This is an excellent question, Liam, and one that people have been wondering about for centuries! There are really two parts that must be addressed in this question, and both are difficult to answer.
The first part to think about before you can try to answer this question is this:
What IS life?
Before you can answer how something came to be alive, you must decide what being alive means! In science, we define life in a way that can be objectively understood and measured. Therefore a scientist will determine whether something is alive by subjecting it to tests, and if it passes all of the tests we will say that it is alive. These tests include whether or not the thing can grow, whether or not it can respond to different environmental stimuli, and whether or not it can reproduce itself. If something can grow, react to different conditions, and reproduce, according to the criteria of science it is probably alive. Of course, in philosophy life might have a very different meaning. You did not ask the question how life emerged, but rather how did the first living things get life. This suggests that you may be thinking of life not in this strict scientific sense, but rather in terms of being aware, or conscious-- to have a soul, to use a common term.
So, whether or not you prefer to think of life in terms of the biological definition or the philosophical view, you still have the second part of the question to contend with, and that is where life came from. Science, unfortunately, does not have a good understanding of that yet. We know that there are many different forms of life, from bacteria through plants and animals. We also know that there are some things that act as if theyre alive but really are not since they cannot pass all of the tests, such as a virus or even a computer. Based on these observations, some ideas have been formed about how life may have been born.
The most prevalent idea is that some very simple molecules combined to form a more complex molecule that was able to make copies of itself. These copies got modified into many different types of molecules that were also able to copy themselves. These molecules might have joined together to form a group of molecules that could function together, and the group could reproduce itself. Eventually the complexity could have increased until the whole thing was able to pass all of the tests of life. These new forms of life may have continued to increase in complexity until eventually consciousness also emerged.
Does this answer seem very vague? Thats because it is! A lot of research is being done to see how life could have emerged, but we still do not have any solid answers about how molecules could have come together to form life. Maybe one day we will have a better answer!
In science, the thought is that life began in very simple, primitive terms and grew more complex. The simplest form of true life that we know of is a bacterial cell. Therefore, most scientists agree that bacteria were the first living things. We do still have lots and lots of bacteria around today. However, whether the bacteria we have today are the same as the ones that existed long ago is unknown, but again most scientists would say probably not. Unfortunately, its impossible to really definitively answer the question of what was the first living thing on Earth without using a time machine to go back and actually look!In order for the principles of mutation and natural selection in the theory of evolution to work, there have to be living things for them to work on. Life must exist before it can to start diversifying. Life had to come from somewhere, and the theory of evolution proposes that it arose spontaneously out of the inert chemicals of planet Earth perhaps 4 billion years ago. Could life arise spontaneously? Even a primitive cell like an E. coli bacteria -- one of the simplest life forms in existence today -- is amazingly complex. Following the E. coli model, a cell would have to contain at an absolute minimum.
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