WHAT IS LIFE? (big questions need big letters)

“So Albert, what's next?”

Next is that little planet called Earth.

“Where is it?”

It's that little gleaming dot ahead of us.

“But it's tiny.”

We're still eight minutes away, about 90 million miles. At this distance the Earth looks like a tiny bright pearl lost in a sea of darkness.

“Don't get poetic on me, Albert. Tell me what makes the Earth so special?”

Life. The Earth’s special because it’s full of life. It’s just the right distance from the sun so that it is not too hot and not too cold. It’s also the right size to have enough gravity to keep a breathable atmosphere. Remember Mars, it’s a bit smaller and has been losing it atmosphere for billions of years. The huge planets like Jupiter and Saturn have so much gravity they have a huge, dense atmosphere that isn’t very friendly to life. So the Earth is just in the right place and is just the right size. Right in the middle of that goldilocks zone we talked about before.

"I know this is a silly question…..”

Some of the best questions are, so go on.

“But what is life? How can you tell that something is alive?"

Hmmm…now that is an interesting question. Let’s think about that. Well, most living things move, but not all of them and anyway cars move. So movement can't be the key. Living things grow but then so can stalactites in caves and volcanoes. How about reproducing. That's it; living things reproduce and make new living things.

"So anything that's not reproducing isn't alive?"

Well, you don't have to be always in a constant reproductive frenzy to be alive. I suppose it's the ability or potential to reproduce that's the key to life?

“So if someone couldn’t have babies for some medical reason they wouldn’t be alive?”

Of course humans or any creature can be unable to have children and be alive. I guess reproductive potential is out. What does that leave? Evolution, living things evolve getting better each generation.

"So do soap powders."

They need energy.

"So does an iPod."

They can think and react to things.

“I don’t think grass thinks and that’s alive.”

Hmmm. When you think about, it's surprising tricky to define what life is. It’s easy to say that humans are alive and trees are alive in a different sort of way. To describe life in a simple way that includes everything we think of living but doesn’t include any machines or anything else is surprising difficult.

“What about DNA? Isn't that in all living things?”

That is true and it is amazing that, on Earth at least, all forms of life use this chemical DNA, which is short for DeoxyriboNucleic Acid. But on its own DNA is just a molecule, a whole load of atoms arranged in this long spiral shape – the famous double helix. DNA contains all the instructions about to how make a living thing and keep it working. There is a very simple sort of alphabet stored in DNA, so simple that it only has four types of letters A, C, G and T. These letters stand for Adenosine, Cytosine, Guanine and Thymidine which are the names of the molecules that DNA is made from. These letters make up three letters words which stores all the information in the DNA. There are only 64 word combinations of these letters so the DNA language is very simple compared to most human languages. The Oxford English Dictionary has 290,000 words. But every bug, person and blade of grass on Earth uses the same DNA language of just 64 words and three of those are ‘stop’.

“So is life DNA?”

It’s used by life on Earth but there could be life on other planets that uses something other than DNA. Even on Earth DNA isn’t alive. You could take the DNA from any living creature and have it in a test-tube but it wouldn’t be alive. When an animal dies you could still extract the same DNA as was there when the animal was alive. So life has to be more than just DNA. DNA is like music printed on a page. All the instructions are there to make a beautiful sound but it needs an orchestra to play the notes. Life is the whole thing, the instructions, the musicians, and the music.

“So if you just record the music on a CD would that be life?”

No because the sound of the music is just the end result. A CD of music is just a copy of what the music sounds like, just like a DVD of a film shows living actors but isn’t alive. The whole complex process of actually making the music is more what life is about. One extra twist is that the printed music in the form of DNA also includes the instructions about how to make the instruments, musicians and everything else needed to make music. Part of the problem in defining life is that there are lots of type of life and so lots of different ways of living.

“What do you mean?”

