Steven A. Benner

Steven Albert Benner
Born (1954-10-23) October 23, 1954[1]
Nationality United States
Fields Chemistry, synthetic biology
Institutions Harvard University
ETH Zurich
University of Florida, Foundation for Applied Molecular Evolution
Alma mater Yale University
Harvard University
Doctoral advisor Robert Burns Woodward, Frank Westheimer
Website
http://www.ffame.org/

Steven Albert Benner (born October 23, 1954) has been a professor at Harvard University, ETH Zurich, and the University of Florida where he was the V.T. & Louise Jackson Distinguished Professor of Chemistry. In 2005, he founded The Westheimer Institute of Science and Technology (TWIST) and the Foundation For Applied Molecular Evolution. Benner has also founded the companies EraGen Biosciences and Firebird BioMolecular Sciences LLC.

Benner and his colleagues were the first to synthesize a gene, beginning the field of synthetic biology. He was instrumental in establishing the field of paleogenetics. He is interested in the origin of life and the chemical conditions and processes needed to produce RNA. Benner has worked with NASA to develop detectors for alien genetic materials, using the definition of life developed by the NASA Exobiology Discipline Working Group in 1992, “a self-sustaining chemical system capable of Darwinian evolution”.[2][3][4][5]

Education

Benner attended Yale University, receiving his B.S./M.S. in Molecular Biophysics and Biochemistry in 1976. He then went to Harvard University, receiving his Ph.D. in Chemistry in 1979.[6] He worked under the supervision of Robert Burns Woodward, completing his thesis work with Frank Westheimer after Woodward's death. His Ph.D. thesis was Absolute stereochemistry of acetoacetate decarboxylase, betaine-homocysteine transmethylase, and 3-hydroxybutyrate dehydrogenase.[7]

Career

After graduating from Harvard University, Benner became a fellow at Harvard, receiving the Dreyfus Award for Young Faculty in 1982. He was an Assistant Professor in the Department of Chemistry at Harvard University from 1982-1986.[8]

In 1986, Benner moved to ETH Zurich, the Swiss Federal Institute of Technology in Zurich.[9] He held the positions of Associate Professor of Bio-organic Chemistry from 1986 to 1993 and Professor of Bio-organic Chemistry from 1993 to 1996.[8]

By 1996[10] Benner joined the faculty at the University of Florida, as a professor in both chemistry and cell & molecular biology. He was appointed the V.T. & Louise Jackson Distinguished Professor of Chemistry at the University of Florida's Department of Chemistry in 2004.[11]

Benner left University of Florida in late December 2005 to found The Westheimer Institute of Science and Technology (TWIST) in Honor of Frank Westheimer. It is part of the Foundation For Applied Molecular Evolution (FfAME) in Alachua, Florida, which Benner founded in 2001.[12]

Benner founded EraGen Biosciences in 1999. The company was acquired by Luminex in 2011.[13][14] He founded Firebird BioMolecular Sciences LLC in 2005.[12][15][16]

Research

Benner's research falls into four major areas:

  1. expanding the genetic alphabet by synthesizing artificial structures
  2. pre-biotic chemistry, the recreation of the chemical origin of life
  3. paleogenetics, the study of ancient proteins from long-extinct species
  4. detection of extraterrestrial life[17]

The Benner laboratory is an originator of the field of "synthetic biology", which seeks to generate, by chemical synthesis, molecules that reproduce the complex behavior of living systems, including their genetics, inheritance, and evolution. Some high points of past work in chemical genetics are listed below.

Gene synthesis

In 1984, Benner's laboratory at Harvard was the first to report the chemical synthesis of a gene encoding an enzyme,[18][19][20] following Khorana's synthesis of a shorter gene for tRNA in 1970.[21] This was the first designed gene of any kind, a pioneering achievement that laid the groundwork for protein engineering.[22] The design strategies introduced in this synthesis are now widely used to support protein engineering.[23]

Artificial genetic systems

Efforts toward the goal of artificial genetic systems were first reported by Benner and coworkers in 1989, when they developed the first unnatural base pair.[24][25][26][27] Benner and his colleagues have since developed a six-letter artificially expanded genetic information system called Artificially Expanded Genetic Information System (AEGIS) which includes two additional nonstandard nucleotides (Z and P) in addition to the four standard nucleotides (G, A, C, and T).[28][29][30][31] AEGIS has its own supporting molecular biology.[5] It enables the synthesis of proteins with more than the naturally-encoded 20 amino acids, and provides insight into how nucleic acids form duplex structures, how proteins interact with nucleic acids,[32] and how alternative genetic systems might appear in non-terran life.[33]

