My name is Hoshi and I am a professor at Keio University. Dr. Matsui just delivered a talk on astrobiology. And now I would like to talk to you about how we look at the population problem from the viewpoint of biology (fundamental biology).

　

In response to the talk given by Dr. Matsui just now, I would like to begin by telling you about how complex the biological system is.

　

Our body, the body of an adult person, is made of 60 trillion cells. They start out as a single cell, a fertilised egg, and this fertilised egg divides successively into 60 trillion cells inside our body. This is an incredible number; it is said that, out of these 60 trillion cells, you string together only the blood circulatory system cells, for example, and it would be 100,000 kilometres long. We can live for 60, 80, and 100 years without any problem in particular as our blood cells move inside the blood circulatory system of 100,000 kilometres long. That is the kind of system we have. Also, as you may know, our genetic information consists entirely of substance called DNA. It is said that DNA comprising a cell would amount to 2 metres in length when unwound. Although there are cells like red blood cells that have abandoned their DNA, the majority of cells have DNA. If we string together a person's entire DNA, its length would exceed the orbital diameter of Pluto, the most distant planet in the Solar System. Pluto's orbital diameter is about 1010 kilometres. The length of a person's DNA far exceeds this and amounts to 1011 kilometres. Its actual length is beyond our imagination. All of that is actually contained here inside our bodies and stores all the genetic information in them. That is the kind of system biology deals with.

　

A moment ago, Dr. Matsui talked about the need for complex system, a systematic or a bird's-eye point of view instead of a simple idealism. That is how biology is structured in the first place. To begin with, organisms have a history of about 3.8 billion years, just a little short of 4 billion years.

　

What I am referring to here is "organisms living on Earth" because − even though there may be organisms outside this Earth − we are presently not aware of them.

　

To think of them in terms of size, starting from the largest, a very tall tree would only be about 100 metres high at the most. Whales, which are very large among animals, are only a few tens meters long. On the smaller end of the spectrum are small organisms that are about a millionth of a metre in size.

　

Such diverse world of organisms works as a very orderly system and is manifested in certain hierarchies. For example, here is a wolf, but a wolf never exists by itself. They always form these packs, or wolf communities. They ultimately form their own ecosystem and the world of organisms further expands to biosphere, stretching out of the surface of entire Earth.

　

To look at this from the other end, you keep going down, down to the level of cells, intracellular organelles, supramolecular structures, molecules and atoms. Each level has its hierarchy with unique logic at work at each hierarchical structure, in the structure of individual, on the level of organs or on the level of cells. Simply adding up what's working at one level would therefore not allow you to go up to the next hierarchy. New characters are being added one after another as a result of moving up the hierarchy. Therefore, biology deals with many subjects and it is not easy to overview the entirety even you if limit the scope to biology alone.

　

Where are organisms living now? They are living on the surface of Earth. How thick is this surface? It exists approximately between 10,000 metres above sea level and about 10,000 metres below the water surface. By the way, we have many guests from overseas attending this conference. When they come to Japan, they fly at altitude of 10,000 metres or so. It is said that we Homo sapiens are the most commonly found species at this altitude. Since we fly at an altitude of 10,000 metres when you travel on a jumbo jet, we can say that Homo sapiens live there.

　

Some bacteria are found at even higher altitudes, but they do not actually live there. They just happen to be there. The thickness of the 20 kilometre layer inhabited by life is hardly visible to the naked eye if you shrink the Earth to the size of a globe and hold it in your hand. It is only a thin layer of skin on the surface of the Earth.

　

In terms of weight. the weight of all life put together is said to amount to a mere one-10 billionth of the Earth's mass. One-10 billionth means that all life combined − including giant trees and everything − weighs less than an eyelash if you liken the weight of the Earth to our body weight. So I jokingly say that the world of organisms is a world of an eyelash. It is an extremely insignificant world on the physical level. Although some exceptions have been found lately in the deep sea, all life is dependent on photosynthesis. All life activities are carried on using about 1 percent of the energy coming in from the sun and reaching the Earth's surface. It is also extremely insignificant in this sense.

　

To put it in another way, organisms are like dust, a completely trivial existence from the viewpoint of physicochemistry. But this dust-like existence has been affecting the entire planet and changing its surface in a significant manner. In fact, the oxygen we are breathing now has been created entirely by life and did not originally exist in atmosphere.

　

Impact of organism activity is also seen clearly on iron ore, for example. In other words, the activities of life which is extremely trivial and comparable to dust in quantity are drastically changing the Earth at least on the surface.

