Looking from the astrobiology perspectives, life on Earth started early—just about as soon as it could. The more we learn about the origin of life, the more we realize it may be a likely outcome any time you have the right ingredients. However, if you look at the history of life on Earth, let’s say you put it on a twelve-hour clock, up until four o’clock it was just a world of microorganisms, from four to five o’clock that’s the era of plants coming onto land and animals and creatures in the sea, then after five o’clock until about ten o’clock this will be a world of only microorganisms again. So, in fact, our planet is in its late middle ages in terms of life on the surface. Then from ten o’clock until about midnight, the world will be completely desolate, devoid of life as the sun is running out of its nuclear fuel in the center and its outer atmosphere is expanding. The point is that our world has had big life for only a small slice of its existence and the portion of that which has had technology is even smaller. I think life is presumably abundant everywhere; the most common form is likely going to be microbial life. In addition, the distances are so vast that unless other civilizations have developed both a means of crossing those distances quickly and the desire to do so, plus the energy capability, I don’t know if we’ll see alien intelligence in our lifetime.

The origin of life happened in significantly less than 500 million years. ...Six days was once a popular hypothesis. ...A process that happens quickly is a process that is in some sense likely... this evidence suggests that the origin of life was in some sense easy, in some sense sitting in the laws of physics and chemistry.

An honest man, armed with all the knowledge available to us now, could only state that in some sense, the origin of life appears at the moment to be almost a miracle, so many are the conditions which would have had to have been satisfied to get it going. But this should not be taken to imply that there are good reasons to believe that it could not have started on the earth by a perfectly reasonable sequence of fairly ordinary chemical reactions. The plain fact is that the time available was too long, the many microenvironments on the earth's surface too diverse, the various chemical possibilities too numerous and our own knowledge and imagination too feeble to allow us to be able to unravel exactly how it might or might not have happened such a long time ago, especially as we have no experimental evidence from that era to check our ideas against. Perhaps in the future we may know enough to make a considered guess, but at the present time we can only say that we cannot decide whether the origin of life on earth was an extremely unlikely event or almost a certainty — or any possibility in between these two extremes.

An honest man, armed with all the knowledge available to us now, could only state that in some sense, the origin of life appears at the moment to be almost a miracle, so many are the conditions which would have had to have been satisfied to get it going. But this should not be taken to imply that there are good reasons to believe that it could not have started on the earth by a perfectly reasonable sequence of fairly ordinary chemical reactions. The plain fact is that the time available was too long, the many microenvironments on the earth's surface too diverse, the various chemical possibilities too numerous and our own knowledge and imagination too feeble to allow us to be able to unravel exactly how it might or might not have happened such a long time ago, especially as we have no experimental evidence from that era to check our ideas against.

The first living organisms on Earth doubtless arose in an aqueous environment, and the course of evolution has been shaped by the proper­ ties of the aqueous medium in which life began.

Bacteria … have been here for three and a half billion years, and without them we have no chance whatsoever of survival. Humans are something very recent, like the froth on top of a glass of beer.

600 million years ago, the monopolizing grip of the algae was broken and an enormous proliferation of new lifeforms emerged, an event called the Cambrian explosion. Life had arisen almost immediately after the origin of the Earth, which suggests that life may be an inevitable chemical process on an Earth-like planet.

For the first half of geological time our ancestors were bacteria. Most creatures still are bacteria, and each one of our trillions of cells is a colony of bacteria.

"The problem of the origin of life is, at bottom, a problem in organic chemistry — the chemistry of carbon compounds — but organic chemistry within an unusual framework. Living things, as we shall see, are specified in detail at the level of atoms and molecules, with incredible delicacy and precision. At the beginning it must have been molecules that evolved to form the first living system. Because life started on earth such a long time ago — perhaps as much as four billion years ago — it is very difficult for us to discover what the first living things were like. All living things on earth, without exception, are based on organic chemistry, and such chemicals are usually not stable over very long periods of time at the range of temperatures which exist on the earth's surface. The constant buffeting of thermal motion over hundreds of millions of years eventually disrupts the strong chemical bonds which hold the atoms of an organic molecule firmly together over shorter periods; over our own lifetime, for example. For this reason it is almost impossible to find "molecular fossils" from these very early times."

Take the entire 4.5-billion-year history of the earth and scale it down to a single year, with January 1 being the origin of the earth and midnight on December 31 being the present. Until June, the only organisms were single-celled microbes, such as algae, bacteria, and amoebae. The first animal with a head did not appear until October. The first human appears on December 31. We, like all the animals and plants that have ever lived, are recent crashers at the party of life on earth.

