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.

From microbe to man, life prevailed generation to generation by sneaking past the jealous powers of destruction. Just like the Tzivkever smugglers in World War I, who stuffed their boots and blouses with tobacco, secreted all manner of contraband about their bodies, and stole across borders, breaking laws and bribing officials — so did every bit of protoplasm furtively traffic its way from epoch to epoch. It had been so when the first bacteria appeared in the slime at the ocean's edge and would be so when the sun became a cinder and the last living creature on earth froze to death, or perished in whichever way the final biological drama dictated. Animals had accepted the precariousness of existence and the necessity for flight and stealth; only man sought certainty and instead succeeded in accomplishing his own downfall.

Not only did life originate on earth very early in its history as a planet, but for the first two billion years, Earth was inhabited solely by bacteria.

I would say that if there's a probability of life being cellular, which I think there is. Life being based, which I think there is. Life starting out with CO₂ because it's so common in planetary atmospheres, and , which is very common, from the kind of s which I'm talking about... and liquid water. They need liquid water for , but we know of it on ... on Europa... [Serpentinization] is giving rise to alkaline fluids with hydrogen gas. Most hydrogen gas you find in planetary atmosphere are coming from serpentinization. , which is the mineral required for that... is ubiquitous in interstellar dust... So all of this pushes you down a certain avenue, and if that's correct it gives you bacteria... and if that's correct then bacteria have a structural problem, and they're not going to get beyond bacteria except with an endosymbiosis, and that in itself is improbable, unlikely... because it only happened once, to our knowledge, on earth.

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.

The microbial brain—gifted with long-range transport, data trading, genetic variants... and the ability to reinvent genomes—began its operations some 91 trillion bacterial generations before the birth of the Internet. Ancient bacteria, if they functioned like those today, had mastered the art of worldwide information exchange. ...The earliest microorganisms would have used planet-sweeping currents of wind and water to carry the scraps of genetic code...

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.

The oxygen crisis that began only two billion years ago prompted the evolution of respiring bacteria. These microbes that used oxygen to derive biochemical energy more efficiently than ever before eventually took over most of the world. Some of the oxygen-breathing bacteria became symbiotic, merging with different (oxygen-eschewing) bacteria to form nucleated cells, which, after becoming sexual, evolved into fungi, plants, and animals.

Let's set the existence-of-God issue aside for a later volume, and just stipulate that in some way, self-replicating organisms came into existence on this planet and immediately began trying to get rid of each other, either by spamming their environments with rough copies of themselves, or by more direct means which hardly need to be belabored. Most of them failed, and their genetic legacy was erased from the universe forever, but a few found some way to survive and to propagate.

Life today is an autopoietic, photosynthetic phenomenon, planetary in scale. A chemical transmutation of sunlight, it exuberantly tries to spread, to outgrow itself. Yet by reproducing, it maintains itself and its past even as it grows. Life transforms to meet the contingencies of its changing environment and in doing so changes that environment. By degrees the environment becomes absorbed into the processes of life, becomes less a static, inanimate backdrop and more and more like a house, nest, or shell—that is, an involved, constructed part of an organic being.

It would be a little disappointing if we didn't even find bacteria in our own solar system. I would be rather surprised to find what I would describe as large, morphologically complex life. ...It only arose once on earth. That doesn't necessarily mean that it's improbable... but it does raise some interesting questions about why, and... I think we can apply principles to it, and those principles effectively are why do bacteria and , as assistive groups to the bacteria... They're biochemically very complex. They're genetically very complex. They're kind of structured in a different way where they have large, complex metagenomes, but... I doubt very much that we'll ever find anything of the [morphological] level of complexity of a flea, composed of bacterial cells.

Bacteria represent the world's greatest success story. They are today and have always been the modal organisms on earth; they cannot be nuked to oblivion and will outlive us all. This time is their time, not the “age of mammals” as our textbooks chauvinistically proclaim. But their price for such success is permanent relegation to a microworld, and they cannot know the joy and pain of consciousness. We live in a universe of trade-offs; complexity and persistence do not work well as partners.

What, after all, is so special about genes? The answer is that they are replicators. The laws of physics are supposed to be true all over the accessible universe. Are there any principles of biology which are likely to have similar universal validity? When astronauts voyage to distant planets and look for life, they can expect to find creatures too strange and unearthly for us to imagine. But is there anything which must be true of all life, wherever it is found, and whatever the basis of its chemistry? If forms of life exist whose chemistry is based on silicon rather than carbon, or ammonia rather than water, if creatures are discovered which boil to death at -100 degrees centigrade, if a form of life is found which is not based on chemistry at all, but on electronic reverberating circuits, will there still be any general principle which is true of all life? Obviously I do not know but, if I had to bet, I would put my money on one fundamental principle. This is the law that all life evolves by the differential survival of replicating entities. The gene, the DNA molecule, happens to be the replicating entity which prevails on our own planet. There may be others. If there are, provided certain conditions are met, they will almost inevitably tend to become the basis for an evolutionary process.

Life was given to us a billion years ago. What have we done with it?