HOROWITZ, NORMAN HAROLD

(b. Pittsburgh, Pennsylvania, 19 March 1915; d. Pasadena, California, 2 June 2005),

genetics, biochemistry of metabolism, prebiotic chemistry, origin of life, astrobiology.

Horowitz did pioneering work in two main areas: in the biochemical genetics of Neurospora and in exobiology (later renamed astrobiology), the search for extraterrestrial life and for the origins of life. In a series of experiments from 1942 through 1946, Horowitz, along with others in George Beadle’s group, worked out much of the detail of the experimental work supporting the “one gene–one enzyme hypothesis.” Horowitz was the first to use that expression in print, though he credited Beadle with the concept and with coining the phrase. The pioneering stage of that work was essentially completed by the late 1940s. During the Neurospora work, Horowitz had an important insight into the early evolution of multistep metabolic pathways, which he published in 1945.

Because the origin of metabolic pathways pointed backward logically to the origins of life, this was the beginning of his serious involvement in origin-of-life research, and the new discipline of exobiology into which such research was incorporated after 1959. Horowitz was a seminal thinker on the origins of life and an important experimentalist in the other part of exobiology (later astrobiology), the search for extraterrestrial life that began in earnest with the dawn of the space age. This research dominated Horowitz’s career from 1960 onward. In his capacity as chief of bioscience at the Jet Propulsion Laboratory (JPL) he supervised the design of experiments, including some of his own, which flew on spacecraft Mariner 6 and 7 (1969) and on the Viking Mars Landers (1975–1977). He oversaw the design of the gas chromatograph–mass spectrometer and the pyrolytic release experiment (PR) on Viking 1 and 2.

Horowitz received a BS in biology from the University of Pittsburgh in 1936 and a PhD in biology from the California Institute of Technology (Caltech) in 1939, where Thomas Hunt Morgan assigned him to work under Albert Tyler on marine animals. As an undergraduate, Horowitz had already published a paper on transplantation of tissue in salamanders in the Journal of Experimental Zoology. During 1939 to 1940 he was a National Research Council fellow at Stanford University (at which time he first began work on Neurospora), then a research fellow at Caltech from 1940 to 1942. From 1942 to 1946 Horowitz was a research associate in biology with Beadle’s group at Stanford. Then he returned to Caltech for the remainder of his career, as associate professor in biology (1947–1953), professor of biology (1953–1982), and professor emeritus from 1982 until his death. He was a Guggenheim Fellow at Boris Ephrussi’s lab in Paris from 1954 to 1955. In addition, while continuing as professor at Caltech, from 1965 to 1970 Horowitz simultaneously served as chief of the bioscience section of the National Aeronautics and Space Administration’s (NASA) JPL in Pasadena, California. Horowitz was a member of the National Academy of Sciences (NAS) since 1969 and a member since its founding in 1972 of the International Society for the Study of the Origin of Life. He remained active in research and administrative activities until his death.

The One Gene–One Enzyme Hypothesis . Horowitz’s interest in the genetics and metabolism of the red bread mold Neurospora crassa was piqued by a 1941 talk Beadle gave on his discovery with Edward Tatum that mutants of the mold showed a direct correlation between a single gene mutation and deficiency of the ability to synthesize a single nutrient. The wild-type mold could grow on a very simple growth medium, synthesizing almost every nutrient it needed. Beadle and Tatum irradiated the mold with x-rays to produce mutants, screening them afterward for which single nutrient they could not survive without.

Within a year of the discovery, Horowitz joined Beadle’s group at Stanford. Horowitz, David Bonner, and others in this research group sought to systematically demonstrate that each and every nutrient-deficient mutant was altered by only a single-gene mutation and to show what specific enzyme the gene was responsible for. Usually the mutant strain produced some nonfunctional enzyme for a single step in the metabolic pathway to synthesizing some nutrient, such as biotin, another B vitamin, or an amino acid. Horowitz found, for example, that a mutation of the gene for the enzyme tyrosinase was inherited as a simple Mendelian trait; a mutation that produced a tyrosinase molecule having different thermostability characteristics than the wild-type enzyme. The pathway in Neurospora for synthesis of the amino acid arginine was found to have seven steps, each controlled by a specific enzyme coded for by a single gene. Horowitz found that some of the biochemical synthetic pathways had branching steps in them, which complicated the attempt to demonstrate a one gene–one enzyme correlation. In the end, however, in eight papers published between 1943 and 1945, the group showed that the one gene–one enzyme correlation was exactly as predicted by the hypothesis.

