What kind of planet is our Earth? Its characteristics are easily visible when seen from the outer space. With blue oceans and trailing clouds, Earth is a planet covered with water. When a star like the Sun is born, remaining nearby dust can clump together and form small planets like Earth. When one is formed closer to such a star, it becomes a hot, burning planet of fire. When one is formed far from the star, it becomes a cold, frozen planet. Earth has liquid oceans simply because it was coincidentally formed at exactly the right proper distance from the Sun, where it is not too hot or too cold.
The chances of an ocean planet, like Earth, being formed a proper distance from a star is believed to be about 1%. Earth was born under a probability of only one in one hundred.

Meanwhile, there are 200 billion stars in the Milky Way, the galaxy that includes our Solar System. A simple estimation suggests that there may be 1% of 200 billion, or two billion, ocean planets in the Milky Way. Moreover, the Milky Way is just one of the innumerable galaxies in the universe. Probability suggests there are many more Earth-like planets in the universe.
But are these other Earths full of life like ours?
Today, I’d like to talk about how life was born on our planet Earth, and how it achieved its current form, teeming with life. By watching the evolution of both Earth and life itself, let’s consider whether we really are alone in the universe, or if life like ours could be incredibly common.

The four-billion-year evolution of both life and Earth was no gentle stroll up an even slope. Instead, it involved dramatic convulsions and several epochs of ups and downs. I am going to take us through these epochs, each one key to the destiny of the Earth. We’re also going to evaluate the present from the perspective of evolution of life on Earth, and then think about the future, which starts from this moment.

Today, Earth is home to several tens of millions of species of life. It includes huge terrestrial mammals like elephants, fish and crustaceans deep in the sea, flying insects, and practically invisible microorganisms like bacteria. Life is diverse not only in size but also in habitat.

This diversity is a result of the evolution of life, which has spanned the extraordinarily long period of four billion years. At the same time, it is not the result of the self-willed evolution of organisms. Instead, it is a consequence of the dramatically evolving Earth, each challenging new environment thus created, and life having to continue to change flexibly and vigorously to keep up. Those that could not adapt disappeared—what we call becoming “extinct.” Far more species have gone extinct in the history of the Earth than currently live on our plant today.
The first epoch in the story of life on Earth is the beginning of life, or the origin of life. Much like how a big river starts from a drop of water seeping out from a mountain, all our diverse lifeforms started from one single common origin.

Life on Earth is believed to have been born about 4.2 to 4 billion years ago. I say “is believed” as there is no absolute evidence that proves when life began on Earth. The oldest marine sedimentary rocks remaining on our planet are 4 billion years old, meaning there is no older data. Because life existed already four billion years ago, life must have been born at least that long ago.
Meanwhile, Earth was formed 4.5 million years ago. Early Earth was first covered by high-temperature magma oceans. Later, when the magma started to cool down, the atmosphere and oceans were formed. During the first several hundred million years, however, Earth was subjected to frequent massive asteroid impacts, which likely caused all the oceans to evaporate. It was only about 4.2 billion years ago when an environment ready for the beginning of life was finally shaped.
Exactly where and how the life on Earth was born is a question for which no one knows the true answer, as there’s no conclusive evidence. Despite this lack, many scientists agree on a location that is likely the key—these being underwater hot springs, scientifically called “hydrothermal vents.”

Three elements are essential for life to survive; organic matter, water, and energy. Comparing life to an analog clock, organic matter are the parts constituting the clock, such as gears and screws; water is the lubricating oil that smoothly connects the components; and energy is the battery. A clock can function only when these three are assembled, and the same applies to life. Life can perform biological activities—those activities particular to life, such as cell division and metabolism—when the three elements are available.
One of the reasons why hydrothermal vents are a key in studying the origin of life is that these three vital elements are naturally available there. Of them, energy, is the most important. Like a clock, which stops functioning when the battery goes dead, life dies quickly when there is no energy available.

In and around hydrothermal vents, hot water touching rocks causes chemical reactions that involve the formation of molecules such as hydrogen. The products of such reactions between hot water and rocks serve as food for primitive lifeforms. All primitive organisms that currently live on Earth feed on hydrogen, produced through reactions between hot water and rocks, and gain energy. They settle on rocks and receive a tiny proportion of heat energy from Earth to power the analog clock of life.
At the eve of abiogenesis, when life emerged from non-living matter, diverse kinds of molecules were formed in and near hydrothermal vents through reactions between rocks and hot water. From such molecules. complicated organic substances that had functions much like life emerged and are believed to have evolved into primitive life. This was the “beginning of life”.

Primitive lifeforms born in hydrothermal vents first lived a weak and meagre existence. Soon they became accustomed to the environment and started to propagate. The propagation continued until the hydrogen and geothermal energy available at the thermal vents became insufficient to support them. Individual organisms that actively propagated left many offspring. As a result, such aggressive individuals could preferentially survive. The survivors were not weak and meagre anymore.

