amino acids.
Coccoid and filamentous cyanobacteria and fungi from the meteorite Efremovka
And in February 2014 – Jason Dworkin and his colleagues from NASA examined not the debris, but literally the specks of the Murchison meteorite, particles that are thousands of times smaller than the fragments that have been studied so far. They showed that even in such dust particles, amino acids and nitrogen bases can survive and quite safely survive very long space flights. Perhaps once, in the distant past, the Merchison meteorite swept through the gas-dust cloud from which the solar system was born – and how a dusty cloth collected various substances. Or maybe he brought the “spores of life” from the more distant worlds? At least, this is indicated by a curious fact.
Let us recall that proteins – the basis of life on Earth – are chains of amino acids. In turn, amino acids consist of a carbon atom to which four different elements are attached: an amino group, an acid carboxyl group, a hydrogen and a functional group, for each amino acid. These four components can be joined in a different order, creating an amino acid of one of two mirror-like shapes, just as the fingers are joined on the palms of our hands. These forms of amino acids are called enantiomers, and their approximately equal mixture is racemic.
One has to remember that ordinary light is a combination of electromagnetic waves that oscillate in different planes. If they put a special filter on their way, we can cut off all the extra fluctuations and get the light, the waves of which fluctuate in a strictly coordinated way, say, vertically. With such a polarized light, amino acids have a special relationship.
Pair of granules of meteoritic matter
If we take a racemic mixture of amino acids, the polarized radiation will pass through it without a hitch. But if in our solution only certain definite enantiomers are contained, the plane of polarization … will start to spin! Some enantiomers – they are called L-amino acids – will turn it to the left, counter-clockwise, others, D-amino acids – to the right.
It is worth saying that you can not separate the racemate by chemical methods: from the point of view of chemistry, they behave quite the same, and in synthesis they are formed in equal amounts. But for living organisms everything is different: all our protein machinery is set to work with spatial forms of molecules, and the shape of enantiomers is fundamentally different – a mirror image, like that of the right and left hands. And, as you can not pull the left glove on the right hand, so can not the proteins work with the inappropriate isomer of the amino acid. All life on Earth, from the E. coli to the senator, is built from L-amino acids: right-handed enantiomers for us simply do not exist.
The famous astrobiologist, academician of the Russian Academy of Sciences Alexei Rozanov is one of the most active adherents of the hypothesis of the extraterrestrial origin of earthly life. According to him, not only in the Murchison meteorite, but also in the meteorite Efremovka with the help of an electron microscope, you can see traces of microorganisms reminiscent of the simplest fungi and bacteria.
At the same time, Academician Rozanov is not talking about petrified remains, but about some kind of prints left by “alien microbes” in nature. These conclusions look rather bold for not too reliable observations, and the majority of the scientific community is not accepted.
Theoretically, nothing prevents life from being based on D-isomers, but once started with one type of amino acid, it is forced to remain forever attached to it. Why the Earth became the kingdom of L-, not D-amino acids – one of the most intriguing mysteries of biochemistry. The answer to it again can give the Murchison meteorite and the interpretation of its amazing composition. The fact is that the L-amino acids in it are 2-3 times larger than their D-forms. It turns out that this asymmetry is peculiar not only to the Earth, but also, at least, to the entire Solar system? .. Where does it come from?
How Symmetry Dies
Amino acids of non-biological origin, formed as a result of reactions in space, will give a racemic mixture of L and D forms approximately equally. But, as we have already said, the relationship of such isomers with polarized light is by no means simple.
First, they rotate the plane polarized radiation. Secondly, if they themselves illuminate circularly polarized light (the plane of which rotates, twisting in a spiral), one of the mirror antipodes will gradually be destroyed. Which one – depends on the direction of rotation of the polarization: if the light is polarized clockwise, the D-isomers are destroyed, and if against – L-amino acids. If the Murchison meteorite does not lie, this should have happened even before the first amino acids hit the Earth – in space. It remains to find there a source of circularly polarized radiation that would destroy the D-amino acids in it, without touching their L-forms.
It seems that the Murchison meteorite can really be a rare example of how the “embryos of life” could appear in the distant past somewhere in the depths of space, make a dizzying flight – simultaneously retaining more L-isomers – and eventually get to Earth. And maybe on other planets? In the end, nothing prevents such “embryos” from constantly moving from one planetary system to another. And there is such a hypothesis.