Horowitz was also involved during the early 1940s in using nutrient-deficient Neurospora mutants to assay quantitatively for nutrients in foods, for example, choline and other B vitamins. He also screened for mutants of Penicillium that could produce a higher yield of penicillin.

Origin of Multistep Biosynthetic Pathways . Given that multistep synthetic pathways were common in Neurospora and that many of the intermediate compounds were not themselves useful nutrients, Horowitz pondered how such a complex pathway could be produced by natural selection, when apparently most of the intermediate, shorter-chain stages would not be functionally adaptive; only the full pathway with all the steps could actually supply the needed nutrient. He reasoned that simple pathways of just a few steps could occasionally arise by chance combinations of mutations. But this would be statistically impossible for pathways such as the seven-step arginine synthesis, more common in Neurospora and most other organisms.

The paradox, however, lent itself to an ingenious solution, which Horowitz was first to recognize. Suppose, he suggested, the essential end product, molecule A, was originally freely available in the external environment. Such an environment abundant in organic molecules had been proposed by Aleksandr Oparin as part of his heterotroph hypothesis for the origin of life. Then the ancestral organism would be under no selective pressure to develop a means to synthesize that nutrient biochemically. However, as the nutrient was gradually depleted once living heterotrophs arose and steadily consumed it, then any mutant that appeared with the capacity to synthesize A with an enzyme that could make it from B + C (two other freely available organic precursors) would be at a substantial selective advantage. So much so that in the continued absence of A soon only those descendants with the new mutation would survive. Eventually B and C would also become depleted, and then selective pressure would favor mutants that by chance arose with the ability to enzymatically synthesize, say, molecule B, from other common precursor molecules D and E.

The process could repeat many times over evolutionary time. “Given a sufficiently complex environment and a proportionately variable germ plasm, long reaction chains can be built up in this way. … This model is thus seen to have potentialities for the rapid evolution of long chain syntheses in response to changes in the environment,” said Horowitz (1945, p. 156). Thus an apparently “irreducibly complex” system within cells was not so in fact and could be built up in reverse by simple natural selection. The first organism was, in Horowitz’s view, a “self-duplicating nucleoprotein molecule” that originated as a step in Oparin’s suggested process of chemical evolution.

To summarize, the hypothesis presented here suggests that the first living entity was a completely heterotropic [sic] unit, reproducing itself at the expense of prefabricated organic molecules in its environment. A depletion of the environment resulted until a point was reached where the supply of specific substrates limited further multiplication. By a process of mutation a means was eventually discovered for utilizing other available substances. With this event the evolution of biosyntheses began. The conditions necessary for the operation of the mechanism ceased to exist with the ultimate destruction of the organic environment [as the initial organics were all consumed]. Further evolution was probably based on the chance combination of genes, resulting to a large extent in the development of short reaction chains utilizing substances whose synthesis had been previously acquired. (p. 157)

Horowitz, then, had not only solved a significant puzzle about early evolution; he had simultaneously been led to stake out a position in agreement with Hermann J. Muller’s (1926) claim that a “gene” must have been the first living thing.

Origin of Life: Genes versus Metabolism . Horowitz had stepped into a debate that had been developing for the previous thirty years, over whether a primitive chemical system that counted as “living” needed first to exhibit metabolism, or rather self-duplication (called “replication” after Watson and Crick’s 1953 DNA structure made clear that genetic duplication was a much more high-fidelity process than many had previously thought). As mentioned, famed Drosophila geneticist H. J. Muller had insisted unequivocally that a naked gene was the first living organism.