Before long, the number of microorganisms reached the limit of the life-nurturing capacity of the origin hydrothermal vent. Some left the vent, being displaced or simply being engulfed by oceanic currents. These microorganisms drifted about in the sea. They could not swim and so were adrift, leaving their life to fate. A few lucky ones reached another hydrothermal vent. Most of the microorganisms perished in the foodless desert of the sea.
Interestingly, there emerged some that expressed a miraculous ability that we call photosynthesis. An organism’s substances for detecting light underwent changes and became able to use solar energy to break down the surrounding water and produce hydrogen. Hydrogen is the food produced in a hydrothermal environment—in other words, an energy source. For the first time, life acquired the capability to create energy without hot water, from sunlight.
Photosynthesis is one miracle in the evolution of life. The chances must be minute that a mutation occurs in the gene of a drifting microorganism so that molecules enabling photosynthesis are created!

Photosynthesis is believed to have emerged about three billion years ago and is one of the most important biological innovations by life on Earth. Until then, life relied on the heat of the Earth for its energy. With photosynthesis, it became possible to use sunlight as energy instead. This innovation freed organisms from life crowded around small and local hydrothermal vents deep undersea, and allowed them to simultaneously spread to all parts of the world exposed to sunlight. This is known as the spread of life.
However, the emergence of photosynthetic organisms alone did not immediately lead to the entire Earth becoming full of the life. Another major event also had to occur. This was the global glaciation that occurred about 2.5 billion years ago, called a snowball Earth event, and during which the entire global surface was covered by ice.

The snowball Earth glaciation was a key event because of the intense greenhouse effects that occurred as a reaction immediately after the glaciation period. In this resulting ultra-greenhouse climate, the population of photosynthetic organisms exploded. As prior primitive organisms once propagated to the upper energy limit of their hydrothermal vent homes, photosynthetic organisms propagated to the upper limit offered during the greenhouse Earth. Photosynthetic organisms spread throughout the globe and rapidly filled the atmosphere with oxygen, a byproduct of photosynthesis.

This Great Oxidation Event occurred about 2.5 billion to 2.2 billion years ago and still marks the worst environmental pollution in the history of the planet. Oxygen was highly toxic to microorganisms. Most of them died out, and those survived shifted their habitat to the silt of the deep-ocean seabed, where oxygen could not reach. Atmospheric oxidation—the increase in oxygen content of the atmosphere—also caused changes in the chemical composition of oceans. Oxygen caused reduced iron dissolved in the oceans to precipitate on the seabed as oxidized red iron deposits. Sulfur became a major component in the oceans because the presence of oxygen increased its supply. Such drastic changes in marine water composition caused far-reaching changes to the ecosystem of the time.
Meanwhile, life also emerged that adapted to the oxygen-rich environment and efficiently used the gas for respiration. These are the ancestors of humanity, eucaryotes. Respiration using oxygen produces enormous amounts of energy. Comparing life with our analog clock once again, if the energy acquired at a hydrothermal vent is deemed a button battery, then aerobic respiration—using oxygen—produces energy enough energy to move an automobile. Just like a larger battery being able to move a larger machine, life that acquired aerobic respiration became larger and more complex by using that same energy.

Earth experienced global glaciations and became the snowball Earth again about 600 million years ago. Immediately after the glaciation periods, a similar massive propagation of photosynthetic organisms and second oxidation of the atmosphere occurred. The second oxidation event caused multicellular life to emerge. Multicellular organisms expanded their habitat from the sea to the land, backed by their motor capabilities powered by high energy from the higher oxygen concentration. Finally, the spread of life came to cover the entire globe, including the land.

The three essential elements for life: organic matter, water, and energy. A revolution in gaining energy, called photosynthesis, in turn causing environmental pollution of global scale. And life that adopted to the change being able to rapidly expand their habitat.
Similar phenomena can be said to be happening due to the human industrial revolution and mechanized civilization. From the perspective of the four-billion-year history of life on Earth, if we call the period when life relied on the heat energy from Earth the first epoch, and the period of using sunlight energy the second epoch, we currently find ourselves in the third epoch.
Humanity uses fossil fuels and atomic power to gain energy. This means that, which previous organisms used energy that was available only in the moment, humanity can freely obtain energy from the past and future, surpassing the flow of time. Fossil fuels are preserved solar energy from the past. Atomic power involves gaining energy by artificially accelerating nuclear fission that may occur in future.

Acquisition of energy that surpasses time has enabled humankind to power automobiles and trains, construct huge cities and tall buildings, and launch space ships into outer space. It is basically the same as previous organisms increasing the size and complexity of their bodies and earning higher motor capabilities.
Human civilization is one of the most important innovations made by life on earth, closely equivalent to the acquisition of photosynthetic ability by those simple lifeforms in the distant past. Current global warming and environmental changes may be comparable to the environmental pollution caused by the Great Oxidation Events.

As the stages of energy acquisition have advanced, life has expanded its habitat dramatically. From that perspective, it may be an inevitable consequence of the evolution of life that we have developed our current civilization and are spreading our living sphere to the Moon and eventually to Mars.

I am neither negative nor pessimistic about human civilization, which marks the third epoch. My point is that the period called the “present” may give a different impression when it is seen from the history of life on Earth.
The future of life and the third epoch has just started, and we are all are a wonderful, integral part of this period. Humanity is fundamentally different from hydrothermal vent-dwelling microorganisms and photosynthetic organisms, in terms that helping each other and sympathy for each other have become basic building blocks for us to live as social multicellular organisms. I believe that humankind will be able to survive in a manner different from those of lifeforms that marked the past two epochs.
There must be a vast number of ocean planets, like Earth, out in the universe. There may even be some on which civilization has evolved. An awareness of these universal possibilities may come to alter our way of living in interesting ways.