What Makes Something Alive?

Wind up a toy frog, set it on the floor, and watch it hop across the room in little, jerky leaps. A minute later, a real frog on a garden wall does something that looks almost identical: it hops. Both moved. Only one of them is alive. If movement alone decided the question, you would have to call the toy frog living too, and that feels wrong. So movement by itself cannot be the real test. There has to be something else, something a toy frog can never do no matter how well it is built, that a real frog does automatically, without even trying. Finding that something else is really the aim of this whole chapter: not just to point at living things when you see them, but to understand the actual, complete checklist of things that only life can do.

Look around any room and try sorting everything into two piles: alive, and not alive. A pencil, a chair, a mobile phone go in one pile. A pigeon on the windowsill, the potted plant in the corner, you yourself, go in the other. Now for the harder test: a car can move faster than any pigeon, so why does it stay in the not-alive pile? A car cannot eat and turn food into energy the way you do after a meal; it needs someone to pour in fuel from outside. A car cannot grow one centimetre taller no matter how many years you keep it. It cannot make baby cars, and it does not breathe. Compare that against yourself: you eat food and your body uses it to grow and to power everything else you do; you have grown taller than you were four years ago, without trying, just by being alive; and if you were ever cut off entirely from air, you could not survive more than a few minutes. Movement, eating for energy, and growing are three of the clues scientists use to separate the living from the non-living, and a car already fails on two of the three.

Breathing feels so automatic that it is easy to forget it is happening at all, but try this: count how many breaths you take in one minute while sitting still, then count again right after running on the spot for thirty seconds. The second number will be noticeably higher, because your body is working harder and needs more air. This taking in and letting out of air is called respiration, and every living thing does it, even though it does not always look like human breathing. Dogs and cats do not have chests that visibly rise and fall the way ours do, but watch a dog resting on a hot afternoon and you will see its belly moving in and out rapidly, its own version of the very same process. Plants respire too, just without lungs or a nose: the surface of a leaf is covered in thousands of tiny pores, far too small to see with the naked eye, called stomata, which let air move in and out. Breathing in is only half the story. Whatever a living thing takes in, it eventually has to let some of it back out as waste, a process called excretion. In humans that shows up as sweat, which is mostly water and salt, and as urine. You have probably noticed pale patches forming on a shirt under the arms after a hot day; that is excreted sweat drying on the fabric. Plants excrete too, quietly and without any fuss: look closely at grass or a rose bush early in the morning and you may spot tiny droplets sitting on the very tips of the leaves, a plant pushing out extra water and minerals it does not need.

Step barefoot on a sharp thorn by accident, or brush your hand against a hot cup of tea, and your body reacts before you have even finished thinking about it: your foot lifts, your hand pulls back. The thorn and the hot cup are both examples of a stimulus, anything in the surroundings that triggers a living thing to respond, and pulling away instantly is your response. This ability to sense something and react to it is called response to stimuli, and it is not limited to animals with nerves and brains. The touch-me-not plant, called chhui-mui or lajjalu in many Indian languages, folds its feathery leaves shut within seconds of being touched, an unmistakable, visible reaction with no brain involved at all. Some plants respond to a completely different kind of stimulus: the amla tree folds its leaflets together every evening, closing up in response to fading light and reopening again at sunrise, almost as if it can tell day from night. Neither the touch-me-not nor the amla tree is thinking about what to do; they simply react, automatically, the same way your foot automatically lifts off a thorn.

Look at a litter of kittens, and it is obvious at a glance that they belong to a cat: smaller, wobblier, but unmistakably cat-shaped. No pencil has ever produced a smaller pencil, and no chair has ever given birth to a chair. Producing new individuals of one's own kind is called reproduction, and every living thing does it in some way, because without it, every kind of plant and animal on Earth would eventually vanish the moment its last member died. Reproduction is really the answer to the deepest question in this chapter: why does life bother continuing at all? It continues because living things actively make more of themselves. This finally brings us back to the toy frog from the start of the chapter. A toy frog can never make a smaller toy frog. It never truly eats, never grows a single millimetre, and if you take its battery out, it does not die the way a living thing does, because it was never alive to begin with; it simply stops. A real, living frog is different: if it ever permanently stops moving, eating, growing, breathing, excreting, responding and reproducing, all at once and for good, even though food, air and water are still available to it, then it has died. Death is not the same as being non-living. A toy was never alive in the first place. A dead frog, on the other hand, was alive right up until the moment its checklist of life finally, permanently, stopped.