Oparin, however, tended to emphasize complex, membrane-bounded metabolizing entities that could only come about through a long, stepwise evolutionary process. He and his supporters were deeply skeptical that any molecule as complex as a gene could possibly develop suddenly, de novo, outside such a preexisting, membrane-bounded, metabolizing structure. They saw this as tantamount to a claim of “spontaneous generation,” that is, the old exploded doctrine that a living thing could appear very suddenly in a short time in the right chemical environment.

This debate became only more entangled and complex when molecular biology discovered by the early 1960s that nucleic acids cannot replicate or perform their functions without the assistance of a suite of protein enzymes, usually only found in an enclosed metabolizing cell. Thus a “naked gene” became an oxymoron, and an insoluble “chicken and egg” dilemma emerged. If proteins are absolutely necessary for the functioning of DNA, and DNA is necessary to code for the making of all proteins, then how can such a system have ever come into being without both components? Even assuming the original self-duplicating molecule was much less complex and copied itself with much less fidelity than DNA, it still remains a subject of intense debate in the origin-of-life research community, which of the two probably came first and served as the “scaffolding” upon which the “free-hanging arch” was later created.

When the first International Conference on Origin of Life Research was held, organized by Oparin in 1957 in Moscow at the height of the Cold War, many Western scientists attended, including Horowitz. Some American and British scientists supported the “metabolism-first” view of Oparin (e.g., Sidney Fox, Erwin Chargaff, John Desmond Bernal, H. J. Muller, and Norman Wingate Pirie); nonetheless, the debate about “genes first” became

somewhat politicized because of the domination of Soviet biology by Trofim Denisovich Lysenko’s anti-Mendelian rhetoric, of which Oparin had been supportive.

Along with Heinz Fraenkel-Conrat and Wendell Stanley, Horowitz forcefully insisted, contrary to Pirie’s assertion that “life” is indefinable, that a self-duplicating gene capable of mutability and catalysis of metabolic reactions was a perfectly clear-cut definition. Such living things “arose as individual molecules in a polymolecular environment,” Horowitz stated in 1957 (1959, p. 107), much as he had in 1945. In numerous subsequent conferences on the origin of life, Horowitz stuck to this position and was, along with Muller, one of its most steadfast champions. He coauthored a major review of recent origin-of-life research with Stanley Miller (1962). This paper was highly critical of many “test tube” experiments showing possible prebiotic chemistry, because it was common for experimenters to add organic components to their mixture, which were highly unlikely to have ever existed in a realistic prebiotic Earth environment.

Exobiology and Astrobiology . From the earliest attempts by microbiologist Joshua Lederberg to organize the Space Sciences Board of the NAS in support of work on exobiology in 1959, Horowitz was a major helper in this effort. He was an active member of the West Coast branch of the NAS’s subpanel on extraterrestrial life (WESTEX), led by Lederberg.

The two men worked together once NASA supported exobiology from 1960 on, to prevent the Cold War politicization of their new science in the way that space exploration had widely become subservient to such interests, especially the human space program. Horowitz and Lederberg both felt that the search for the origin of life and for extraterrestrial life were marginal enough to begin with and that all hope of scientific respectability and high-quality scientific work would be lost if the field of exobiology came to be seen as just one more attempt to “catch up with the Soviets” technologically in the wake of Sputnik. They called for high levels of NASA money for research, but in addition they were crucial engineers of an emerging NASA exobiology program that eventually gave half or more of its funds to independent researchers in the academic community (even when it came to design of experiments that would fly on NASA spacecraft), rather than becoming a research effort staffed only by U.S. government employees. Both men became active in early attempts to design life-detection instruments intended to fly on a Mars probe, Horowitz at first when he was asked in 1962 to be a scientific consultant on the “Gulliver” instrument being designed by Gilbert Levin. He was an active participant at a series of meetings sponsored by the NAS (May 1964 through October 1965) on Biology and the Exploration of Mars, cochaired by Colin Pittendrigh and Lederberg.

