Life is important to astrobiologists. They spend a lot of time looking for it, on Mars and even further afield. But what is life? Surely that’s a simple question to answer, you can tell if something is alive just by looking at it, rocks aren’t alive, cats are. For most people that’s enough. But this can be an interesting question, even for people who aren’t pedants.
Science textbooks have slightly different variations of the definition of life, but the common features are thus, a living organism must be able to:
1. Absorb energy and material from the external world in order to repair itself and grow (basically eat food)
2. Reproduce in order to create offspring (sex)
3. Pass on information and characteristics to its offspring (inheritance)
4. Be capable of undergoing Darwinian evolution (natural selection)
All forms of life can satisfy these criteria, from bacteria to oak trees. For example, plants get their food mostly from absorbing water and carbon dioxide, and get their energy from absorbing Sunlight. Almost everything else gets its food and energy by eating another organism, some eat plants, others eat animals. Humans eat both (I don’t eat animals though, because I’m a communist)
All forms of life reproduce; many do this by having sex. Almost all animals have sex, and many plants do to. When you see a bee carrying pollen between plants, you are actually seeing plants having sex, as pollen is plant sperm, and bees help transport this pollen from the male part of a flower (the stamen), to the female part of a flower (the carpel), which contain ovaries. Once fertilization occurs flowering plants typically produce seeds, often encased in fruit, so that animals, like humans, carry the seeds away to new locations. Sure there’s less groaning and hair-pulling involved, and no actual movement on the participant’s part, but this is plant sex. There’s also asexual reproduction, in which an organism simply divides in two, or sheds a small part of itself which then grows. Some plants can do this. Bamboo and strawberry plants create buds; smaller extensions of themselves that grow into new plants. Lots of simpler organisms reproduce like this too, like bacteria. With this method of reproduction only one parent is involved, so all progeny are clones.
The third criteria is passing-on inherited characteristics, this is done via DNA. DNA is a molecule that has some unique properties, it can be arranged in many different sequences of its components (bases), it can be used to create the building blocks of proteins, and it can copy itself. The different sequences of bases allow different proteins to be created, giving different organisms their unique physical characteristics, whilst the ability to copy itself allows parents to pass DNA to their progeny and because the progeny will inherit the same base sequences, they will inherit the physical characteristics of their parents.
The last is the capacity to undergo Darwinian evolution. This is evolution by natural selection. In the natural world life is tough. Food and other resources are scarce and individuals must compete to stay alive. Those individuals with characteristics that enable them to compete more effectively will be more likely to survive and bare offspring. And due to inheritance, these characteristics can be passed onto the next generation. Over time, more-and-more beneficial characteristics will emerge and subsequent generations will accumulate more-and-more changes, eventually leading to the emergence of new species (this is a massive simplification, but pretty accurate).
The definition above sounds conclusive and simple; basically eating, growing, sex, similar-looking offspring and evolution. But some things we wouldn’t consider alive can satisfy some of the requirements. Crystals for instance. A solution of chemicals in water can begin to crystallize, as atoms in the water join and become part of the crystal their energy is bound in chemical bonds. The crystal thus grows by absorbing energy and material from the external world. Once the crystal reaches a certain size it may become unstable and parts may drop off and start growing into new crystals, similar to asexual reproduction, and the ‘progeny’ crystals will even look like their ‘parents’. Obviously it won’t be able to undergo evolution by natural selection though. To make this weirder, viruses can’t actually do either of the requirements above, unless they can infiltrate a living cell. Viruses are built out of some of the same materials as living cells, and can grow and reproduce and ultimately evolve, but only when inside other organisms. Even weirder, what if we simulated a perfect copy of a human brain using software? We don’t have the technology to do this yet, as the human brain is the most complex structure we know of, in the entire Universe! But one day we may have the technology to create a human mind in a computer. If this mind had feelings, thoughts and dreams like a typical human would it be alive? It wouldn’t satisfy any of the three criteria, but you’d have to be pretty narrow-minded to say no.
So crystals are almost alive, viruses aren’t and one day my computer might be? The point is, that where life is concerned strict definitions are difficult. It’s probably the case that we’ll judge something as being alive or not when we see it. So right back to where we started then. This whole post could have ended at the first paragraph, but then I wouldn’t have had the chance to talk about plant sex, clones and conscious computers.