Defining life is surprisingly difficult. The obvious criteria include (1) ability to reproduce, (2) ability to metabolize, i.e., take in energy, (3) possession of DNA (or RNA), (4) ability to move, (5) ability to grow, (6) being comprised of cells (or a cell), and (7) being constructed of carbon. The problem is that when you take them one at a time, they all have exceptions. For example, mules are sterile, so they don’t satisfy #1 above, yet mules are obviously alive. Ability to metabolize may seem to be a good one, yet biologists can create chemical compounds that perform the same steps found in plant photosynthesis, i.e., they use light to make sugar and then reverse the process, or “metabolize” the sugar. This satisfies #2, but no one would say this stuff in a test tube is actually “alive.” For the same reason, possession of DNA is not enough by itself, because a test tube containing DNA is not alive either. Crystals grow, but are clearly not alive. And of course a prion (the subject of a future blog post) has the capacity to reproduce and is made of carbon, but it is not considered “alive” either.
The list of things that meet some, but not all, of the current criteria for life goes on and on, but the most interesting one is the virus. Many biologists say that viruses are not “alive.” This has always struck me as arbitrary because they meet so many of the criteria for life: they have either DNA or RNA; they reproduce; they have something like a cell (at least they are contained by a membrane); and they take in energy and use a host’s metabolism to grow and reproduce. True, they don’t metabolize by themselves—and that is why many biologists (but not all!) say viruses are not alive. I say it’s pretty clever of viruses to harness a host’s metabolism for their own benefit. So what if they don’t metabolize directly? It seems reasonable to me that over evolutionary time, some organisms could be “intermediate” between “alive” and “not alive.” Viruses may be in that stage and just have never evolved out of it.
The virus question may not have a universally-accepted answer, but it has certainly led to some great quotations:
“Viruses are parasites that skirt the boundaries between life and inert matter.”
“Viruses lead a kind of borrowed life.”
and my favorite—
“Whether or not viruses should be regarded as organisms is a matter of taste.”
I’m sure there will never be a consensus on the question as to what is “life.” The universe is just too multi-dimensional for simple definitions. But even so, a definition of life that I like, coined in 1975 by the great biologist John Maynard Smith, is: “any population of entities which has the properties of multiplication, heredity and variation.”
But even if we can’t exactly define life, the fact remains that in 1982, scientists created SYNTHETIC life, assuming you are in the camp that believes viruses are alive. It was made completely artificially, using a poliovirus as a model. And it actually functions like a “normal” virus regarding infectivity!
In fact, its chemical formula is:
C 332,652 H 492,388 N 98,245 O 131,196 P 7,501 S 2,340.
Is that wild? To reduce a “living thing” to a chemical formula? Ha!
Since we don’t seem to be able to get away from the virus questions, here is another one: are computer viruses alive? Well, they don’t have DNA but they do have a code that allows them to reproduce. They don’t have cells, but they certainly “live” on a hard drive. They don’t metabolize, but they get their “energy” from the computer they reside on. And they can evolve, or change with time. It seems to me that a computer virus comes pretty close to being alive, and given Smith’s definition above, they would have to be considered alive. I wonder what he would have thought about computer viruses, not considered by him in 1975. Oh, dear.
And then there is one of the most astonishing biological achievements of all time: in 2010 Craig Venter and his colleagues actually figured out the minimum number of genes (382) that are necessary to “make a bacterium live,” and then they artificially constructed these 382 genes and introduced them into a bacterial cell that was “dead” by any one’s definition (all of its DNA had been removed—so it was just an inert sack of protoplasm). And this artificial construct booted up, and was, well, alive! It grew, reproduced, and is a NEW organism. They even gave it a new species name—Mycoplasma laboratorium. Ever the showman, Craig Venter says this is “the first self-replicating species we’ve had on the planet whose parent is a computer.”
So in this case I think you would (or at least I would) have to say that Venter MADE life. He took a “dead cell” and made it into a new bacterium with the minimum number of genes for continued life.
Say what you will, but this is the future. There are roadblocks, however. Mycoplasma laboratorium cost $40 million to make, not a research budget most scientists can muster. And of course there are criticisms, one of which is that Venter had to stick the new synthetic DNA in an already existing cell (though lacking DNA) so he did not REALLY make “life.” I think this is a quibble—he has figured out the minimum number of genes for something to be “alive”, and he has synthetically made a new life form that meets all the criteria for “life”. (Venter and two other inventors applied for a patent on their methodology in 2005; their patent application is still pending.)
Given the difficulty of defining life, how would you know you have found it on, say, the moon? Assuming, of course, that something didn’t tap you on the shoulder while you were standing in the Sea of Tranquility. Take the moon landing of 1969. There was, as you may recall, fear that an unknown organism would be picked up by the astronauts, and to ensure there were no harmful passengers hitching a ride back to earth, they were quarantined for 21 days after they got home. This was in accordance with a 1969 federal law (the Extraterrestrial Exposure Law) stating that returning persons or property would be quarantined for a period of time. That law was revoked in 1977.