Plants do not get up and walk across a room, and it is tempting to conclude they do not move at all, but that is not quite true. A flower slowly opening its petals in the morning sun is a plant moving. A climbing vine curling itself around a fence or a neighbouring branch, tightening its grip a little more each day, is a plant moving. The most dramatic plant movement of all belongs to the sundew, or Drosera, an insectivorous plant that gets part of its nutrition by trapping insects instead of only making its own food. Its leaves are covered in hair-like projections tipped with a sticky, glistening liquid that looks almost like dew, which is where its name comes from. The moment a small insect lands and touches these hairs, they slowly curl inward, trapping the insect against the sticky surface, in one of the few plant movements fast enough to actually watch happen. Even a plant's overall growth is a kind of movement, and it happens in a very particular, predictable direction. Roots almost always grow downward into the soil, while shoots grow upward toward the sky, and if a young seedling is deliberately turned upside down, its root bends and corrects course to keep growing down while its shoot bends the opposite way to keep growing up. Shoots go further still: they actively lean and grow toward whichever direction sunlight is strongest, which is why a potted plant left on a windowsill for weeks ends up leaning visibly toward the glass. An Indian scientist named Jagadish Chandra Bose was fascinated by exactly this kind of plant behaviour over a century ago, and built a device called a crescograph that could measure a plant's growth with incredible precision, proving scientifically that plants really do sense and respond to light, touch and gravity, just far too slowly for the human eye to catch in the act.

A dry bean seed sitting in a kitchen jar looks about as lifeless as a pebble, yet given the right conditions it can turn into a sprouting plant within days. What exactly does it need to wake up? The most important requirement is water. A seed's outer covering, called the seed coat, is tough and hard, mainly to protect the tiny, curled-up embryo sleeping inside it during storage. Water slowly softens this seed coat and seeps in, allowing the embryo to finally begin the internal changes it needs to grow, in much the same way a stiff, dried-out sponge only starts working again once it properly soaks up water. A seed also needs air, specifically the air trapped in the tiny gaps between particles of soil, which is one reason packing soil down too tightly around a seed can stop it from germinating even with plenty of water present. Interestingly, most seeds do not actually need light to germinate at all; a seed buried in the dark underground still sprouts perfectly well, since light only becomes essential afterwards, once a green seedling needs to make its own food. You can test all of this yourself with a simple, fair experiment: plant identical bean seeds in four separate pots, then treat each pot differently - one with no water at all, one with far too much water, one kept in complete darkness but with a moderate amount of water, and one with a moderate amount of water and normal sunlight. After a week or two of watching and comparing, a clear pattern emerges: the pot with no water fails to germinate no matter how sunny its spot is, the waterlogged pot also tends to fail because the excess water pushes all the air out of the soil, and both the dark, moist pot and the sunny, moist pot succeed - the experiment's real punchline: sunlight decided nothing here, but water and air decided everything.

Follow a single bean seed for three months and you get to watch an entire life story unfold in fast motion. It begins as stage one, a small, dry, seemingly lifeless seed. Given water and air, it germinates: a tiny root pushes downward out of the splitting seed coat, closely followed by a shoot pushing upward, and this is stage two. Within days, the shoot unfurls its first pair of leaves and starts turning green, stage three, at which point it finally starts making its own food using sunlight rather than relying only on the food stored inside the original seed. The plant then keeps growing taller and bushier for weeks, until buds appear and open into flowers, stage four, usually the most colourful and eye-catching part of the whole story. If the flowers are successfully pollinated, each one slowly changes shape and swells into a pod containing a fresh new generation of seeds, stage five. Finally, once the pods and the seeds inside them are fully mature, the original plant's leaves yellow, dry out, and it dies, even if you never once stop watering it, because a plant's life cycle, like every living thing's, has a natural end. The word cycle is the important part of the term life cycle: this is not simply a straight line from birth to death. The fresh seeds produced inside those mature pods can themselves be planted, and they will go through the exact same five stages all over again, seed to germination to leaves to flowers to fruit and seeds once more, generation after generation, for as long as conditions remain favourable.

Not every animal grows up looking like a smaller version of its adult self. A mosquito's life begins as an egg laid directly on the surface of still water, which is precisely why doctors and health workers keep repeating one piece of advice: never let water stagnate anywhere around your home, in a flowerpot's saucer, an old tyre, or a desert cooler left unemptied. Within the water, the egg hatches into a worm-like larva, which grows and eventually transforms into a pupa, an even stranger-looking comma-shaped stage; both the larva and the pupa live entirely underwater yet must repeatedly wriggle up to the surface to breathe air, since neither of them has gills like a fish. This is exactly why a thin layer of kerosene oil poured onto stagnant water works as a mosquito-control measure: the oil film seals the surface and blocks the larvae and pupae from reaching the air they still need, even though they are underwater. Finally, the pupa splits open and a fully formed adult mosquito emerges, rests briefly on the water's surface while its body hardens, and then flies off, living for only about ten to fifteen days before the female lays eggs of her own and the four-stage cycle, egg to larva to pupa to adult, begins again. A frog's story runs on a slower, gentler version of the same idea. It begins as spawn, a jelly-like clump of eggs laid in a pond, which hatches into a tadpole with a long swimming tail but no legs at all. Over the following weeks, the tadpole gradually grows hind legs, and then front legs, while its tail slowly shrinks, since a tail built for swimming becomes far less useful once four working legs are ready for hopping instead. This in-between stage, tiny, four-legged, still carrying the last remnant of its tail, is called a froglet, and it has started venturing onto land even though it still spends plenty of time in water. Only once the tail has disappeared completely does it become a fully adult frog, at home both on land and in water.