This question comes to us from Jeri in New York. Thanks for submitting a question Jeri!
As we’ve talked about in our article: “What makes a good scientific question?”, to have a strong scientific question, and therefore a strong scientific answer, we need all the terms to be well defined. Because the term “Life” is not very well defined, it makes this a very difficult to answer.
When you really start to think about it, how would you define life? How do you know if something is alive or not? One might be tempted to just say something that is alive must eat and breathe, but that definition would include fire, so you can see how complex defining life might be.
Life is not really a “thing”; it’s a bunch of processes. One of the hardest parts of defining life is that the processes by which different organisms live are different between organisms. The way different organisms eat, breathe, reproduce, create energy, and move, all varies from organism to organism. A single anaerobic celled organism has completely different life processes compared to say an antelope; a jellyfish has much different processes than an earthworm.
There is no simple measurement that we can do that says “Yes… This thing is alive”. Life is one of those places where the universe does not like to fit into tiny little boxes as we would like them to. The universe tends to work in gradients and evolution, not in hard lines and clear distinctions which at times can make things difficult to define.
A scientist by the name of Martin Hanczyc has been doing research on the line between life and not life. He has been doing work on protocells which are different colored chemicals that behave like living cells. His TED talk is particularly interesting and can be seen here:
This research and similar research from other scientists all goes towards the goal of trying to define the process we call life; including determining how life first started here on earth, and if there could be other forms of life on other planets or even our own.
Our current definition of life from the perspective of biology is generally agreed to center around certain characteristics that an organism must have to be considered alive:
- Organization- Life is highly organized, and complex and has been defined as being made up of one or more cells.
- Metabolism-Life must have a way to use and convert energy know as metabolism. This metabolism can be any number of different processes depending on the living organism, but the end results of that process needs to be the making, storing and converting energy.
- Homeostasis- Life must maintain an internal environment.
- Growth- A living thing must be able to increase in size . Reproduction- A living thing must be able to produce new living things
- Response- A living thing must be able to respond the environment
- Evolution – A living thing must be able to change over time and respond to changes in the environment.
This is how the scientific community has come to define life. So far the only organisms that fit this definition that we’ve found, are DNA based so I might argue that this is how we defined DNA and cellular based life but at the moment this is the definition we’re working with.
To take this logic puzzle one step further, since DNA is based on carbon, our definition of life is carbon based. And there is some logic to this in terms of the chemistry as well. If life is indeed organized and complex, carbon is the easy answer as the uniting element of life as we have defined it for several reasons.
There is a whole branch of study devoted to carbon-based chemistry: organic chemistry. Of the 10 million or so different molecules that we know of today, about 90% of them are organic and contain carbon. Carbon bonds to itself very easily, and can make complex chains, rings, balls, and bonds easily with hydrogen, oxygen, and other elements. And since life needs to have a metabolism, processes to produce energy, get rid of waste, and maintain homeostasis, carbon is an obvious answer because there are so many chemicals that can be made and converted etc. with carbon depending on where the organism is and what it needs to do.
That being said, there is another element that shows similar bonding preferences to carbon. If you look at the periodic table:
You may notice that Silicon (#14) is right below Carbon (#6). This means that Silicon has the same number of electrons in its outer shell as carbon does and therefore will bond similarly to carbon. And since silicon can make similar bonds as carbon, the same complex molecules could be made out of silicon instead of carbon. This has been discussed in Science fiction quite a bit over the years. There are a lot of differences between silicon and carbon though, and a silicon based organism would have to have wildly different processes than carbon.
For example, trees on earth rely on carbon dioxide as a major role in photosynthesis. For a silicon based “tree”, if it needed silicon dioxide for a similar process that might be a bit of a problem. Carbon dioxide at earth temperatures and pressures is a gas, and is readily available in our atmosphere to be absorbed. Silicon dioxide meanwhile, is quartz, and thus a solid, so a similar process to photosynthesis based on silicon would be very foreign to us, and perhaps hard to look for.
The outlier in category of “life” that break the DNA rule are viruses. They are DNA or RNA based, and essentially, they are just very complex chemicals. Viruses are not cell based, they have no metabolism, and they cannot reproduce on their own. They need a host cell to reproduce, and basically, they are just large organic chemicals that will hijack a cell, take over its ability to reproduce; make many copies of itself before killing the cell.
As of right now there is still debate in the scientific community on whether or not viruses are alive. Many think they are on the border between that which is a very complex form of chemistry, and that which is alive. Which begs the question, if there is not a hard line between living and not living, and chemicals themselves can have some will to reproduce, will we ever have a hard definition of what is alive?
As you can see, the scientific community is still learning about what is alive and what is not so determining whether or not there are lifeforms in the universe that are not carbon based is very hard to say. There may be other places in the universe where life has evolved that is not the carbon based life we are used to here on earth. Maybe it’s energy based, or like a more complex virus that doesn’t have a cell. What we do know is that life, to quote the first Jurassic Park movie… “finds a way”.