But the real news concerning the search for life on another planet occurred in 1976 with the touchdown of two Viking landers on Mars. A key experiment to test for life on Mars was to determine whether some unknown microbe could convert a cocktail of ingredients into CO2. Now, these ingredients were compounds believed to occur in space, since they were formed from the famous Urey experiments of 1951 (another story). Suffice it to say that it is believed with a high degree of certainty that the compounds formaldehyde, hydrogen cyanide, acetylene, cyanoacetylene, and the amino acid glycine reside in the Martian soil. So any microbes that were there MAY have been adapted to using these compounds in some form of metabolism. So, the Viking lander picked up a soil sample (hopefully containing microbes), and added acetylene, formaldehyde, etc., each radioactively labeled with C14.. The idea was that the microbes would metabolize these radioactive C14-labeled compounds and release radioactive CO2. Pretty sweet experiment. It would have worked perfectly on any earth soil, but for it to work on Martian soil, the assumptions had to be made that life on Mars would have a metabolic system like life on earth and that there was not another ingredient in the Martian soil that would convert these compounds into C02 in the ABSENCE of metabolism. When the news first hit the press that they got C02 from the experiments on Mars, it caused quite a stir. HOWEVER it was discovered in 2008 that there indeed IS another compound (perchlorate) in the Martian soil that could cause the cocktail ingredients to convert to C02 in the absence of metabolism. So a theory is that the C02 release was a geological event rather than a biological one and thus not evidence of “life”.
And this is all still controversial today—some astrobiologists think that the 1976 Viking experiments showed metabolism (and hence LIFE), while others think it is all still inconclusive. As recently as 2012 there was a reanalysis of the 36-year-old data from the Viking expedition, and the authors concluded that the data was consistent with biological activity and not geological activity.
This is how science goes—analyze, criticize, hypothesize, reject, and experiment.
Also note that the Viking experiment did not test for a form of metabolism found in “Chemolithotrophs” (Latin for “rock eating”), which are bacteria that get their energy from ammonia, hydrogen, or iron. Who knows what types of bacteria could live in a Martian soil—and if you don’t know the type of bacteria, you certainly don’t know what kind of metabolism they have, and you won’t be able to set up an experiment to look for their metabolic by-products.
And, as everyone knows, since August 2012 the Mars Science Laboratory, nicknamed Curiosity, has been happily roaming around the Martian surface taking incredible photographs, collecting samples, searching for evidence of past or present water, and, of course, looking for life. To date no results have been reported on the big “life” question. Nor have I been able to find out what kinds of experiments they are using to search for life, other than looking for water (found), and for organics (which they reported in fall 2012). Since dozens of different organic molecules have been found in space, it would be a shock if there were NO organics on Mars. And if Mars has liquid water and organics to metabolize, can life be far behind??! But as far as metabolic experiments, I’m not so sure. Nor am I certain they have any microscopes powerful enough for direct observations. Anyway, reporting such results will have to be done very carefully, as no one wants to make a positive report and then have to retract it.
My geeky bones just wait, and wait, and wait . . . .
The list of things that meet some, but not all, of the current criteria for life goes on and on, but the most interesting one is the virus. Many biologists say that viruses are not “alive.” This has always struck me as arbitrary because they meet so many of the criteria for life: they have either DNA or RNA; they reproduce; they have something like a cell (at least they are contained by a membrane); and they take in energy and use a host’s metabolism to grow and reproduce. True, they don’t metabolize by themselves—and that is why many biologists (but not all!) say viruses are not alive. I say it’s pretty clever of viruses to harness a host’s metabolism for their own benefit. So what if they don’t metabolize directly? It seems reasonable to me that over evolutionary time, some organisms could be “intermediate” between “alive” and “not alive.” Viruses may be in that stage and just have never evolved out of it.
The virus question may not have a universally-accepted answer, but it has certainly led to some great quotations:
“Viruses are parasites that skirt the boundaries between life and inert matter.”
“Viruses lead a kind of borrowed life.”
and my favorite—
“Whether or not viruses should be regarded as organisms is a matter of taste.”
I’m sure there will never be a consensus on the question as to what is “life.” The universe is just too multi-dimensional for simple definitions. But even so, a definition of life that I like, coined in 1975 by the great biologist John Maynard Smith, is: “any population of entities which has the properties of multiplication, heredity and variation.”
But even if we can’t exactly define life, the fact remains that in 1982, scientists created SYNTHETIC life, assuming you are in the camp that believes viruses are alive. It was made completely artificially, using a poliovirus as a model. And it actually functions like a “normal” virus regarding infectivity!
In fact, its chemical formula is:
C 332,652 H 492,388 N 98,245 O 131,196 P 7,501 S 2,340.
Is that wild? To reduce a “living thing” to a chemical formula? Ha!
Since we don’t seem to be able to get away from the virus questions, here is another one: are computer viruses alive? Well, they don’t have DNA but they do have a code that allows them to reproduce. They don’t have cells, but they certainly “live” on a hard drive. They don’t metabolize, but they get their “energy” from the computer they reside on. And they can evolve, or change with time. It seems to me that a computer virus comes pretty close to being alive, and given Smith’s definition above, they would have to be considered alive. I wonder what he would have thought about computer viruses, not considered by him in 1975. Oh, dear.
