Their posted summary says:
Sir Martin Rees describes how for the first time, humans as a species may start to change in observable ways within single lifetimes and under some loose control of our own influence. If this future plays out, the future itself becomes more difficult to forecast.
Rees offers two brief scenarios, with one glass half-empty and one half-full:
One extreme, pessimistic scenario is that, during this century, we suffer disasters which foreclose all future technological progress and perhaps make it difficult for civilization to survive on Earth.
The optimistic scenario is that, during this century, human communities spread beyond the Earth for the first time. Self-sustaining groups established a hundred years from now would not be destroyed whatever happened to the Earth. That could be the first step towards evolution beyond the Earth.
A feature of this century, which I emphasize in the book, is that not only are traditional technologies changing faster than ever, but the world is changing in different ways. Humans beings themselves are going to change. For several thousand years, the one thing that hasn't changed has been human nature and the human physique. But in this century we have targeted drugs, genetic manipulation, and maybe even implants in the brain.
His point about "technologies changing faster than ever" is consistent with CRN's concerns. Accelerating progress in enabling technologies likely will make development of general-purpose molecular manufacturing possible far sooner than most people expect -- and if it happens before adequate policy has been prepared, the results could be very bad.
Rees also does a nice job in this interview of helping us put ourselves, as a species, in perspective:
Most educated people are aware that we are the outcome of nearly four billion years of Darwinian selection, and I think many tend to think humans are the culmination of all that. But astronomers know that our sun is less than halfway through its life span. Our sun will flare up and die six billion years from now, a period of time longer than the sun's history so far. Some people imagine that there will be humans watching the sun's demise six billion years from now, but any creatures that exist then will be as different from us now as we are from bacteria or amoebae.
We should think of ourselves as still in the early stage of the emergence of complexity and intelligence. It's hard to conceive what forms that might take on Earth or far beyond Earth. But I think we should see ourselves as nowhere near the culmination of evolution.
Whether or not humanity survives long enough to evolve into something more complex and intelligent may be contingent upon how well we manage our entry into the nano era.