Well if we start with the smallest living things, there are bacteria. These are tiny little balls of life, so small, about a millionth of a metre across, they can only been seen with powerful microscopes. They have DNA, can move, and can make new baby bacteria. They live almost everywhere. Some bacteria when they start growing inside humans cause disease or infections, but there are lots, literally billions, that live quite happily inside every earthling. But most bacteria live in soil. There is an even simpler form of life that is smaller than bacteria, some are only a few billionths of a metre across and not everyone even thinks they are actually alive. These are viruses that can only live inside other living things. There are even viruses that live inside bacteria.

“So what is the difference between a virus and a bacteria?”

A virus is just a package of DNA (or its first cousin called RNA, ribonucleic acid) surrounded by a capsule usually made of protein. On their own they can’t do anything. They can’t grow, move, reproduce or anything else. All they can do is get inside other living things. The virus then takes them over from the inside and makes new copies of itself.

“Is that all they do?”

That’s it. Going back to my orchestra example, it’s like someone sneaking in and changing the music on the music stands without the musicians noticing so they all start playing a different tune. Sometimes viruses don’t cause much harm to the thing they invade but often they can kill the cell and can cause diseases from the common cold and the Flu to and HIV and the deadly Ebola which is probably the nastiest virus on the planet for humans.

“Why do they exist? What’s the point of just being DNA and making copies of yourself if you don’t do anything else?”

I suppose they exist just because they can. A scientist called Richard Dawkins has come up with the idea of the ‘Selfish Gene’. A gene is set of instructions coded in DNA. His idea is that all living things are just very complicated ways for genes to spread themselves around the earth.

“So viruses are one of the best ways of doing just that.”

That’s right so the real question is why bother with great big complicated animals like humans. It takes humans 30 or so years just to make a few new humans in the form of kids. Then they have to grow up and go through 13 years of school just so they can make some more copies of their DNA by having more kids. During that time they even have lessons on how to avoid making new humans.

“I guess it’s more fun being human than being a virus, but I’m not sure how this fits into the selfish gene theory. Anyway what about all the other living things?”

One up from bacteria in size and complexity is a whole range of living things made up of things called cells, which are still tiny but much bigger than bacteria at a few tens of millionths of a metre. There are three types of these cells; animals, plants and fungi. Even single celled animals (like an amoeba), single celled plants (like algae) and single celled fungi (like yeast in bread) are different enough to tell apart. Single cell plants are green, single animal cells can have little hair-like things on their surface called cilia which lets them move. Fungi are in the middle, not quite an animal but not a plant. From tiny single cell versions, much more complicated living things have been made just by putting together lots of different types of cells. The cells then become specialised so you have brain cells, skin cells and cells that help make bones. That is all humans are, collections of lots of cells working together like a massive city.

“So we're not really all that different from other forms of life. What’s the best type of life to be?”

It depends on how you judge it. For the health and safety of planet then definitely trees; they’re good for the environment and trees have never started a war or created toxic waste dumps. For cleverness humans win, but you would wonder how clever it is pollute the planet you live in. The biggest life form is a fungus. Most earthlings would say the blue whale is the biggest living thing on Earth but they’d be wrong. A fungus has been discovered that lives in the forests of Oregon in North America. This monster fungus, Armillaria ostoyae or the Honey Mushroom, lives mostly underground and is over 3 miles across and could cover more than 1,500 football pitches. A blue whale is only about the quarter of the length of one football pitch.

“So do bigger things have more DNA?”

Surprisingly not. Humans have 3 billion letters in their genetic code but onions have 17 billion. The animal with the largest amount of DNA is the marbled lung fish. This is the sort of thing we are supposed to have evolved from when life left the sea and started living on land. This fish has 130 billion letters in its DNA. But the record so far is a single celled amoeba, so small that you need a microscope to see it, it has 670 billion letters.

“Why?”

There is a lot we don't understand about DNA, like why some creatures have so much and what most of it does. We have a better idea of how the universe started than we do about why a lungfish and an amoeba have so much more DNA than us.