And then there is one of the most astonishing biological achievements of all time: in 2010 Craig Venter and his colleagues actually figured out the minimum number of genes (382) that are necessary to “make a bacterium live,” and then they artificially constructed these 382 genes and introduced them into a bacterial cell that was “dead” by any one’s definition (all of its DNA had been removed—so it was just an inert sack of protoplasm). And this artificial construct booted up, and was, well, alive! It grew, reproduced, and is a NEW organism. They even gave it a new species name—Mycoplasma laboratorium. Ever the showman, Craig Venter says this is “the first self-replicating species we’ve had on the planet whose parent is a computer.”
So in this case I think you would (or at least I would) have to say that Venter MADE life. He took a “dead cell” and made it into a new bacterium with the minimum number of genes for continued life.
Say what you will, but this is the future. There are roadblocks, however. Mycoplasma laboratorium cost $40 million to make, not a research budget most scientists can muster. And of course there are criticisms, one of which is that Venter had to stick the new synthetic DNA in an already existing cell (though lacking DNA) so he did not REALLY make “life.” I think this is a quibble—he has figured out the minimum number of genes for something to be “alive”, and he has synthetically made a new life form that meets all the criteria for “life”. (Venter and two other inventors applied for a patent on their methodology in 2005; their patent application is still pending.)
Given the difficulty of defining life, how would you know you have found it on, say, the moon? Assuming, of course, that something didn’t tap you on the shoulder while you were standing in the Sea of Tranquility. Take the moon landing of 1969. There was, as you may recall, fear that an unknown organism would be picked up by the astronauts, and to ensure there were no harmful passengers hitching a ride back to earth, they were quarantined for 21 days after they got home. This was in accordance with a 1969 federal law (the Extraterrestrial Exposure Law) stating that returning persons or property would be quarantined for a period of time. That law was revoked in 1977.
But the real news concerning the search for life on another planet occurred in 1976 with the touchdown of two Viking landers on Mars. A key experiment to test for life on Mars was to determine whether some unknown microbe could convert a cocktail of ingredients into CO2. Now, these ingredients were compounds believed to occur in space, since they were formed from the famous Urey experiments of 1951 (another story). Suffice it to say that it is believed with a high degree of certainty that the compounds formaldehyde, hydrogen cyanide, acetylene, cyanoacetylene, and the amino acid glycine reside in the Martian soil. So any microbes that were there MAY have been adapted to using these compounds in some form of metabolism. So, the Viking lander picked up a soil sample (hopefully containing microbes), and added acetylene, formaldehyde, etc., each radioactively labeled with C14.. The idea was that the microbes would metabolize these radioactive C14-labeled compounds and release radioactive CO2. Pretty sweet experiment. It would have worked perfectly on any earth soil, but for it to work on Martian soil, the assumptions had to be made that life on Mars would have a metabolic system like life on earth and that there was not another ingredient in the Martian soil that would convert these compounds into C02 in the ABSENCE of metabolism. When the news first hit the press that they got C02 from the experiments on Mars, it caused quite a stir. HOWEVER it was discovered in 2008 that there indeed IS another compound (perchlorate) in the Martian soil that could cause the cocktail ingredients to convert to C02 in the absence of metabolism. So a theory is that the C02 release was a geological event rather than a biological one and thus not evidence of “life”.
And this is all still controversial today—some astrobiologists think that the 1976 Viking experiments showed metabolism (and hence LIFE), while others think it is all still inconclusive. As recently as 2012 there was a reanalysis of the 36-year-old data from the Viking expedition, and the authors concluded that the data was consistent with biological activity and not geological activity.
This is how science goes—analyze, criticize, hypothesize, reject, and experiment.
Also note that the Viking experiment did not test for a form of metabolism found in “Chemolithotrophs” (Latin for “rock eating”), which are bacteria that get their energy from ammonia, hydrogen, or iron. Who knows what types of bacteria could live in a Martian soil—and if you don’t know the type of bacteria, you certainly don’t know what kind of metabolism they have, and you won’t be able to set up an experiment to look for their metabolic by-products.
And, as everyone knows, since August 2012 the Mars Science Laboratory, nicknamed Curiosity, has been happily roaming around the Martian surface taking incredible photographs, collecting samples, searching for evidence of past or present water, and, of course, looking for life. To date no results have been reported on the big “life” question. Nor have I been able to find out what kinds of experiments they are using to search for life, other than looking for water (found), and for organics (which they reported in fall 2012). Since dozens of different organic molecules have been found in space, it would be a shock if there were NO organics on Mars. And if Mars has liquid water and organics to metabolize, can life be far behind??! But as far as metabolic experiments, I’m not so sure. Nor am I certain they have any microscopes powerful enough for direct observations. Anyway, reporting such results will have to be done very carefully, as no one wants to make a positive report and then have to retract it.
My geeky bones just wait, and wait, and wait . . . .
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