Benner is one of a number of researchers, including Eric T. Kool, Floyd E. Romesberg, Ichiro Hirao, Mitsuhiko Shionoya and Andrew Ellington, who have created an extended alphabet of synthetic bases that can be incorporated into DNA (as well as RNA) using Watson-Crick bonding (as well as non-Watson-Crick bonding). While most of these synthetic bases are derivatives of the A, C, G, T bases, some are different. While some are in Watson-Crick pairs (A/T, C/G), some are self complementing (X/X). Thus the genetic alphabet has been expanded.[15][25][27][34][35][36][37][38]:88–98

The number of possible nucleotide triplets, or codons, available in protein synthesis depends on the number of nucleotides available. The standard alphabet (G, A, C, and T) yields 43 = 64 possible codons, while an expanded DNA alphabet with 9 DNA bases would have 93 = 729 possible codons, many of them synthetic codons. For these codons to be useful, Aminoacyl tRNA synthetase has been created such that tRNA can code for the possibly synthetic amino acid to be coupled with its corresponding synthetic anti-codon. Brenner has described such a system which uses synthetic iso-C/iso-G DNA which uses the synthetic DNA codon [iso-C/A/G] which he calls the 65th codon. Synthetic mRNA with synthetic anti-codon [iso-G/U/C] with synthetic aminoacyl-tRNA synthetase results in an in vivo experiment that can code for a synthetic amino acid incorporated into synthetic polypeptides (synthetic proteomics).[38]:100–106

A "second generation" model for nucleic acids

Benner has used synthetic organic chemistry and biophysics to create a "second generation" model for nucleic acid structure. The first generation model of DNA was proposed by James Watson and Francis Crick, based on crystallized X-ray structures being studied by Rosalind Franklin. According to the double-helix model, DNA is composed of two complementary strands of nucleotides coiled around each other.[39] Benner's model emphasizes the role of the sugar and phosphate backbone in the genetic molecular recognition event. The poly-anionic backbone is important in creating the extended structure that helps DNA to replicate.[40][41][42]

In 2004, Benner reported the first successful attempt to design an artificial DNA-like molecule capable of reproducing itself.[22]

Genome sequencing and protein structure prediction

In the late 1980s, Benner recognized the potential for genome sequencing projects to generate millions of sequences and enable researchers to do extensive mapping of molecular structures in organic chemistry. In the early 1990s, Benner met Gaston Gonnet, beginning a collaboration that applied Gonnet's tools for text searching to the management of protein sequences.[43][44] In 1990, in collaboration with Gaston Gonnet, the Benner laboratory introduced the DARWIN bioinformatics workbench. DARWIN (Data Analysis and Retrieval With Indexed Nucleic acid-peptide sequences) was a high-level programming environment for examining genomic sequences. It supported the matching of genomic sequences in databases, and generated information that showed how natural proteins could divergently evolve under functional constraints by accumulating mutations, insertions, and deletions.[45] Building on Darwin, the Benner laboratory provided tools to predict the three dimensional structure of proteins from sequence data. Information about known protein structures was collected and marketed as a commercial database, the Master Catalog, by Benner's startup EraGen.[45]

The use of multiple sequence information to predict secondary structure of proteins became popular as a result of the work of Benner and Gerloff.[46][47][48] Predictions of protein secondary structure by Benner and colleagues achieved high accuracy.[49] It became possible to model protein folds, detect distant homologs, enable structural genomics, and join protein sequence, structure, and function. Further, this work suggested limits to structure prediction by homology, defining what can and cannot be done with this strategy.[45]

Practical genotyping tools

Benner's approach opened new perspectives on how nucleic acids work, as well as tools for diagnostics and nanotechnology. The FDA has approved products that use AEGIS DNA in human diagnostics. These monitor the loads of virus in patients infected with hepatitis B, hepatitis C and HIV.[50] AEGIS has been the basis of the development of tools for multiplexed detection of genetic markers such as cancer cells[51] and single nucleotide polymorphisms in patient samples. These tools will allow personalized medicine using "point-of-care" genetic analysis,[52] as well as research tools that measure the level of individual mRNA molecules within single processes of single living neurons.[53]

Interpretive proteomics

Interpreting genomic data and projecting back to a common genetic ancestor, "Luca", the Benner laboratory has introduced tools that analyze patterns of conservation and variation using structural biology, study variation in these patterns across different branches of an evolutionary tree, and correlate events in the genetic record with events in the history of the biosphere known from geology and fossils. From this have emerged examples showing how the roles of biomolecules in contemporary life can be understood through models of the historical past.[54][55]

Experimental paleogenetics

For more details on this topic, see Ancestral gene resurrection.

