What is Life?

What is Life? August 7, 2020



Recently, NASA launched Perseverance on a mission to determine whether there is life on Mars and, also, I had the privilege of making a show on Abiogenesis with Professor Lee Cronin (see my last post).  I was set thinking…

Last century, when I was a young science teacher, a lesson that I gave to year seven was on The Characteristics of Life. I used to start by drawing a chalk line down the middle of the blackboard (yes it was that long ago), heading the left column ‘Living Things’ and the right column ‘Non-living Things’. Then I would ask the class to put up their hands and make some suggestions to list under the two headings. With some coaxing, we would compile a diverse list on both sides. It seemed easy to distinguish the living from the non-living. I would then go on to the more difficult bit – I’d ask them how they knew; what was it about all the things on the left that made them different from all the things on the right? That made them think.

With a bit of coaching we would come up with the idea that living things could do things that non-living things couldn’t, so we made another list – The Characteristics of Life. As they gave me their ideas for activities that living things can perform but non-living things cannot, I discussed them and wrote them on the board if we agreed that they fitted all the items in the two columns. I didn’t put them down in the order they came in because I wanted to form a memory word from the initial letters… It was MARRIAGE.


M for Movement 

A for Assimilation

R for Respiration

R for Reproduction

I for Irritability

A for And Water balance

G for Growth

E for Excretion

We then discussed each characteristic. This lesson went down well because it was interactive – it engaged their brains in a productive way and allowed them to see that science was about collaboration and having ideas and that there was no stigma to ‘being wrong’.

That lesson was fine for eleven year olds back then, but we now know it’s not quite as simple as that… Not all living things do all of those activities all of the time.

Let’s look more closely at Movement. There are degrees of it. Some organisms are motile – they can move from place to place under their own locomotion. Plants can’t and neither can barnacles or sea anemones for most of their lives; the adults are sessile; the only movement they are capable of is intracellular

What about Assimilation? Again there are degrees of it. Animals are mostly holozoic – they eat, digest and eliminate the undigestible, while most plants use sunlight to synthesize food, although there are exceptions to that – think of parasites and saprophytes or symbionts…

Irritability is variable too – the ‘higher’ organisms have good vision but ‘lower’ ones can barely distinguish day from night, although some trees are certainly aware of the seasons. ‘Consciousness’ is a spectrum…

All of life has to have a relationship with Water but not all of the time, the tardigrades and cereal seeds seem to be able to withstand high levels of dehydration for extended durations and Growth is an intermittent requirement too…

Even Respiration is not performed all of the time at a detectable rate by some creatures – the tardigrades can exist for long periods of inanimation and then revive… 

And, of course, if a creature is having a holiday from some of the activities covered above, it is not producing the waste materials of metabolism, the poisonous carbon dioxide and urea that needs to be Excreted…

A tapeworm is incapable of most of the things on our list; it has no need for them, being bathed in the products of digestion inside the gut of a host animal. Then there are viruses, which are pretty much like crystals except when they hijack a cell… 

So what are we left with as the one criterion, the single identifying feature that can determine what is alive?

Reproduction. That’s all… 

The ability to replicate distinguishes life from non-life.


Image credit: NASA/JPL-Caltech

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