Dawkins, Richard. River Out of Eden: A Darwinian View of Life. New York: Basic Books, 1995.

All organisms that have ever lived – every animal and plant, all bacteria and all fungi, every creeping thing, and all readers of this book – can look back at their ancestors and make the following proud claim: They all reached adulthood, and every single one was capable of finding at least one heterosexual partner and of successfully copulating. (p. 1)

Not a single one of our ancestors was felled by an enemy, or by a virus, or by a misjudged footstep on a cliff edge, before bringing at least one child into the world. Thousands of our ancestors’ contemporaries failed in all these respects, but not a single solitary one of our ancestors failed in any of them. (pp. 1-2)

Since all organisms inherit their genes from their ancestors, rather from their ancestors’ unsuccessful contemporaries, all organisms tend to possess successful genes. They have what it takes to becomes ancestors – and that means to survive and reproduce. This is why organsisms tend to inherit genes with a propensity to build a well-designed machine – a body that actively works as if it is striving to become an ancestor. (p. 2)

Do good by stealth. A key feature of evolution is its gradualness. This is a matter of principle rather than fact. T may or may not be the case that some episodes of evolution take a sudden turn. There may be punctuations of rapid evolution, or even abrupt macromutations – major changes dividing a child from both its parents. There certainly are sudden extinctions – perhaps caused by great natural catastrophes such as comets striking the earth – and these leave vacuums to be filled by rapidly improving understudies, as the mammals replaced the dinosaurs. Evolution is very possibly not, in actual fact, always gradual. But it must be gradual when it is being used to explain the coming into existence of complicated, apparently designed objects, like eyes. For if it is not gradual in these cases, it ceases to have any explanatory power at all. Without gradualness in these cases, we are back to miracle, which is simply a synonym for the total absence of explanation. (p. 83)

Exponential growth, if not checked by lack of resources, always leads to startlingly large-scale results in a surprisingly short time. In practice, resources are limited, and other factors, too, serve to limit exponential growth. (p. 146)

Threshold 1 is, of course, the Replicator Threshold itself: the arising of some kind of self-copying system in which there is at least a rudimentary form of hereditary variation, with occasional random mistakes in copying. (p. 151)

To begin with, success among rival replicators will be judged purely on the direct properties of the replicators themselves – for example, how well their shape fits a template. But now, after many generations of evolution we move on to Threshold 2, the Phenotype Threshold. Replicators survive not by virtue of their own properties but by virtue of causal effects on something else, which we call the phenotype. On our planet, phenotypes are easily recognized as those parts of animal and plant bodies that genes can influence. (p. 152)

This is Threshold 3, the Replicator Team Threshold, which may on some planets be crossed before, or at the same time as, the Phenotype Threshold. In early days, replicators are probably autonomous entities bobbing about with rival naked replicators in the headwaters of the genetic river. But it is a feature of our modern DNA/protein information-technology system on Earth that no gene can work in isolation. (pp. 153-154)

Perhaps the next major threshold to be crossed as a replication bomb gathers momentum on a planet is the Many-Cells Threshold, and I’ll call this Threshold 4. Any one cell in our life form, as we saw, is a little local sea of chemicals in which a team of genes bathe. Although it contains the whole team, it is made by a subset of the team. Now, cells themselves multiply by splitting in half, with each one growing to full size again. When this happens, all the members of the team of genes are duplicated. If the two cells do not separate fully but remain attached to one another, large edifices can form, with cells playing the role of bricks. The ability to make many-celled edifices may well be important on other worlds as well as our own. (p. 155)

The next major threshold I want to consider, because I suspect that it, too, is probably of more than local planetary significance, is the High-Speed Information-Processing Threshold. On our planet this Threshold 5 is achieved by a special class of cells called neurons, or nerve cells, and we might locally call it the Nervous System Threshold. However it may be achieved on a planet, it is important, because now action can be taken on a timescale much faster than the one that genes, with their chemical levers of power, can achieve directly. (pp. 156-157)

Among these consequences may be large aggregations of data-handling units – “brains” – capable of processing complex patterns of data apprehended by “sense organs” and capable of storing records of them in “memory.” A more elaborate and mysterius consequence of crossing the neuron threshold is conscious awareness, and I shall call Threshold 6 Consciousness Threshold. (p. 157)

Whether or not consciousness requires language, let us anyway recognize the Language Threshold as a major one, Threshold 7, which may or may not be crossed on a planet. (pp. 157-158)

Language, from this point of view, is the networking system by which brains (as they are called on this planet) exchange information with sufficient intimacy to allow the development of a cooperative technology. Cooperative technology, beginning with the imitative development of stone tools and proceeding through the ages of metal-smelting, wheeled vehicles, steam power and now electronics, has many of the attributes of an explosion in its own right, and its initiation therefore deserves a title, the Cooperative Technology Threshold, or Threshold 8. (p. 158)

Indeed, it is possible that human culture has fostered a genuinely new replication bomb, with a new kind of self-replicating entity – the meme, as I have called it in The Selfish Gene – proliferating and Darwinizing in a river of culture. There may be a meme bomb now taking off, in parallel to the gene bomb that earlier set up the brain/culture conditions that made the take-off possible. (p. 158)

I must return to the main theme of the planetary explosion and note that, once the stage of cooperative technology has been reached, it is quite likely that somewhere along the way the power to make an impact outside the home planet will be achieved. Threshold 9, the Radio Threshold, is passed, and it now becomes possible for external observers to notice that a star system has newly exploded as a replication bomb. (p. 158)

After radio waves, the only further step we have imagined in the outward progress of our own explosion is physical space travel itself: Threshold 10, the Space Travel Threshold. (p. 160)