Benner was an originator of the field of experimental paleogenetics, where genes and proteins from ancient organisms are resurrected using bioinformatics and recombinant DNA technology.[56] Experimental work on ancient proteins has tested hypotheses about the evolution of complex biological functions, including the biochemistry of ruminant digestion,[57][58]:209 the thermophily of ancient bacteria, and the interaction between plants, fruits, and fungi at the time of the Cretaceous extinction.[58]:17 These develop our understanding of biological behavior that extends from the molecule to the cell to the organism, ecosystem, and planet, sometimes referred to as planetary biology.[58]:221

Astrobiology

Benner is deeply interested in the origin of life, and the conditions necessary to support an RNA-world model in which self-replicating RNA is a precursor to life on Earth. He has identified calcium, borate, and molybdenum as important to the successful formation of carbohydrates and the stabilization of RNA.[59] He suggested that the planet Mars may have had more desirable conditions than Earth for the initial production of RNA,[60][61] but more recently agreed that models of early Earth showing dry land and intermittent water, developed by Stephen Mojzsis, present sufficient conditions for RNA development.[12]

The Benner group has worked to identify molecular structures likely to be universal features of living systems regardless of their genesis, and not likely products of non-biological processes. These are "biosignatures", both for terrean-like life and for "weird" life forms.[3][62][63]