　

As mentioned by Dr. Matsui in his presentation earlier, various calculations have been made on the number of varieties this trivial existence can be divided into. However, even a moderate estimate is in tens of millions. Just recently, everyone involved in the taxonomic community in Japan got together and formed the Union of Japanese Societies for Systematic Biology. According to the declaration of establishment at the time, there are estimated 200 million species of life forms on the planet. Some say there are even more. As for the ones we are aware of, the pamphlet prepared by the Union of Japanese Societies for Systematic Biology says that 1.75 million species have been described so far. A slightly older data says 1.5 million. The fact that this many species are known only means that they have been named; it does not mean we know how they live in details.

　

As I mentioned earlier, we have named 1.75 million species so far. However, people in the taxonomic community are saying that there are 200 million species in this world, so we don't even know 1 percent of the entire species in reality. The fact that they have not been named means we do not know they exist, so we don't even know the existence of the remaining 99 percent. We do know that they exist but we do not know who they are. In other words, it means that people say biologists know well about organisms, but we actually do not know even 1 percent of the species existing on Earth. Various people have made strenuous efforts over the past 250 years or so to give scientific names to life, but this is all that we know. To put it differently, various organisms are disappearing one after another before we even know they exist.

　

Organisms have been divided into five major groups. The world of bacteria is one. Actually, there are two types of bacteria, but we'll just call it the "world of bacteria" for now. There is also the world of eukaryotes like us. As I mentioned earlier, eukaryotes have their DNA, the substance that carries genetic information, stored in a safe we call "nucleus". These eukaryotes having this "nucleus" consist of 4 groups. Animals are one of them. Animals have to eat to live. Plants form another group. They can synthesize organic matter from inorganic matter through photosynthesis. Fungi, represented by mushrooms, are another. They play a very important role in the sense that they take dead organisms, such as dead trees and dead animals, and transform them into forms that can be reused. That is, they are the recyclers of resources in the living world. Organisms in all of these groups have multiple cells. The last group, protista, is mainly made up of unicellular organisms.

　

Among these five groups, vast majority of those that have been named are animals. This means that we have given more names to creatures that are deeply related to our lives and creatures that we are interested in. I would like to stress here again there are vast number of species that have not even been named.

　

These creatures are living in their niches in a very complex manner. For example, many organisms are segregated on a rock surface in shallow sea that is receiving a lot of sunlight. Ocean surface has the strongest light and abundant oxygen. Amount of light and oxygen decreases as you go deeper. Different bacteria are segregated according to slight difference in environment inside this 1 millimetre layer of a biological mat covering rock surface and are living in it by creating their own system. This is the kind of life system that exists on a rock surface

　

To give you an example that is more familiar, cherry blossom is very pretty right now at the Meiji Shrine. As you know, Meiji Shrine is an artificial forest that was built after the demise of Emperor Meiji. In addition, it is located in the middle of Tokyo, one of the largest cities in the world. Moreover, Tokyo is located in the temperate latitudes and is relatively low in terms of biodiversity. Actually, more than 90 percent of biodiversity exist in the tropics. However, you'll find more than 3,000 ticks, almost 2,000 earth worms and nearly 75,000 nematodes under your foot once you walk into this forest that was built less than a hundred years ago in the centre of one of the largest cities of the world in the temperate latitudes. These are the numbers of various organisms living under your foot.

　

These values have been actually studied by a group led by Dr. J. Aoki, professor emeritus Yokohama National University and the director at Kanagawa Prefecture's Planet of Life Museum. There are this many ticks once you step into this artificial forest. You may not like them too much, though. Dr. Aoki is famous for his study of ticks. Incidentally, the majority of ticks are not parasitic on humans. Parasitic ones are extremely exceptional. These small ones are called nematodes. These majority of these odds and sods actually do not even have a name. So if you want to find new species, you can do so by going to the Meiji Shrine and looking under your foot. This beautiful forest exists thanks to the existence of all of these creatures. We say that "we fail to see the forest for the tree" and that "we fail to see the tree for the forest". But actually, these staggering quantities of creatures live under our feet. Forests do not exist had it not been for them.

　

Another characteristic of life is the fact that all life forms are descendants of a single lineage. If you look at a genealogical tree of life forms, it branches off from a single organism. Some have reached a dead end and become extinct. To be more precise, 99 percent of species that existed have become extinct. It has now branched off into people and to various animals and plants. I believe that among the species that existed in the past, there were those that originated from different lineage but have become extinct along the way. So, at least all organisms that we know of have originated from a single lineage, i.e. single ancestor. So in this sense, all life forms are connected with one another. I am now past 60. But from the standpoint of life, my life itself is connected with all life forms since life first began. So, in a sense, you can say that my life is 3.6 billion years old.