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Alien Origin of Life?

A recent publication in the journal, The Proceedings of the National Academy of Science (Proc Natl Acad Sci U S A. 2011 Aug 11.) entitled; “Carbonaceous Meteorites Contain a Wide Range of Extraterrestrial Nucleobases,” has set off a firestorm of excitement among origin of life researchers and scientists looking to evolution for answers as to how living things came into existence. Why the excitement? NASA researchers at the Goddard Center for Astrobiology have analyzed a portion of 12 meteorites for carbon-based molecules. Since all life is built on carbon as the fundamental skeleton for biological chemistry, it has been hoped that finding carbon based molecules of any kind might indicate the source of the chemicals needed for life to arise spontaneously by chemical evolution. The researchers found evidence of a group of chemicals called nitrogenous bases or nucleobases that are required to synthesize another class of chemicals, the nucleotides. The latter class of chemicals are the smallest subunits needed for the synthesis of DNA and RNA; molecules that contain the genetic blueprint for creating life.

Life in outer space or in this case even organic-like molecules from outer space have been a focus of researchers for some time as current theories for the spontaneous development of life on planet Earth have been compromised. Major scientific difficulties exist in considering Earth as the birth place of the chemical factory required to produce the fundamental building blocks of life. If an undirected, non-intelligent mechanism brought life into existence, it is logical that the chemical building blocks essential for life must have come into being before any living thing could have been produced; but how?

A first premise for biological evolution to work is that something must fit the criteria of being alive in order for natural selection to work on genetic information and ‘select’ for complexity; ultimately giving rise to the diversity of living forms now in existence. Even the criteria for defining what life is have been debated over the decades since evolution became popularized. The reason for this is to allow scientists to consider molecular conglomerates as intermediates towards the formation of life. To this end, organic molecules derived from non-organic synthesis¯synthetic chemistry if you will¯are required in order to have the “soup” of molecules available for biological chemistry to form. Organic chemistry is needed to spontaneously form some sort of self-replicating system that is capable of all the functions that meet the criteria of being alive: things like eating, reproducing, moving, regulating metabolism and such. Quite a task! Many scientists would be satisfied that a molecule capable of just self-replication would be sufficient to meet their definition of being “alive.”

While a number of “Earth theories” have been proposed for the synthetic formation of such organic molecules, each has major drawback. This prevents serious consideration of such theories to be used in solving the problem of how the first living thing came into existence. Theories have included the primordial soup, deep sea vents, electrical sparks, claymation, a frozen planet, the RNA world or any combination of these and more; none of which can credibly account for the formation of the organic molecules needed to produce the chemistry leading up to living systems. Looking to the heavens for the answers to life (as many of us do) has become a well funded and fascinating alternative for scientists and hence the excitement over meteorite molecules.

In the PNAS paper, first author Callahan published the lab’s findings on the chemical analysis of 12 meteorites; specifically the discovery of a wide range of nucleobases[1] and nucleobase analogues[2] that were extracted from the interior of certain carbon-rich meteorite rocks.  This finding, at first glance seems extraordinary. Also found were 14,000 other organic molecules found in the extracts and this adds to the excitement, if in fact the rocks have not been contaminated by the Earth’s biology. The excitement was not so much that the rocks contained so many carbon compounds or even that they contained nucleobases since other researchers have had similar results in similar material tested. The excitement was that these studies also found nucleobase analogues that are not common to living things on Earth. 

First, this means that the nucleobases may have been the product of synthetic chemistry and therefore may not be the result of biological contamination of the meteorite rock from our own planet.  This would suggest that organic chemistry occurs by an undirected evolutionary process. Secondly, the finding implies that the meteorite may have contained these nitrogenous compounds before entering the Earth’s atmosphere; therefore they had to be from outer space. Putting the two points together the findings suggest that synthetic chemistry occurred in outer space resulting in the formation of organic compounds from the basic elements of carbon, nitrogen, hydrogen and oxygen. Some of these chemical compounds are important in the construction of the basic chemicals that we know are both created by and used by living cells to form the DNA and RNA molecules; chemicals that genetic information. 