The idea that life was born not on Earth, but was brought on it from outer space, first sounded back in the middle of the XIX century. The hypothesis of panspermia was supported by the famous Russian naturalist and thinker Vladimir Vernadsky. However, when scientists began to better understand the conditions existing in space, their enthusiasm diminished. A deep vacuum, incredible cold and hard radiation did not seem to give the “embryos of life” a chance to survive, not just for millions of years, but for at least a few weeks.
But this was only part of the truth. Already in the late 1960s, evidence of the amazing resilience of some terrestrial organisms appeared. Bacteria survived on the Apollo-12 probe that landed, and more and more organic molecules began to be found in outer space.
Some confirmation of this hypothesis is also provided by laboratory experiments. So, a few years ago, researchers modeled the process of releasing matter from the planet into space – along with accidentally caught bacteria. Scientists led by Gerda Horneck (Gerda Horneck) calculated the pressure, which they will have to transfer at the same time, and tested the possibility of survival of microbes in such aggressive conditions. As it turned out, there are bacteria that can withstand such a tough start and safely survive it.
The first step of such “interplanetary fertilization” can be considered to some extent possible. But what about the space flight itself? Are there organisms – or at least fragments of their biomolecules – that can transfer all the vicissitudes of a long and dangerous journey? There.
A number of experiments, put on the Soviet orbital station “Mir”, and then – and on the ISS, made it possible to detect surprisingly resistant to the space environment, organisms. Champions in this area can be called lichens – symbiotic associations of fungi and microscopic green algae. In 2008, together with samples of other living forms, they spent several months on the outer surface of the ISS. The test was not easy: the temperature jumped from minus to high heat hundreds of times a day, the Sun poured them hard ultraviolet, gravity was almost absent – like pressure and air. Lichens transferred everything quite calmly: they simply went into a “sleep mode”, and upon returning to Earth – again “woke up”.
And the Murchison meteorite, and a host of other experiments and studies, suggest that life could be brought to Earth from space. But – alas! – all this is only circumstantial evidence on the basis of which no honest investigator would have started to build any accusations. Yes, now we can say for sure that there is a mass of various organic substances in space. Moreover – there are even conditions for the appearance of optical isomers of amino acids and some other biomolecules. Yes, and these molecules, and even some living organisms are able to endure extreme conditions of interplanetary flight …
Unexpected (albeit indirect) confirmation of the hypothesis of panspermia was received by genetics. In 2013, Alexey Sharov and Richard Gordon (Richard Gordon) tried to apply the law of Moore, known to us from electronics, to the genome of living organisms.
Back in the late 1960s, Gordon Moore noticed that the power of computers is growing like a snowball: every two years the number of transistors on a chip chip is doubled. And if two years ago there were, say, 100, today there are 200, in two years there will be 400 and so on. If we did not even know when this development started, then, having learned the number of transistors on a modern chip and remembering Moore’s law, we can easily calculate: the number of transistors on the crystal was zero in the early 1960s – since that time modern electronics began.
In the same way grows the complexity of living organisms, from bacteria to fungi, from fish to mammals. Along with the complexity of the body, theoretically, the size of its genome – at least its coding part, carrying information for the synthesis of proteins – should grow. Indeed, the calculations of Gordon and Sharov showed that the length of this useful DNA also grows exponentially. True, much slower than the number of transistors: doubling here takes about 376 million years.
But when the scientists then took the gene for “the top of evolution” – mammals – and began to move into the past, cutting it twice in every 376 million years, then they came to zero … only after 9.7 (± 2.5) billion years! It was then that the DNA complication and the improvement of organisms should begin. This figure does not fit well in the head: our life is two times older than our planet, and the Sun itself. Sharov and Gordon believe that the figures they obtained directly indicate the extraterrestrial origin of life.
But all this – the possibilities, but how much they really realized, it is impossible to say so far. At least until we find real living organisms on some meteorite, or at least – on some other planet. Well, until then, the Earth remains the only known to us, a unique example of a planet inhabited by life. And there is no reason to deprive her of this status, giving the palm to someone unknown from outer space.