References

  1. ↑ "Benner, Steven A. (Steven Albert), 1954-". Library of Congress Authority Records. Retrieved 30 June 2016.
  2. ↑ Mullen, Leslie (August 1, 2013). "Defining Life: Q&A with Scientist Gerald Joyce". Astrobiology Magazine. Retrieved 5 July 2016.
  3. 1 2 Benner, Steven A. (December 2010). "Defining Life". Astrobiology. 10 (10): 1021–1030. doi:10.1089/ast.2010.0524. PMC 3005285Freely accessible.
  4. ↑ Klotz, Irene (February 27, 2009). "Synthetic life form grows in Florida lab". Science. Retrieved 5 July 2016.
  5. 1 2 Lloyd, Robin (February 14, 2009). "New Artificial DNA Points to Alien Life". LiveScience. Retrieved 5 July 2016.
  6. ↑ Impey, Chris Impey; Spitz, Anna H.; Stoeger, William, eds. (2013). Encountering life in the universe : ethical foundations and social implications of astrobiology. Tucson: University of Arizona Press. p. 259. ISBN 978-0-8165-2870-7. Retrieved 30 June 2016.
  7. ↑ "Steven A. Benner". Chemistry Tree. Retrieved 30 June 2016.
  8. 1 2 "Events at Rice". Rice University. Retrieved 1 July 2016.
  9. ↑ Kwok, Roberta (21 November 2012). "Chemical biology: DNA's new alphabet". Nature. 491 (7425): 516–518. doi:10.1038/491516a. Retrieved 1 July 2016.
  10. ↑ Brenner, Steven A. "Non-Standard Base Pairs as Biomedical Research Tools". GRANTOME. National Institutes of Health. Retrieved 1 July 2016.
  11. ↑ "Participants". The Humble Approach Initiative. Retrieved 1 July 2016.
  12. 1 2 3 Clark, Anthony (March 24, 2016). "Local team to head $5.4 million quest to study origins of life on Earth". The Gainesville Sun. Retrieved 30 June 2016.
  13. ↑ Wyzan, Andrew (July 12, 2011). "Former Gainesville biotech sold for $34 million". The Gainsville Sun. Retrieved 1 July 2016.
  14. ↑ Carroll, John. "Luminex snaps up EraGen Biosciences in $34M deal". Fierce Biotech. Retrieved June 22, 2011.
  15. 1 2 Howgego, Josh (25 February 2014). "On stranger nucleotides". ChemistryWorld. Retrieved 1 July 2016.
  16. ↑ "Firebird BioMolecular Sciences LLC".
  17. ↑ "President's Dream Colloquium". Simon Fraser University. Retrieved 1 July 2016.
  18. ↑ Gross, Michael (August 2011). "What exactly is synthetic biology?". Current Biology. 21 (16): R611–R614. doi:10.1016/j.cub.2011.08.002. Retrieved 5 July 2016.
  19. ↑ Nambiar, K.; Stackhouse, J; Stauffer, D.; Kennedy, W.; Eldredge, J.; Benner, S. (23 March 1984). "Total synthesis and cloning of a gene coding for the ribonuclease S protein" (PDF). Science. 223 (4642): 1299–1301. doi:10.1126/science.6322300. Retrieved 5 July 2016.
  20. ↑ D'Alessio, Giuseppe; Riordan, James F. (1997). Ribonucleases structures and functions. San Diego: Academic Press. p. 214. ISBN 9780125889452. Retrieved 5 July 2016.
  21. ↑ Khorana, H.G.; Agarwal, K.L.; Büchi, H.; Caruthers, M.H.; Gupta, N.K.; Klbppe, K.; Kumar, A.; Ohtsuka, E.; RajBhandary, U.L.; van de Sande, J.H.; Sgaramella, V.; Tebao, T.; Weber, H.; Yamada, T. (December 1972). "CIII. Total synthesis of the structural gene for an alanine transfer ribonucleic acid from yeast". Journal of Molecular Biology. 72 (2): 209–217. doi:10.1016/0022-2836(72)90146-5. Retrieved 5 July 2016.
  22. 1 2 Gramling, Carolyn (2005). "For Chemistry Professor Steven Benner, Life As We Know It May Not Be The Only Alternative". Amazing Science. 10 (1). Retrieved 9 July 2016.
  23. ↑ Köhrer, Caroline; RajBhandary, Uttam L., eds. (2009). Protein engineering. Berlin: Springer. pp. 274–281, 297. ISBN 978-3-540-70941-1. Retrieved 5 July 2016.
  24. ↑ Fikes, Bradley J. (May 8, 2014). "Life engineered with expanded genetic code". San Diego Union Tribune. Retrieved 5 July 2016.
  25. 1 2 Matsuda, Shigeo; Fillo, Jeremiah D.; Henry, Allison A.; Rai, Priyamrada; Wilkens, Steven J.; Dwyer, Tammy J.; Geierstanger, Bernhard H.; Wemmer, David E.; Schultz, Peter G.; Spraggon, Glen; Romesberg, Floyd E. (August 2007). "Efforts toward Expansion of the Genetic Alphabet: Structure and Replication of Unnatural Base Pairs". Journal of the American Chemical Society. 129 (34): 10466–10473. doi:10.1021/ja072276d. PMC 2536688Freely accessible.
  26. ↑ Switzer, Christopher; Moroney, Simon E.; Benner, Steven A. (October 1989). "Enzymatic incorporation of a new base pair into DNA and RNA". Journal of the American Chemical Society. 111 (21): 8322–8323. doi:10.1021/ja00203a067. Retrieved 5 July 2016.
