Module 5 - The Search for Life

Ingredients and requirements for life

If we want examine the ingredients of life at the most elemental level, we need to start with, well, elements. We all have a large number of different elements within our bodies, including metals (e.g. you can buy Zinc supplements).  These play important roles but most exist in minute quantities within your body.  If we want to narrow our investigation to the most common or most important elements, two acronyms are worth remembering. CHON stands for Carbon (C), Hydrogen (H), Oxygen (O), and Nitrogen (N) and describes the 4 most common elements.  CHNOPS stands for Carbon (C), Hydrogen (H), Nitrogen (N), Oxygen (O), Phosphorus (P) and Sulfur (S), representing the six most important elements in all terrestrial life. The Sulfur helps to catalyze or facilitate reactions within the body (Selenium may substitute within some organisms). 

Phosphorus, primarily in the form of adenosine triphosphate (ATP), is vitally important for energy transfer during metabolism.  ATP is a complex molecule, with a formula of C10H16N5O13P3, so you can think of it as CHON+P.  The role of P is so universal that some NASA scientists caused quite a stir in 2010 when they reported to have found a strain of bacteria, apparently capable of using arsenic instead of phosphorus.  Their paper was entitled ” A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus” and they stated “Exchange of one of the major bioelements may have profound evolutionary and geochemical significance.”  This report had significant impact  (e.g. see this article in space.com)

However, their report in the journal “Science” was met with significant scepticism from other scientists, prompting 8 official comments from other scientist on the original publication. This is highly unusual for scientific papers and even warranted a note from the Editor-in-chief in 2011. In this Editor's Note, the editor states:

Science received a wide range of correspondence that raised specific concerns about the Research Article's methods and interpretations. Eight Technical Comments that represent the main concerns, as well as a Technical Response by Wolfe-Simon et al., are published online with this issue at the addresses listed below (all were previously published in Science Express on 27 May). They have been peer-reviewed and revised according to Science's standard procedure.

The print version of the Wolfe-Simon et al. paper reflects minor clarifications and copyediting of the Science Express version. We hope that publication of this collection will allow readers to better assess the Research Article's original claims and the criticisms of them. Our procedures for Technical Comments and Responses are such that the original authors are given the last word, and we recognize that some issues remain unresolved. However, the discussion published today is only a step in a much longer process.

Since we are not aware of any further confirmations of Arsenic-using bacteria since then, we assume that the reservations of the other scientists were justified.
 
Before we leave these elemental considerations behind, it should also be pointed out that there has been speculation about life not based on CHON.  We’ll only mention that possibility here, though if you want to read about a Universe with very alien creatures you may want to check the fiction of David Brin. The possibility of silicon-based has been discussed considerably, because the Silicon atom has similar properties to Carbon.  However our understanding of silicon chemistry suggests that it is less versatile than carbon, making silicon-based life more problematic and no doubt very different from us.

Fans of the original Star Trek may want to check out the “The Devil in the Dark” episode which deals with a silcon based life form, the Horta. Here is a 5 min review of that episode:

We’ll leave this topic behind with 2 comments:
1) Life not based on CHON, as a silicon life form would be, may indeed be too strange for us to recognize. This is indeed one of the possible answers for the Fermi-Paradox.
2) Just to be on the safe side, we suggest that you be kind to your pet rock, most likely a silicate, just in case our silicon overlords arrive.
 
So, to recap, we’re all fellow- CHNOPS on this planet. Those elements are primarily responsible for making the amino acids, fats, lipids and the smaller nucleobase molecules from which DNA and RNA inside your cells are made. But how do we go from single elements to cells? 

The next step is to look for simple molecules composed of those elements, which could then be combined to form larger, more complex molecules until you get a molecule or suite of molecules that are capable of self-replication and then life is off and living. 

The simple molecules composed primarily of CHONs are referred to as organic molecules (Carbon based) and they are pretty much everywhere.  Earth, as you may expect, is saturated with them and you buy them by the liter, every time you fill up your car or motorcycle (Tesla owners exempt).  Similarly we have detected them on Mars, the atmospheres of the gas giants, on their icy satellites, on comets and meteorites. A class of meteorites called carbonaceous chondrites are called that because they contain more than 600 organic compounds, including amino acids.  But we have not only found them within the solar system, interstellar space is full of molecules ranging from the very simple (CH) to the very complex. In short, there seems to be no shortage of very basic building blocks.  Lets see what we can do with them.

Update 27 May 2016 - Rosetta's comet contains ingredients for life
And we get more evidence that the needed ingredients are quite common from recent results of the Rosetta mission to the comet.  The important aspect of this news release is that scientists confirmed the presence of the amino acid glycine as well as phosporus in outbursts coming from the comet.  To quote the scientists:

"There is still a lot of uncertainty regarding the chemistry on early Earth and there is of course a huge evolutionary gap to fill between the delivery of these ingredients via cometary impacts and life taking hold," says co-author Hervé Cottin.

"But the important point is that comets have not really changed in 4.5 billion years: they grant us direct access to some of the ingredients that likely ended up in the prebiotic soup that eventually resulted in the origin of life on Earth."

"The multitude of organic molecules already identified by Rosetta, now joined by the exciting confirmation of fundamental ingredients like glycine and phosphorous, confirms our idea that comets have the potential to deliver key molecules for prebiotic chemistry," says Matt Taylor, ESA's Rosetta project scientist.

If you want to read the whole news release, here it is:SaveSave

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This page references:

  1. ATP
  2. Star Trek TOS: The Devil in the Dark
  3. CHONPS