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What is life?

Did you learn about Mrs Gren at school? She was a useful person to know when you wanted to remember that Movement, Respiration, Sensation, Growth, Reproduction, Excretion, and Nutrition were the defining signs of life. But did you ever wonder how accurate this classroom mnemonic really is, or where it comes from?

The definition, though not perfect, has survived for several millennia because its fluidity allows us to deal with all those things that defy neat categorisation. When you look at a limpet how much movement do you see? When you touch a lichen, can it feel any sensation? When you consider a rose, have you ever wondered about its excrement? In reality, few living things plainly exhibit all seven of Mrs Gren’s signs of life. But this flexible system allows us to pick and choose how many of the characteristics must be displayed before we decide that something is or isn’t alive. A crystal can grow, but it isn’t alive. A virus appears to be able to do some of the things on the list (like reproduce) but not without some help from a host, so it isn’t really alive either. On the flipside, some things exhibit fewer than seven of the signs of life, but we do consider them to be alive: most plants, for example, don’t display much sensitivity or movement, but we still class them as living beings.

The idea that living things can be defined by their ability to grow, reproduce, feel, and so on is an ancient one with roots stretching back as far as Aristotle. Aristotle wrote several books about zoology (and a few about botany, but sadly these haven’t survived). In these zoology books, Aristotle listed five markers of animal life: the ability to move, to feel, to reproduce, to digest, and the presence of blood or a similar fluid. We can see instantly that four of these five characteristics are echoed by Mrs Gren. So can we say that to be an animal is to be alive? What about the vegetable kingdom? The word ‘vegetable’ comes from the Latin vegetus, meaning ‘lively’ or ‘vigorous’. Nowadays, the word ‘lively’ connotes being active and energetic; but in a more literal sense, plants really are lively. All plants plainly exhibit three of the seven signs of life: they grow, they reproduce and they need nourishment. They also all respire (although that wasn’t known until the eighteenth century). Some of them can even sense and move.

Today, biologists take it for granted that plants are indeed alive. In fact, this is such a common notion that it was built into the definition of the three kingdoms that dominated eighteenth-and nineteenth-century life sciences. This definition was coined by the famous Swedish botanist Carl Linnæus (1707-1778) who codified the basic differences between three kingdoms of nature in his famous phrase: lapides crescent, vegetabilia crescunt et vivunt, animalia crescunt, vivunt et sentient — rocks grow, plants grow and live, animals grow, live and feel. Here, plants were accorded the property of ‘life’, while animals had ‘life’ and sensation.

Dragonfly, by Enrico Donelli. CC-BY-ND-2.0 via Flickr.
Dragonfly, by Enrico Donelli. CC-BY-ND-2.0 via Flickr.

Unfortunately, Linnæus was vague about how exactly he defined this nebulous concept of life. Though many trace Mrs Gren’s roots to Linnæus’s writings, in fact, Linnæus’s maxim assumed that ‘life’ was a necessary quality of a plant or an animal. Linnæus, perhaps prudently, avoided explicitly listing the attributes of life. Despite his vagueness about the meaning of ‘life’, eighteenth-and nineteenth-century naturalists adopted and elaborated Linnæus’s definition until it expanded into the list of seven characteristics still recognised by schoolchildren today.

Linnæus was the figurehead of an eighteenth-century craze for classifying the natural world. This craze was driven in part by the expansion of European empires which led to huge number of specimens flooding into Europe from far-flung corners of the world. At the same time as these exotic flowers and beasts were being neatly pigeonholed, a different group of life-forms were defying classification. Late in the seventeenth century, tiny animalcules with the power of independent motion had been seen through microscopes. By the nineteenth century, scientists declared that these ‘micro-organisms’ were a group of life forms entirely separate from the animal or vegetable kingdom. Initially, all micro-organisms were placed in the kingdom Protista; but, over time, new kingdoms have been defined to accommodate these small, strange things — Prokaryota, Monera, Fungi, Bacteria, Eubacteria, Archaebacteria, Archaezoa, Archaea, Protozoa, Chromista.

Today, scientists agree that microscopic life-forms far outnumber animals and plants, both in absolute number and as a percentage of the planet’s biomass. What impact have these creatures (which do not always display the traditional seven signs of life) had on how we define life? As we have discovered more living things, the definition has had to become even more fluid. Though many of Mrs Gren’s characteristics still play a role in determining whether something is alive, life can now also be defined by the ability of a thing to metabolise, to evolve, to display homeostasis (internal regularity), to pass on genetic information to offspring, or to maintain negative entropy.

After more than 2000 years of trying, we still don’t have a single, all-encompassing definition of life. Will it ever be possible to fully define this quality, or to give a definitive list of the characteristics of life? It is perhaps more useful to think of life as series of processes, rather than a particular entity or substance. But I suspect the quest to define life will continue for as long as there is life for, as Aristotle said, defining life is part of the process of defining ourselves.

Featured image credit: Seedling, by Ray_from_LA. CC-BY-2.0 via Flickr.

Recent Comments

  1. Pedro bara

    Life Cycle
    Continuous melting
    Rigid water
    Rock water

  2. Pedro bara

    I have a little note
    And her grandmother while distilling grain
    Anise for o oil
    Which can be inferred rotation mechanism
    Frost or ice to create a cell
    Sound …..

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