  27. 1 2 Piccirilli, Joseph A.; Benner, Steven A.; Krauch, Tilman; Moroney, SimonE.; Benner, Steven A. (4 January 1990). "Enzymatic incorporation of a new base pair into DNA and RNA extends the genetic alphabet". Nature. 343 (6253): 33–37. doi:10.1038/343033a0. PMID 1688644.
  28. ↑ Benner, SA; Hutter, D; Sismour, AM (2003). "Synthetic biology with artificially expanded genetic information systems. From personalized medicine to extraterrestrial life.". Nucleic acids research. Supplement (3): 125–6. PMID 14510412. Retrieved 5 July 2016.
  29. ↑ Yang, Z; Hutter, D; Sheng, P; Sismour, AM; Benner, SA (2006). "Artificially expanded genetic information system: a new base pair with an alternative hydrogen bonding pattern". Nucleic acids research. 34 (21): 6095–101. doi:10.1093/nar/gkl633. PMC 1635279Freely accessible. PMID 17074747. Retrieved 1 July 2016.
  30. ↑ Yang, Zunyi; Chen, Fei; Alvarado, J. Brian; Benner, Steven A. (28 September 2011). "Amplification, Mutation, and Sequencing of a Six-Letter Synthetic Genetic System". Journal of the American Chemical Society. 133 (38): 15105–15112. doi:10.1021/ja204910n.
  31. ↑ Merritt, Kristen K; Bradley, Kevin M; Hutter, Daniel; Matsuura, Mariko F; Rowold, Diane J; Benner, Steven A (9 October 2014). "Autonomous assembly of synthetic oligonucleotides built from an expanded DNA alphabet. Total synthesis of a gene encoding kanamycin resistance". Beilstein Journal of Organic Chemistry. 10: 2348–2360. doi:10.3762/bjoc.10.245. Retrieved 1 July 2016.
  32. ↑ Laos, Roberto; Thomson, J. Michael; Benner, Steven A. (31 October 2014). "DNA polymerases engineered by directed evolution to incorporate non-standard nucleotides". Frontiers in Microbiology. 5. doi:10.3389/fmicb.2014.00565. Retrieved 5 July 2016.
  33. ↑ Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life ; Space Studies Board, Division on Engineering and Physical Sciences ; Board on Life Sciences, Division on Earth and Life Sciences ; National Research Council of the National Academies (2007). "4. Alternatives to Terran Biochemistry in Water". The limits of organic life in planetary systems. Washington, D.C.: National Academies Press. ISBN 978-0-309-10484-5.
  34. ↑ Pollack, Andrew (July 24, 2001). "Scientists Are Starting to Add Letters to Life's Alphabet". The New York Times. Retrieved 30 June 2016.
  35. ↑ Singer, Emily (July 10, 2015). "New Letters Added to the Genetic Alphabet". Quanta Magazine. Retrieved 30 June 2016.
  36. ↑ Switzer, CY; Moroney, SE; Benner, SA (5 October 1993). "Enzymatic recognition of the base pair between isocytidine and isoguanosine.". Biochemistry. 32 (39): 10489–96. doi:10.1021/bi00090a027. PMID 7691174.
  37. ↑ Takezawa, Yusuke; Shionoya, Mitsuhiko (18 December 2012). "Metal-Mediated DNA Base Pairing: Alternatives to Hydrogen-Bonded Watson–Crick Base Pairs". Accounts of Chemical Research. 45 (12): 2066–2076. doi:10.1021/ar200313h. Retrieved 30 June 2016.
  38. 1 2 Simon, Matthew (2005). Emergent computation emphasizing bioinformatics. New York: AIP Press/Springer Science+Business Media. ISBN 978-0-387-27270-2.
  39. ↑ Watson JD, Crick FH (1953). "The structure of DNA". Cold Spring Harb. Symp. Quant. Biol. 18: 123–31. doi:10.1101/SQB.1953.018.01.020. PMID 13168976.
  40. ↑ Committee on the Limits of Organic Life in Planetary Systems, Committee on the Origins and Evolution of Life ; Space Studies Board, Division on Engineering and Physical Sciences ; Board on Life Sciences, Division on Earth and Life Sciences ; National Research Council of the National Academies (2007). "4. Alternatives to Terran Biochemistry in Water". The limits of organic life in planetary systems. Washington, D.C.: National Academies Press. ISBN 978-0-309-10484-5.
  41. ↑ Benner, Steven (2004). "Evolution-based genome analysis: An alternative to analyze folding and function in proteins". In Westhof, E.; Hardy, N. Folding and Self-assembly of Biological and Macromolecules : proceedings of the deuxièmes Entretiens de Bures, Bures-sur-Yvette, France, 27 November - 1 December 2001. Singapore: World Scientific. pp. 1–42. ISBN 978-981-238-500-0. Retrieved 6 July 2016.
