Living beings are comprised of cells, working in unison. Cells contain macro-structures built from organic chemicals and organized into organelles which perform cell functions. The aim of each cell is to do its job to keep the colony alive - to keep the processes of life running, to keep the being in an appropriate equilibrium. The ultimate aim of the whole organism is to reproduce, to support its family in so far as its relatives carry the same genes as itself.
But why must cells be about 10 micrometres in size? Why must they use organic chemicals? Why must they exist in this way? The answer is that they do not have to be like that. They did evolve into that form because of Earth's chemical makeup but also because of the properties of elements such as carbon, with its very useful valency of 4. In other words, organic chemicals were the right tools for the job.
Organic structure could have encompassed a completely different set of constituent parts. An alternative to cellular biology could have been crystalline biology. In solution they can grow, divide in half and continue growing. No cells: just molecules. Although crystalline structures are an occasional part of organic chemistry and some have argued that DNA itself started out as a crystal replicator, the laws of physics doesn't quite allow enough at that scale for life to have emerged in that way. Or, perhaps, cellular life proved superior and grabbed all the resources in a much more efficient way than crystalline growth allows, suppressing its simpler competitor before it even had a chance to get going.
Many proceeds of the imagination have created life forms that are not based on organic chemistry at all. Life in other places in the universe could easily be utterly different to ours. Our life is based on organic biochemistry: this is not known to be the only way that life can exist. Where any pattern can reproduce itself, life could evolve, in any medium. Life could exist inside complex echoes of sound waves, in the patterned reflections of radiation, or could form out of the complicated eddies of gravitational fields or in the movement of liquids. Life could be gaseous, liquid or solid, or in combinations that we find it hard to imagine. We could even find that such life exists on our planet and we simply don't see it. Planets themselves could be conscious. Many of these ideas are being increasingly discussed by scientists1. Lovelock imagined that Earth, as a complex planet, is itself conscious and alive, with the whole atmosphere forming its biochemistry (his idea was named The Gaia Hypothesis)2. The point is that we have no grounds on which to assume that life-as-we-know-it is the only form of life.
And what if there was something about the properties of the universe itself that meant that carbon was not suitable for biology? Or, and here you will have to try to stretch your imagination, if the nuclear forces did not allow any molecules at all to form? What if one of the few intrinsic physical forces had a slightly different value, and atoms themselves never formed?
Life may well still have found a way. Instead of forming atoms, the strong and weak nuclear forces could have allowed protons, neutrons, mesons and other large subatomic particles to combine amongst themselves in different configurations to form new kinds of subatomic molecules, in chains. Therefore, genes could have been absolutely miniscule compared to our reality, thousands of times smaller, and they could still have divided, evolved, and built protective cells using subatomic tools. Over time, the same complexification could have resulted in subatomic organic chemistry, and eventually, to self-conscious mega-colonies of cells like us. But comprised of a radically different underlying structure.
If you roll the dice and set the properties of the fundamental forces to random values, it is easy to see that at some scale of organizing, life could still evolve. Photons themselves, given different strengths of electromagnetism and gravity, could form replicating structures - imagine a universe where the speed of light was a meagre 1,000 m/s. When people say that the Universe is fine-tuned for life, what they mean is this: Our Universe is a perfect basis for the life that evolved in our Universe. But what they can't say is that if the Universe were different, then, life wouldn't have appeared. With enough resources, space and time (all things that are not in short supply), all kinds of replicating structures will be possible, and it may come to pass that no matter what you do with the basic properties of the physical laws, some stable replicating structures may come to fruition. In such a strange Universe, beings might emerge and opine subjectively that the Universe, with its speed of light of 1,000 m/s, is fine-tuned for life. For their life. Adjust that Universe one tiny bit, and their entire way of life would not have been possible. Therefore, they conclude, the Universe must have been designed just for them. They do not, and they cannot, calculate the chances of life in a Universe with a different set of fundamental properties.
It is incorrect for denizens of any Universe to state that it is fine-tuned for them. It is incorrect to imagine that we know in what ways life could evolve in situations with radically different physical effects. What we do know is that given time and slowly changing conditions during the life of a Universe, it is quite plausible that life might always appear no matter what the starting conditions were.
A related discussion is that of emergent properties. Here's an introduction:
“Emergent properties are those that appear beyond a certain scale of organisation even though they are not present in the underlying structure3,4. If you microscopically examine a painted picture of a flower, you can catalogue the physical properties of the molecules of paint without ever finding a trace of the outline of a flower: only through their pattern as a whole does form, shape and beauty emerge. In biology and physics there are many complicated features that emerge from simple rules, simple interactions, merely repeated on a large scale5. Our consciousness arises from complex neuronal activity in our brain6,7,8. Some imagine that on larger scales, entire planets might be conscious (the Gaia hypothesis), or even the entire Universe (scientific pantheism).”