Chapter 1. Our story begins with a simple question, the kind of a question that people often find hard to answer: What makes human beings alive? It was not always thus. Until the 1960s it was generally assumed that this question did not need to be answered because common sense said that people were living things and everyone knew that living things breathe and have hearts. At least everyone knew that until around that time. Most people still think that way, but a number of thinkers – biologists and others – were busy debating whether it was a simple matter of common sense or whether it might be possible to build a successful argument to show that all human beings did not breathe and did not have hearts. ### A short history of life The question of what makes living beings alive is connected to a much broader question. Why are we here? Where did we come from and what might happen to us in the future? How did the universe begin and how is it managed? Such questions are not often posed for plants, and we shall see why shortly, but if we look at this question in a very general way we can see that there are two main approaches. The first approach, perhaps the most obvious to modern people, is to look for the evidence of a beginning. Is there anything in the world around us to suggest that everything we see – all plants and animals, not just humans – came into existence in a certain way or over a certain length of time? This way of thinking is based on the idea that the world is made of matter, and that matter comes from something else, which is also made of matter and whose nature is known. The second approach is to say that it makes no sense to look for evidence of a beginning, for that may be true only for a God who is outside the world and not accessible to our observations. Rather, the question is to understand what makes something alive. We know that some things are living, and we have seen them doing things. By making a comparison with animals and plants and imagining what might happen if living creatures disappeared from the earth, one can understand that the ability to make such a comparison is not enough. If we put our thinking cap on, we may see that there is a kind of logic in the idea of a beginning, and it is only logic. We may also see that there is no necessity to stop at the creation of our own planet. It is also a matter of logic that living beings could have evolved from something quite different from what we see around us. ### The beginnings of life If we see ourselves as connected to the rest of the universe, and it is only logical that we are, then it seems logical that we share a common history. But until very recently people did not give a good deal of thought to the origins of the things around them and we do not know anything about their early history, at least not in any detail. From the point of view of philosophy the history of life is still an extremely primitive story. But the first steps on earth were very important, and the nature of those steps has shaped life ever since, so it is worth looking into this period of time in some detail. The story begins around 3.5 billion years ago in what is known as the Hadean, or the earliest of the four stages of earth history. The Hadean was a period of great heat. The moon is a large, dry rock (well, most of it is a dry rock, anyway), but the early earth was a ball of hot rock too. The Hadean was filled with great storms of hot gases that made life impossible on the surface, and the heat was so intense that its intensity was maintained until the cooling processes begun later in the early history of the planet reached a state of equilibrium and stopped being active. During the Hadean the surface of the earth was being shaped and solidified, for the rocks that make the earth today were forming at the same time as the early crust. The formation of the crust began in a number of 'impact basins' that mark the areas where the crust and the core of the earth collided. The evidence is that when the surface began to cool down again, as the early earth settled into the Hadean, the crust and its upper mantle formed through 'volcanism'. This means that the rocks on the surface of the earth were built up by many thousands of geysers of hot molten material that burst through the rock shell. These geysers are called 'craters'. Some of them formed large volcanoes, such as those in Hawaii and the Canary Islands. Others were quite small, or they burst through small holes that formed naturally in the earth's crust. But they all gave evidence of deep pressures and heat beneath the surface, and all of them were formed of materials that we can see in the present state of the earth's crust. The nature of the volcanism during the Hadean, however, also gave evidence of an active interior. As I have already mentioned, the rocks that make up the earth now all had to be made at the same time, around 3.5 billion years ago, and the volcanism that gave rise to these rocks was quite a strange process. Although we think of rocks as solid, it seems that a small number of geysers were active for as long as 150 million years, spewing out molten rock from the earth's interior at a rate of some one hundred million tonnes per second (a hundred million tonnes is about the mass of a thousand Olympic swimming pools). How these volcanoes worked may be hard to imagine, but they released their material slowly over hundreds of millions of years, and thus the basic physical processes that shape the earth were born. The rocks formed at this time also contained traces of a substance called graphite. Graphite is very important because it is so strong and soft at the same time, a property that makes it ideal for use in making pencils. What made the earth possible was the heat that came from the rocky interior, which warmed the rocky outer surface. This was how heat was transferred into the interior, and by understanding the chemistry of the mantle – the layer below the crust – we can get a better understanding of the planet's early history. The Earth's interior is a mix of silicate rock, iron, and nickel. Over time the iron and nickel sinks to the centre and this iron-nickel core forms the inner layer of the earth. Between the core and the mantle lies the liquid layer that allows the convection currents. The mantle is a very thick layer that is mostly made up of silicate, and it is divided into two layers, an outer layer and an inner layer. In the outer layer the rock is made of rocks that have a variety of structures and compositions. In the inner layer, the rocks are like those found in granite, and this indicates that the mantle is a chemically rather simple region. It is not just the mantle that shows great variability; the crust has also been heavily deformed as the lithosphere – the outer shell of the earth – continues to move and break up. The crust is divided into a variety of tectonic plates, and these plates collide and fuse in a large variety of places. So it is that the earth's history has resulted in a diversity of processes and products that can be observed all over the surface of the earth today. The composition of the earth at this time was very different from what it is now, but the processes were working the same way. The early earth was not alone in the Hadean. At least three other planets formed before it, and at least one of those (about 4.6 billion years ago) was very much like our own planet. Indeed, there is evidence that this planet was rather similar to the earth, though a little smaller, and it was named 'Theia' because it was named after a daughter of Prometheus. But, as we know from the evidence of the moon rocks, the earth was the most important of these planets, for it was our planet that took the most detailed pictures of what went on during the first 500 million years of earth's history. ### Earthshine From a little inspection of the moon and the way that the light from the sun is refracted by the moon, it is possible to see the shadows of the moon on the surface of the earth, as though they were shadows from the earth's atmosphere. Even more spectacular, by reflecting the light from the sun there is an earth-shine that illuminates the planet at night, the same way that the reflected sunlight is called 'daylight' when seen from the moon. The first person to observe the earth as it shone by the moon was Arthur Stanley Eddington. He was a very famous scientist of the early twentieth century. Eddington made a number of important discoveries during his lifetime and he is usually regarded as one of the founders of the discipline of astrophysics. Even during his lifetime, Eddington's achievements were impressive, but this achievement was the most remarkable of all. In 1923 he announced his discovery of the fact that there is a constant amount of radiant energy from the sun shining on the earth, and from this he estimated that the earth's surface must receive this energy in the form of heat. At first he thought it was because the earth was hot, but he found it to be more complex than that. He was able to deduce the rate at which the earth received energy, and from that rate he calculated that if all the energy received came from the sun, the temperature of the earth's surface must be about 55°C. The energy balance was almost perfect and Eddington was able to make many more deductions that were quite accurate. But he made the astonishing announcement that the light