When Mariner 4 first successfully made a close flyby of Mars in 1965, however, the photos and other data it returned suggested Mars was much drier, colder, and had a much thinner atmosphere than had been previously thought. The cratered surface of Mars looked much more like the Moon in those photographs than it did Earth. Horowitz had criticized most experimental life-detection designs, which depended upon putting Mars soil into a liquid nutrient broth to see what microbes would grow; no Mars organism could possibly be adapted for life in copious liquid water, he argued, given how dry and cold the planet was. He put more faith than most exobiology scientists in recent Earth-based observations suggesting a very thin and tenuous Martian atmosphere (insufficient pressure for any water to remain in the liquid form on the surface), so he was becoming steadily more skeptical of whether any life could exist on Mars and was less surprised than most by the Mariner 4 results. But he was surprised to find that most “life on Mars” enthusiasts were still just as optimistic as before about sending their instruments to Mars to look for Earth-like bacteria. (James Lovelock was one of the few scientists involved who was as skeptical as Horowitz in 1965.)

Horowitz had argued from as early as 1960 that concern about contaminating Earth with organisms brought back (“back contamination”) from Mars was a waste of time, so unlikely was it that such microbes even existed. He continued to participate in discussions about adequate levels of sterilization for spacecraft before they left Earth, because he thought the possibility of “forward contamination” of other worlds by Earth organisms a scientific problem whose avoidance justified a reasonable level of prudence. Through JPL, Horowitz sponsored research on the microbiology of the dry valleys of Antarctica, showing that these might be the best analogs of Mars-like conditions available on Earth. Ten to 15 percent of the soil samples there contained no bacteria at all, and the rest had very low bacterial counts, confirming Horowitz in his skepticism about Mars soils under much harsher conditions.

Because Horowitz became chief of biosciences at JPL at this time, where the latest Mars spacecraft were being designed and built, he had the opportunity to oversee at close hand the development of the experiments likely to fly on the Viking Mars landers, slated for launch in the early to mid-1970s. In conjunction with Jerry Hubbard and George Hobby, he designed a life-detection device he called the pyrolytic release (PR) experiment, based upon assimilation of a carbon source by microbes in a nonaqueous, cold environment much closer to actual martian conditions. And he was eventually successful in having this chosen as one of the four experiments relevant to life detection that actually flew on the Vikings (Levin’s “Gulliver” or Labeled Release [LR] experiment was another). Horowitz’s device completely burned (pyrolyzed) the sample after incubation to see if any radioactive carbon-14 from the nutrients had been incorporated into living cells.

In the event, when the experiments reached the surface of Mars on Viking 1 and 2 in the summer of 1976, they began to collect data immediately. While hypothesized chemical oxidants in the soil at first gave the mistaken impression of microbial activity in both the LR and PR experiments, the gas chromatograph–mass spectrometer showed no organic compounds in the Martian soil at all, leading the majority of Viking researchers to conclude that the experiments had given “false positive” reactions, and that Mars must be sterile, at least at the surface. Levin was the only researcher to continue insisting that the data were best interpreted as indicating microbial life on Mars. Even former Martian-life optimists such as Carl Sagan in the end were converted to Horowitz’s view that the Martian surface was lifeless. Subsurface water (and life) remain a possibility, as does fossilized life from a much earlier period in Mars history during which the planet was considerably warmer and wetter.

BIBLIOGRAPHY

WORKS BY HOROWITZ

With Adrian Srb. “The Ornithine Cycle in Neurospora and Its Genetic Control.” Journal of Biological Chemistry 154 (1944): 129–139.

“On the Evolution of Biochemical Syntheses.” Proceedings of the National Academy of Sciences of the United States of America 31 (1945): 153–157.

With David Bonner, H. K. Mitchell, E. L. Tatum, et al. “Genic Control of Biochemical Reactions in Neurospora.” American Naturalist 79 (1945): 304–317.

“On Defining ‘Life.’” In Proceedings of the First International Conference on the Origin of Life, Moscow, 19–24 Aug. 1957, edited by F. Clark and R. L. M. Synge. New York: Pergamon Press, 1959.

