More poignant for us, at Laetoli in Tanzania are the companionable footprints of three real hominids, probably Australopithecus afarensis, walking together 3.6 million years ago in what was then fresh volcanic ash. Who does not wonder what these individuals were to each other, whether they held hands or even talked, and what forgotten errand they shared in a Pliocene dawn?

I speak as a private citizen and not as a representative of the Executive Branch of the United States government. The impression that people of faith are uniformly opposed to stem-cell research is not documented by surveys. In fact, many people of strong religious conviction think this can be a morally supportable approach.

Bush and bin Laden are really on the same side: the side of faith and violence against the side of reason and discussion. Both have implacable faith that they are right and the other is evil. Each believes that when he dies he is going to heaven. Each believes that if he could kill the other, his path to paradise in the next world would be even swifter. The delusional "next world" is welcome to both of them. This world would be a much better place without either of them.

We have to find our own purposes in life, which are not derived directly from our scientific history.

We are machines built by DNA whose purpose is to make more copies of the same DNA. ... This is exactly what we are for. We are machines for propagating DNA, and the propagation of DNA is a self-sustaining process. It is every living object's sole reason for living.

Evolution has no long-term goal. There is no long-distance target, no final perfection to serve as a criterion for selection, although human vanity cherishes the absurd notion that our species is the final goal of evolution.

We are going to die, and that makes us the lucky ones. Most people are never going to die, because they are never going to be born. The number of people who could be here, in my place, out number the sand grains in the Sahara. If you think about all the different ways in which our genes could be permuted, you and I are quite grotesquely lucky to be here. The number of events that had to happen in order for you to exist, in order for me to exist... we are privileged to be alive and nd we should make the most of our time on this world.