  42. ↑ Benner, Steven A.; Hutter, Daniel (February 2002). "Phosphates, DNA, and the Search for Nonterrean Life: A Second Generation Model for Genetic Molecules" (PDF). Bioorganic Chemistry. 30 (1): 62–80. doi:10.1006/bioo.2001.1232. Retrieved 6 July 2016.
  43. ↑ "Prof. Gaston Gonnet: when technology holds the key to evolution". ETH Zurich. Retrieved 9 July 2016.
  44. ↑ Gonnet, GH; Cohen, MA; Benner, SA (5 June 1992). "Exhaustive matching of the entire protein sequence database." (PDF). Science (New York, N.Y.). 256 (5062): 1443–5. doi:10.1126/science.1604319. PMID 1604319. Retrieved 9 July 2016.
  45. 1 2 3 "Genomics Meets Geology". AstroBiology Magazine. September 10, 2001. Retrieved 1 July 2016.
  46. ↑ Jones, David T. (1999). "Protein Secondary Structure Prediction Based on Position-specific Scoring Matrices" (PDF). Journal of Molecular Biology. 292: 196. Retrieved 6 July 2016.
  47. ↑ Benner, SA; Gerloff, D (1991). "Patterns of divergence in homologous proteins as indicators of secondary and tertiary structure: a prediction of the structure of the catalytic domain of protein kinases". Advances in Enzyme Regulation. 31: 121–81. doi:10.1016/0065-2571(91)90012-b. PMID 1877385.
  48. ↑ Gonnet, Gaston H.; Korostensky, Chantal; Benner, Steve (February 2000). "Evaluation Measures of Multiple Sequence Alignments". Journal of Computational Biology. 7 (1-2): 261–276. doi:10.1089/10665270050081513. PMID 10890401.
  49. ↑ Russell, R.B.; Sternberg, M.J.E. (May 1995). "Structure Prediction: How good are we?". Current Biology. 5 (5): 488–490. doi:10.1016/S0960-9822(95)00099-6. Retrieved 6 July 2016.
  50. ↑ Spoto, Giuseppe; Corradini, Roberto, eds. (2012). Detection of non-amplified genomic DNA. Dordrecht: Springer. p. 104. ISBN 978-94-007-1226-3. Retrieved 6 July 2016.
  51. ↑ Dambrot, Stuart Mason (January 24, 2014). "The ties that bind: Recreating Darwinian ligand evolution in vitro". Phys.org. Retrieved 6 July 2016.
  52. ↑ Jannetto, Paul J.; Laleli-Sahin, Elvan; Wong, Steven H. (1 January 2004). "Pharmacogenomic genotyping methodologies" (PDF). Clinical Chemistry and Laboratory Medicine. 42 (11). doi:10.1515/CCLM.2004.246. Retrieved 6 July 2016.
  53. ↑ "Award Abstract #0304569 Nanoscale Arrays for Direct RNA Profiling in Single Cells and their Compartments". National Science Foundation. Retrieved 6 July 2016.
  54. ↑ Plaxco, Kevin W.; Gross, Michael (2006). Astrobiology : a brief introduction. Baltimore: Johns Hopkins University Press. pp. 165–170. ISBN 978-0801883675. Retrieved 6 July 2016.
  55. ↑ Benner, Steven A. (June 2003). "Interpretive proteomics—finding biological meaning in genome and proteome databases" (PDF). Advances in Enzyme Regulation. 43 (1): 271–359. doi:10.1016/S0065-2571(02)00024-9. Retrieved 6 July 2016.
  56. ↑ Jermann, TM; Opitz, JG; Stackhouse, J; Benner, SA (2 March 1995). "Reconstructing the evolutionary history of the artiodactyl ribonuclease superfamily." (PDF). Nature. 374 (6517): 57–9. doi:10.1038/374057a0. PMID 7532788. Retrieved 6 July 2016.
  57. ↑ Benner, SA; Caraco, MD; Thomson, JM; Gaucher, EA (3 May 2002). "Planetary biology--paleontological, geological, and molecular histories of life.". Science. 296 (5569): 864–8. Bibcode:2002Sci...296..864B. doi:10.1126/science.1069863. PMID 11988562.
  58. 1 2 3 Liberles, David A. (2007). Ancestral sequence reconstruction. Oxford: Oxford University Press. p. 221. ISBN 9780199299188.
  59. ↑ Ward, Peter; Kirschvink, Joe (2014). A New History of Life: The Radical New Discoveries About the Origins and Evolution of Life on Earth. USA: Bloomsbury. pp. 55–60. ISBN 978-1608199075. Retrieved 6 July 2016.
  60. ↑ Zimmer, Carl (26 June 2004). "What Came Before DNA?". Discover. Waukesha, WI: Kalmbach Publishing. ISSN 0274-7529.
  61. ↑ Zimmer, Carl (September 12, 2013). "A Far-Flung Possibility for the Origin of Life". The New York Times. Retrieved 1 July 2016.
  62. ↑ Boyd, Robert S. (November 11, 2002). "ANY BEING OUT THERE? Extreme Earth environments test astrobiology ideas". Philadelphia Inquirer. Retrieved 6 July 2016.
  63. ↑ Greenwood, Veronique (November 9, 2009). "What Life Leaves Behind What We Know: The search for life beyond our pale blue dot is fraught with dashed hopes. Will the chemical and mineral fingerprints of Earthly organisms apply on other worlds?". Seed Magazine. Retrieved 6 July 2016.
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