The extrapolation of these findings has led the NASA scientist to endorse the idea that life may have had its origin in a pool of organic molecules that may have been created when billions of these particular types of meteors hit the Earth- 4 billion years ago. This scenario may improve the probability that the nucleobases seeded the oceans with the basic carbon structures upon which other important compounds form. These could then react with sugars and phosphates that made the first building blocks needed to form the first genetic material; whether RNA or DNA. Hence the claim made by the popular press, “Alien origin for life on Earth,” and “We’re all aliens . . . how humans began life in outer space” and “Meteorites may have been molecular tool kits, providing the essential building blocks for life on Earth.”

Some Background

These discoveries in meteorites may be important. However, one would expect a level of certainty in the science that leads researchers to consider that the molecules in meteorites have evolutionary significance. Some questions to ask should include: are the molecules found in such space rocks credible precursors to genetic information? What processes in outer space gave rise to organic molecules? Where were the meteorites found on the planet and how long did they lay there before discovery? How were the rocks treated upon discovery and was the surrounding soil, ice, sand and biology examined for potential sources of contamination? Where, when, and how was the ‘parent’ meteor or comet made which gave rise to this meteorite? How do meteors form?  Where do they come from today? Is there evidence that our planet was bombarded by meteorites in the distant past or that such a bombardment delivered sufficient chemicals to create an organic soup for life to start? Did the soup last several 100 million years to jump start life? Can organic molecules survive both the entry into Earth’s atmosphere and the impact onto the surface of the Earth in such rocks? Can such molecules seep out of rock to form an organic ocean of molecules? Does having an ocean of organic molecules improve the genesis of life? Is it credible that life could have been ‘jump started’ from a soup? Were there oceans on Earth 4 billion years ago? 

A Summary of the Current Hypothetical Origins of Carbon Containing Meteorites

Between stars exists a vast expanse of space and in the space, dust and gas. This dust and gas is composed of elements, only 0.1% of which are heavier than hydrogen and helium¯the substance of stars. This dust and gas are the product of star death; typically supernova. Through the eons, the dust and gas interacted with heat, cold, water, and radiation to form more complex molecules; the most interesting of which are carbon based or ‘organic’ molecules. The hypothesis suggests that the interstellar medium of gas and dust that did not form the planets like Venus and Earth or the sun, combined to form larger particles, rocks, planetoids, and asteroids.. Some of these large entities like planets and planetoids collided to create smaller materials such as comets. Asteroids were created from some of these collisions too.  

One particular class of meteor is believed to have grown from the collection of dust. This dust may have acted as a catalyst to the creation of organic molecules, some of which formed microscopic chondrules or balls. The collection of such dust and molecules over the eons of time may have created rocks capable of housing the precursors to the precursors of molecules that we know are found in living things. The reason these particular meteors are thought to be the oldest rocks in our solar system is that they contain organic chemicals and other small molecules which would have been released from the rock if it had been heated up as the sun formed or shortly thereafter. Since these molecules still exist, they must be ‘primordial’ to the primordial Earth.   Therefore, part of the excitement over these new chemical finds is the possibility, remote as it may be, that the compounds found therein are billions of years old. Along with chemical dating methods, assuming all the conditions for radioactive dating are met, the rocks do seem to be very old. 

Chunks of these rocks enter the Earth’s atmosphere as meteors. When they hit the earth they are then called meteorites. Isotope dating methods place these rocks at old ages and also support the idea that they have originated in outer space. Besides, some of the ‘carbonaceous chondrites’ have been collected from ‘falls;’ meteorites collected shortly after they were seen to fall as meteors.

Studies on the survivability of these interesting organic chemicals inside these rocks indicate that a meteor can heat to over 1000o F when entering our atmosphere. A smaller percentage of meteors may only reach 500-700o F and at these lower temperatures some chemicals may not be destroyed; possibly delivering those chemicals to our planet.

If the hypothesis has credibility then the chemicals that have been extracted from these rocks by the NASA scientists become interesting. Some of the chemicals found are familiar to our biological chemistry. Some are chemicals quite uncommon to our biology and some chemicals are even toxic to our biological chemistry. The most interesting group of molecules touted to be the potential precursors to DNA and RNA¯the genetic information of life, have been shown to be present in the rocks through precise molecular analysis. While some nucleobases represent the kinds of chemicals that living cells make to create the building blocks of genetic information, others are so rare on our planet as to indicate that they are probably from outer space. Interestingly, some chemicals are so toxic that medical science has actually created them to deliberately interfere with the ability of living cells to make DNA¯thus they are toxic drugs that stop cells from growing.  These have been used as cancer fighting chemicals.

