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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.
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.
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]
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]
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]
References:
- How To Recognize Different Types Of Tree - Opening Sequence, Season 1 - Episode 3 of Monty Python's Flying Circus, YouTube Video, Dec 22, 2006.
- John S. Wilkins, "Spontaneous Generation and the Origin of Life," Abiogenesis FAQs - Articles on the Origin of Life, April 26, 2004.
- Erwin Schrödinger, "What is Life?" PDF download from Stanford University.
- Francis Crick, "Life Itself: Its Origin and Nature," Simon and Schuster; First Edition edition, January 1, 1981, 192 pp. (via Amazon).
- Defining Life, Astrobiology Magazine staff, June 19, 2002
- 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.
- Detecting extraterrestrial life through motion, Ecole Polytechnique Fédérale de Lausanne Press Release, December 29, 2014.
Permanent Link to this article
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