You think man can destroy the planet? What intoxicating vanity. Let me tell you about our planet. Earth is four-and-a-half-billion-years-old. There's been life on it for nearly that long, 3.8 billion years. Bacteria first; later the first multicellular life, then the first complex creatures in the sea, on the land. Then finally the great sweeping ages of animals, the amphibians, the dinosaurs, at last the mammals, each one enduring millions on millions of years, great dynasties of creatures rising, flourishing, dying away — all this against a background of continuous and violent upheaval. Mountain ranges thrust up, eroded away, cometary impacts, volcano eruptions, oceans rising and falling, whole continents moving, an endless, constant, violent change, colliding, buckling to make mountains over millions of years. Earth has survived everything in its time. It will certainly survive us. If all the nuclear weapons in the world went off at once and all the plants, all the animals died and the earth was sizzling hot for a hundred thousand years, life would survive, somewhere: under the soil, frozen in Arctic ice. Sooner or later, when the planet was no longer inhospitable, life would spread again. The evolutionary process would begin again. It might take a few billion years for life to regain its present variety. Of course, it would be very different from what it is now, but the earth would survive our folly, only we would not. If the ozone layer gets thinner, ultraviolet radiation sears the earth, so what? Ultraviolet radiation is good for life. It's powerful energy. It promotes mutation, change. Many forms of life will thrive with more UV radiation. Many others will die out. Do you think this is the first time that's happened? Think about oxygen. Necessary for life now, but oxygen is actually a metabolic poison, a corrosive glass, like fluorine. When oxygen was first produced as a waste product by certain plant cells some three billion years ago, it created a crisis fo

What I am trying to do is to present a unified scientific view of life; that is, a view integrating life's biological, cognitive, and social dimensions. I have had many discussions with social scientists, cognitive scientists, physicists and biologist who question that task, who said that this would not be possible. They ask, why do I believe that I can do that? My belief is based largely on our knowledge of evolution. When you study evolution, you see that there was, first of all, evolution before the appearance of life, there was a molecular type of evolution where structures of greater and greater complexity evolved out of simple molecules. Biochemist who study that have made tremendous progress in understanding that process of molecular evolution. Then we had the appearance of the first cell which was a bacterium. Bacteria evolved for about 2 billion years and in doing so invented, if you want to use the term, or created most of the life processes that we know today. Biochemical processes like fermentation, oxygen breathing, photosynthesis, also rapid motion, were developed by bacteria in evolution. And what happened then was that bacteria combined with one another to produce larger cells — the so-called eukaryotic cells, which have a nucleus, chromosomes, organelles, and so on. This symbiosis that led to new forms is called symbiogenesis.

If you imagine the 4,500-bilion-odd years of Earth's history compressed into a normal earthly day, then life begins very early, about 4 A.M., with the rise of the first simple, single-celled organisms, but then advances no further for the next sixteen hours. Not until almost 8:30 in the evening, with the day five-sixths over, has Earth anything to show the universe but a restless skin of microbes. Then, finally, the first sea plants appear, followed twenty minutes later by the first jellyfish and the enigmatic Ediacaran fauna first seen by Reginald Sprigg in Australia. At 9:04 P.M. trilobites swim onto the scene, followed more or less immediately by the shapely creatures of the Burgess Shale. Just before 10 P.M. plants begin to pop up on the land. Soon after, with less than two hours left in the day, the first land creatures follow.

Thanks to ten minutes or so of balmy weather, by 10:24 the Earth is covered in the great carboniferous forests whose residues give us all our coal, and the first winged insects are evident. Dinosaurs plod onto the scene just before 11 P.M. and hold sway for about three-quarters of an hour. At twenty-one minutes to midnight they vanish and the age of mammals begins. Humans emerge one minute and seventeen seconds before midnight. The whole of our recorded history, on this scale, would be no more than a few seconds, a single human lifetime barely an instant. Throughout this greatly speeded-up day continents slide about and bang together at a clip that seems positively reckless. Mountains rise and melt away, ocean basins come and go, ice sheets advance and withdraw. And throughout the whole, about three times every minute, somewhere on the planet there is a flash-bulb pop of light marking the impact of a Manson-sized meteor or one even larger. It's a wonder that anything at all can survive in such a pummeled and unsettled environment. In fact, not many things do for long.

Where and under what form life first appeared, whether at the bottom of the deep sea, as bathybius , or whether with the co-operation of the still excessive ultra-violet solar rays, with still higher pressure of carbonic acid in the atmosphere, who can tell?