The evolutionary story suggests that 100 million years after a super nova exploded, the early Earth was formed, and then bombarded by billions of meteors, some of which would be carbonaceous chondrites and, if there was an ocean in place to receive these meteors, the chemistry of life could have gotten a jump start towards life itself. 

 Missing Pieces of the Puzzle

 Many pieces of the puzzle are missing to make this scenario work. One requirement for precursor molecules is the need for two types of nucleobases, purines and pyrimidines.  DNA and RNA, the genetic information of life, require 4 nucleobases, two are purines and two are pyrimidines. Meteorites have shown no pyrimidine molecules or pyrimidine-analogues. If pyrimidines were to be found in such a rock, would that improve the chance of DNA being formed in some primordial soup? This is not likely as biological chemistry is far more complex than this.

Both of these types of molecules must be connected to a certain type of sugar, of which there are dozens, none of which have been found in meteorites –yet! Only a 5 carbon sugar is used by life to build the blocks of molecules needed for constructing genetic information. Such dimeric molecules (a molecule made of two molecules) are called nucleosides. To this 5 carbon sugar and nucleobase, inorganic phosphate must be attached creating a triune molecular entity called a nucleotide.  Cells are able to build and to use nucleotides with, not one, but three phosphate molecules attached to the nucleoside in order assemble genetic information. Such building blocks of DNA and RNA are called nucleotide triphosphates.

If we were to suppose that pyrimidine and purine nucleobases, of the right kind, were in great supply for several hundred million years, along with plenty of phosphate, and the right kind of sugar molecules, by what mechanism would these three molecules collide in just the right ways to create a nucleotide? More important, by what mechanism would these nucleotides collide in just the right way to form a perfectly linear chain of deoxyribose nucleic acid (DNA)?  Furthermore, by what means would this DNA be protected from destruction before all of the other necessary biochemicals were fortuitously tossed together to form life? What would prevent the other 14000 types of organic molecules from interfering with these specific chemical reactions? Even more important than creating a strand of DNA, by what mechanism would the sequence of nucleotides be ordered to make up the strand so that it would contain the information needed to communicate how to build a cell? And even more important than this; by what mechanism would this new information be communicated to the world around it such that a minimum of 250 proteins AND dozens of RNA molecules would be either formed or recruited in order to unite, in a finely tuned orchestration, to give rise to living substance? And even more important than all of this is the need for a lipid membrane to capture all of these components into a convenient package that we can recognize as a living cell?  The complexity of the issues that arise even if a living cell was formed by “accident” only escalates the multitude of conditions that must be invoked in order for such a thing to continue to survive.

Considering the unknown parameters that must be assumed in order to enable a scientist to seriously consider the idea that molecules inside meteors may have contributed in some way to the evolution of life, it would be irrational to draw such a conclusion from these scientific findings. The ingredients and conditions required for life to spontaneously come into existence and then give rise to the millions of living forms that currently populate our planet are not known. The multitude of conditions in space, on the Earth and in subsequent ages of planetary/solar system development are also unknown and these add to the burden of proof that is needed to give support to the interpretation of these chemical findings.

Finally, science must not ignore our current understanding of biochemistry, genetics, cell biology, astronomy, ecology, cosmology, physics and statistical probabilities which individually deny the possibility of life arising randomly and collectively represent a proof positive that the creation of life demands an intelligent agent. 

Keep in mind, the demand for a creator does not arise because life is complex. Rather the complexity of life is so fully understood that consideration of any scenario other than a designer borders dangerously on fantasy. Considering what has been discovered in a very few meteorite rocks, is it honest for popular media to make the statement: “We’re all aliens . . . how humans began life in outer space”?

 

Dan Moran is a Fulbright Scholar who earned a Ph.D. in Molecular and Cellular Biology from Ohio University.

 

[1] A nitrogen containing compound essential to create the building blocks of genetic information.

[2] Analogues are chemically similar to nucleobases.  Some analogues of nucleotides (which are the building blocks for genetic information) are developed into drugs to combat cancer.  Fast growing cells like cancer cells will incorporate these analogues into their DNA resulting in the death of the cell.