Preface: 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 or “organic chemicals” to form. Biological chemistry is needed to spontaneously form some sort of self replicating molecular entity 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 scientist would be satisfied that a molecule capable of 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 drawbacks. 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 precursors 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 research lab’s findings on the molecular analysis of 12 meteorites; specifically the discovery of a wide range of nucleobases (1) and nucleobase analogs that were extracted from the interior of certain carbon-rich meteorites. This finding, at first glance seems extraordinary. The further finding that 14,000 other organic molecules have been found adds to the excitement, if in fact the rock has not been contaminated by the Earth’s biology. The excitement was not so much that the rock contained so many carbon compounds or even 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; suggesting organic chemistry occurs by an undirected evolutionary process. Secondly, the finding implies that the meteorite may have contained these compounds before entering the Earth’s atmosphere; therefore they had to be from outer space. Putting the two points together the findings may 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 for a fact are both created by and used by living cells to form the DNA and RNA molecules; chemicals that we know carry genetic information.
The extrapolation of these interesting findings has led the NASA scientist to endorse the idea that life may have had its origin in a global pool of organic molecules created when billions of these particular types of meteors hit the Earth 4 billion years ago. This scenario possibly improved the probability that the nucleobases then seeded the oceans with the basic carbon structures upon which nitrogenous bases form. These then could react with sugars and phosphates resulting in the first nucleotide building blocks used to synthesize 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 new 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 ‘space’ processes gave rise to organic molecules? Where was the meteorite found on the planet and how long did it lay there before discovery? How was the rock 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 even embedded in a rock? 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? It is vital that these questions be asked and that a rational answer be given or it is all smoke and mirrors and no amount of chemistry, time or meteor bombardment is useful in developing origin life research.
A summary of on the 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 is heavier than hydrogen and helium, the substances 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 media of gas and dust that did not form planets and stars combined to form larger particles, rocks, planetoids, and planets. Some of these large entities like planets and planetoids collided to create smaller materials such as comets and asteriods. Asteroids might have been created from some of these collisions and some asteroids are believed to represent the earliest substances from which our solar system or at least the rocky planets of Mercury, Venus, Earth and Mars were made from. EVERYTHING IN THE ABOVE IS HYPOTHETICAL except the very first sentence: stars exist and space is vast.
One particular class of meteor is believed to have grown from a collection of interstellar dust. This dust may have been 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. PLEASE NOTE THAT NO EVIDENCE EXISTS THAT METEORS FORM IN THIS FASHION OR THAT SUCH CHEMISTRIES AS DESCRIBED ABOVE ARE KNOWN TO OCCUR.
Chunks of these rocks enter the earth’s atmosphere as meteors and when they hit the earth they are then called meteorites. Some isotope dating methods and their assumptions place these rocks at very old ages and support the idea that they have originated in outer space. Also, some of the ‘carbonaceous chondrites’ have been collected from ‘falls’; meteorites collected shortly after they were seen to fall as meteors. Such finds are very important as they exclude contamination from living substances on the earth.
Studies of the volatility and or the “endurance” of these interesting organic chemicals inside these rocks indicate that a meteor can heat to over 1000 degrees F when entering our atmosphere. A smaller percentage of meteors may only reach 500-700 degrees F and these temperatures may not be sufficient to penetrate the interior of the rock and thus destroy the ancient organic molecules; possibly delivering those to our Earth unaltered.
If all these conditions were true, the chemicals that have been extracted from these rocks by the NASA scientists are, in some cases, familiar to our biological chemistry. Some are chemicals quite uncommon to our biology (though not absent) 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 confirm that they are probably from outer space. Interestingly, some chemicals are so toxic that medical science has actual 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.