Thinking about life
I recently read a book that slipped by me while I was growing up, Little Fuzzy, by H. Beam Piper. It is one of the classics of science fiction genre, first published in 1962 and is now available via Project Gutenberg. It is a very approachable story, and details the meeting between an independent prospector named Jack Holloway and the titular “Little Fuzzy” of the novel. Jack believes the alien to be intelligent, a position not taken by the Zarathustra Mining Corporation, whose hold planet Zarathustra is contingent on there being no indigenous intelligent population. The second half of the novel is taken up by an extensive discussion on what intelligence entails, and how sapience or self-awareness can be determined. Many of the arguments can ultimately be summarized in the same manner as recognizing pornography: “I’ll know it when I see it.” This turns out to be significantly more difficult that you might imagine.
Our understanding of what life itself looks like is also subjected to the same blinders above; we can look for and study life that looks like life on Earth, and expanding our parameters thinking outside the box in the search for life is difficult. One of my favorite treatments of this topic comes from an old episode of Star Trek, which posited a race of silicon-based creatures. The rationale for silicon-based life comes from the ability of the element silicon to form chemical bonds reminiscent of those found in organic (carbon-based) compounds, although the scientific basis from Star Trek is reasonably weak. In fact, most discussions of an alternative biochemistry such as in this episode quickly turn to the science-fictional. Most of our current searches for life outside of Earth tend to focus on life as we know it, since that is what we are best equipped to study.
I find scientific reports which extend the possibilities of life very exciting then. Longtime followers of ycpmicro recall how excited I was last fall when I read about terrestrial life that was able to incorporate arsenic into biomolecules. The study made the extraordinary claim that they had identified a bacterium from a hypersaline lake in California that was able to grow in the laboratory with essentially all phosphorus replaced with arsenic. Arsenic and phosphorus share the same relation on the periodic table that carbon and silicon do, and arsenic can undergo many of the same chemical reactions in molecules that phosphorus can. Phosphorus is an element essential for life; it is incorporated into lipids and nucleic acids, and the high energy phosphate bond in ATP is one of the most important energy storage compounds in all living systems. However, arsenic does not function to the same degree from a biochemical standpoint, and therefore actually is poisonous to living cells.
The data in Wolfe-Simon et al can be briefly summarized as follows: the authors isolated a novel species of bacterium from a high arsenic environment in Mono Lake, California, then in the laboratory they subjected the isolate to increasingly higher levels of arsenic for multiple generations. They then looked to see if arsenic was being incorporated into biomolecules using several approaches:
- Cells grown in high arsenic containing media contained 10 times the amount of arsenic in comparison to phosphate and very low levels of phosphorus, as measured by mass spectrometry. In contrast, the same strain grown in low arsenic containing media had undetectable levels of arsenic inside the cell, and normal levels of phosphorus.
- Using radioactive arsenic and standard cell fractionation procedures, they detected incorporated arsenic in proteins, lipids, and nucleic acids.
- The weight of DNA isolated from the bacterium grown in high arsenic conditions was estimated to be heavier than normal DNA, consistent with the higher molecular mass of the arsenic atom in comparison to the phosphorus atom.
The report was rapidly picked up by the mainstream media, and became the topic of a fair amount of critique via the Internet. Many critics took the position that extraordinary claims require extraordinary proof. I disagree this statement; in science, proof is not the aim, but the best possible hypothesis to explain the current set of data. The authors of the original paper took the stance that they wanted the discussion to be conducted using the process of peer review, an imperfect process that is our best method for conducting science today. A fair enough position, and this week Science magazine took the unusual step of publishing 8 letters to the editor (all peer-reviewed) along with a response by the original authors of the manuscript. The upshot of the discussion is that the authors maintain that the best explanation of the observations is that strain CFAJ-1 is able to incorporate arsenic into biomolecules. Critics maintain that the observations may be explained by incomplete purification and contamination during their biomolecule analysis, and that strain CFAJ-1 is growing in very low levels of phosphorus, and making ‘normal’ biomolecules.
No new data was presented in this week’s scientific conversation, and independent confirmation of the ability of CFAJ-1 to incorporate arsenic has not been accomplished yet. This has been a fascinating discourse to watch unfold, and although I certainly agree with Wolfe-Simon et al‘s desire to defend their work in an open forum in a peer-reviewed form, most science occurs outside of scientific journals, in open discussion at meetings, in conferences, between laboratories, and today via Internet conversations.