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Signs of Life

April 2, 2014

Most science fiction contains technology-based deus ex machina elements that help move the plot along. Gene Roddenberry, when first pitching Star Trek to television networks, explained how "beaming" eliminated the special effects cost of always needing to land a spacecraft on planets. Later, beaming became an essential plot element in extricating characters from underground chambers, etc.

Another Star Trek technology is the ability to scan for "life signs" at great distances, something that Monty Python could do, but only for trees.[1] One Internet commentator has summarized how Star Trek life sensing could be done. However, it's best to follow Clarke's law that any sufficiently advanced technology is indistinguishable from magic.

The nature and genesis of life are questions that have existed since the earliest times, as are their explanations. The pre-Socratic Greek philosopher, Anaximenes of Miletus (c. 585 - c. 528 BC), was close to the modern version of abiogenesis when he wrote that a mixture of earth and water, influenced by the Sun's heat, gave rise to life.[2] The identification of life was not discussed, since it was like the problem of identifying art; namely, "I know it when I see it."

Not just philosophers and biologists, but physicists wondered about life. Erwin Schrödinger, in his 1944 book What Is Life? wrote that the propagation of life was likely by means of an "aperiodic crystal" containing genetic information.[3] Thus, Schrödinger anticipated by nearly a decade, in very general terms, the DNA origin of life discovered by James D. Watson and Francis H. Crick.

Francis Crick, Life Itself book cover.

Life Itself: Its Origin and Nature, is a 1981 book by Francis Crick.

Chapter four of this book, "The General Nature of Life," begins with the statement, "It is not easy to give a compact definition of either "life" or "living."

Crick then attempts a definition in the chapter's fourteen pages.

(Scan of my copy.)[4)]

In his 1981 book, "The General Nature of Life," Francis Crick considered the definition for "life."[4] Crick relies on the survival of the fittest principle to decide that replication and the ability to harvest both materials and energy from the environment are the hallmarks of life. Additionally, these processes must happen at a rapid pace; that is, you aren't going to generate complex organisms if the reaction rates are too slow.

Crick's definition is close to the textbook definition I learned in elementary school, although parts of the definition were presented in the more general terms of "capacity for growth," and "functional activity." Part of "functional activity" is metabolism, through which the living organism changes its environment. This leads us to the question of whether viruses are alive. Viruses are not considered to be living.

Viruses are not alive, since they are just DNA or RNA snippets protected by protein sheathes. They have no inherent ability to replicate on their own; instead, they rely on the reproductive mechanisms of living cells to make copies of themselves. Wendell M. Stanley shared a 1946 Nobel Prize, interestingly in chemistry, not in physiology or medicine, for proving the chemical nature of viruses.

Structure of Myoviridae viruses

Looking like a robotic spacecraft, this is the basic structure of the virus family, Myoviridae.

(Wikimedia Commons image by Ninjatacoshell.)

Scientists were understandably curious as to whether Mars had life, and there was an experiment to check for life during our first visit to the surface of that planet by the Viking Lander in 1976. The Viking Lander‘s Labeled Release Experiment was designed to look for the waste byproducts of metabolism. The idea of the experiment was to provide radioactively-tagged nutrients to a Martian soil sample, and then look for traces of radiation in released gases.[5]

One important part of experimentation is the design of the experiment. You need to ensure that you've controlled for all extraneous variables. In the case of the Labeled Release Experiment, there was a rapid emission of radioactive carbon dioxide gas. Carbon dioxide is a metabolic byproduct of organisms on Earth, but the superoxidative properties of the Martian soil produced the gas through a chemical reaction, only. Thus, metabolism is not a good way to verify the presence of life.[5]

Carl Sagan with a model of the Viking Lander

Amazingly done with 1970s technology.

Carl Sagan, posing with a model of the Viking Lander in Death Valley, California.

(NASA image.)

One feature of bacterial life is taxis; that is, movement, generally towards nutrient sources or light, and away from undesirable environs. Scientists at the École Polytechnique Fédérale de Lausanne (Lausanne, Switzerland), the Université de Lausanne (Lausanne, Switzerland), and the Vlaams Instituut voor Biotechnologie (Leuven, Belgium) have investigated using taxis as a life sign.[6-7]

This research team argues that movement is a common signature of life, and even microorganisms, such as bacteria, will vibrate because of their metabolic activity.[6] Since the motion sensor developed by the team is not a chemical sensor, it's immune to false positives like the one detected on Viking. Furthermore, there is no need to guess at an organism's metabolic reactions to determine a proper nutrient mixture, etc., since the sensor responds to the physical manifestation of metabolism, not its chemical pathways.[6]

The technology for such detection is based on the now common atomic force microscope. Such microscopes incorporate a small cantilever beam (see photograph) formed from a wafer of crystalline silicon, and the size is such that about 500 bacteria will fill its surface area.[7] Bacterial movement, either through locomotion via their flagella or metabolism, imparts vibration to the cantilever. This vibration is detected by reflected light from a laser.[7]

An AFM cantilever

A micrograph of an atomic force microscope cantilever beam.

The sharp tip, used for surface scanning, is not needed in the bacterial motion detector described in the text.

(Via Wikimedia Commons.)

The research team has tested this sensor with isolated bacteria, yeast, mouse and human cells, and also bacteria from environmental sources. They tested soil samples taken near the fields around their laboratory, and from water from a nearby river. In all cases, the vibration signature of living cells was detected, and these signals were extinguished with application of a lethal drug.[7]

Says Giovanni Dietler of the École Polytechnique Fédérale de Lausanne,
"The system has the benefit of being completely chemistry-free... That means that it can be used anywhere - in drug testing or even in the search for extraterrestrial life."[7]
Arrays of such sensors, coated with cancer cells, could be exposed to various drugs to see which are most effective. Such a high-throughput system would be welcome by pharmaceutical companies.[7] It would be applicable for detection of exotic lifeforms, such as those that might be present in the methane lakes of Titan.[7]


  1. How To Recognize Different Types Of Tree - Opening Sequence, Season 1 - Episode 3 of Monty Python's Flying Circus, YouTube Video, Dec 22, 2006.
  2. John S. Wilkins, "Spontaneous Generation and the Origin of Life," Abiogenesis FAQs - Articles on the Origin of Life, April 26, 2004.
  3. Erwin Schrödinger, "What is Life?" PDF download from Stanford University.
  4. Francis Crick, "Life Itself: Its Origin and Nature," Simon and Schuster; First Edition edition, January 1, 1981, 192 pp. (via Amazon).
  5. Defining Life, Astrobiology Magazine staff, June 19, 2002
  6. Sandor Kasas, Francesco Simone Ruggeri, Carine Benadiba, Caroline Maillard, Petar Stupar, Hélène Tournu, Giovanni Dietler, and Giovanni Longo, "Detecting nanoscale vibrations as signature of life," Proc. Natl. Acad. Sci., vol. 112, no. 2 (January 13, 2015), pp. 378-381, doi: 10.1073/pnas.1415348112.
  7. Detecting extraterrestrial life through motion, Ecole Polytechnique Fédérale de Lausanne Press Release, December 29, 2014.

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