With Stanley Miller. “Current Theories on the Origin of Life.” Fortschritte der Chemie Organischer Naturstoffe [Progress in the chemistry of organic natural products] 20 (1962): 423–459.

“The Design of Martian Biological Experiments.” In Life Sciences and Space Research 2, edited by Marcel Florkin and A. Dollfus. Amsterdam: North Holland Publishing, 1964.

With Gilbert Levin, A. H. Heim, M. F. Thompson, et al. “‘Gulliver’: An Experiment for Extraterrestrial Life Detection and Analysis.” In Life Sciences and Space Research 2, edited by M. Florkin and A. Dollfus. Amsterdam: North Holland Publishing, 1964.

“The Evolution of Biochemical Synthesis—Retrospect and Prospect.” In Evolving Genes and Proteins, edited by Vernon Bryson and Henry J. Vogel. New York: Academic Press, 1965.

“Impact of Manned Spacecraft on the Exobiology Program.” In Biology and the Exploration of Mars: Report of a Study Held under the Auspices of the Space Science Board, National Academy of Sciences–National Research Council, 1964–1965, edited by Colin S. Pittendrigh, Wolf Vishnac, and J. P. T. Pearman. National Research Council publication 1296. Washington, DC: National Academy of Sciences, 1966.

“The Search for Extraterrestrial Life.” Science 151 (18 February 1966): 789–792.

With Robert P. Sharp and Richard W. Davies. “Planetary Contamination I: The Problem and the Agreements.” Science 155 (24 March 1967): 1501–1505.

With Jerry S. Hubbard and James P. Hardy. “Photocatalytic Production of Organic Compounds from CO and H₂O in a Simulated Martian Atmosphere.” Proceedings of the National Academy of Sciences of the United States of America 68 (1971): 574–578.

With Roy E. Cameron and Jerry S. Hubbard. “Microbiology of the Dry Valleys of Antarctica.” Science 176 (21 April 1972): 242–245.

With Jerry S. Hubbard. “The Origin of Life.” Annual Review of Genetics 8 (1974): 393–410.

With Harold P. Klein, Joshua Lederberg, Alex Rich, et al. “The Viking Mission Search for Life on Mars.” Nature 262 (July 1976): 24–27.

With Harold P. Klein, Gilbert Levin, Vance Oyama, et al. “The Viking Biological Investigation: Preliminary Results.” Science 194 (17 December 1976): 1322–1329.

Oral history interview by Rachel Prud’homme, 9–10 July 1984. Caltech Archives. Available from http://resolver.caltech.edu/CaltechOH:OH_Horowitz_N. To Utopia and Back: The Search for Life in the Solar System. San Francisco: W. H. Freeman, 1986.

OTHER SOURCES

Beadle, George, and Edward L. Tatum. “Genetic Control of Biochemical Reactions in Neurospora.” Proceedings of the National Academy of Sciences of the United States of America 27 (1941): 499–506.

Dick, Steven J., and James E. Strick. The Living Universe: NASA and the Development of Astrobiology. New Brunswick, NJ: Rutgers University Press, 2004.

Farley, John. Chapter 9 in The Spontaneous Generation Controversy from Descartes to Oparin. Baltimore, MD: Johns Hopkins University Press, 1977.

Fry, Iris. The Emergence of Life on Earth: A Historical and Scientific Overview. New Brunswick, NJ: Rutgers University Press, 2000.

Kamminga, Harmke. “The Protoplasm and the Gene.” In Clay Minerals and the Origin of Life, edited by Alexander Graham Cairns-Smith and Hyman Hartman. Cambridge, U.K.: Cambridge University Press, 1986.

Margulis, Lynn, ed. Origins of Life. New York: Gordon and Breach, 1970.

———, ed. Origins of Life, II. New York: Gordon and Breach, 1971.

Muller, Hermann J. “The Gene as the Basis of Life.” [1926]. In Proceedings of the Fourth International Congress of Plant Biology, vol. 1. Menasha, WI: Banta Publishing, 1929.

Oparin, Aleksandr. The Origin of Life. Translated by Sergius Morgulis. New York: Macmillan, 1938.

James E. Strick