BY MATTHEW HERBERT

In The Magic of Reality, the biologist Richard Dawkins asks the reader to imagine placing postcard-thick photographs of a single line of her ancestors one on top of another in a stack going back 185 million generations. (It’s a thought experiment; it doesn’t matter that we’ve only had photography for the last six or seven generations.)

The stack of photos, laid on its side for easy reference, will be 40 miles long. To find your first ancestor who was appreciably of a different species (Homo erectus), you would have to thumb your way back to about your 50,000-greats grandfather.

At the far end of the stack, all the way at mile 40, you would find one of your earliest vertebrate ancestors, which Dawkins points out, with relish, is–a fish.

There are two important things to notice about the series of photos going back to your fishy ancestor. First, it is almost unimaginably long. You find your Homo erectus ancestor well before the one-mile mark, which means you still have more than 39 miles of pictures to go through before reaching the fish who gave you your backbone. For anyone who has trouble imagining the pace at which species-producing mutational change happens, this ought to help illustrate just how much time we are talking about

The second thing to notice is that your ancestors go on for tens or even hundreds of thousands of generations with no noticeable changes. Dawkins estimates you would have to go back 4,000 generations to find the first Homo sapiens who showed the slightest family resemblances to his Homo erectus forbears–“a slight thickening of the skull, especially under the eyebrows.” So enveloped withing the nearly-unimaginably long spans of time required for speciation, there are shorter but still vast spans of time during which no changes occur to the gross structure of organisms.

Put these two observations together, and you have the outline of what I think of as the biological conception of time–the picture of time’s passage that accounts for the fantastically long series of recombinations of four bases of DNA that have put us humanoids here. Evoltuion is the outcome of combinatorics and time–almost more time than we can possibly imagine.

And that’s just to account for the existence of living things. If the biological conception of time stumps you, chances are you are, like me, hopeless when it comes to the cosmological conception of time. I “know” that the universe is just shy of 14 billion years old, and I “understand” how physicists have reached this conclusion–by reverse modeling the expansion of the universe back along the original trajectories of its component parts, a formula that leads back to the theoretical starting point (and starting time).

None of this abstract knowledge does much good, though, at least for me, when it comes to understanding how the billions of galaxies in existence formed into their prooper parts of planets and moons, many of which likely resemble our own arrangement here in our solar system. We are left to satisfy ourselves with the information that masses of gas and dust naturally spin, and when they do so, the dust tends to accumulate into bodies that become planets. This started happening in our neighborhood sometime after the formation of our sun, about 9 billion years ago.

Why this brief history of time? The better to frame the third, and in my mind most important, way we are uttrely unable to take the passage of time seriously. I think of this notion as the moral conception of time. You may want to call it the religious conception, but I prefer my term, for reasons I’ll come to.

In his short story “The Immortal,” Jorge Luis Borges imagines a group of men who have achieved the Kingdom of Heaven but are subsequently discovered by a cosmic wanderer to have abandoned it. Why? To put Borges’s answer with violent brevity, the blessed men realize, a few generations into their experience, that Heaven is a nightmare because of its moral pointlessness. We spend less than 100 years on earth trying to appease a cosmic moral judge, and if we pass his test, we find ourselves, forever, in a realm utterly divested of moral meaning. Sickened at the thought that this nightmare is programmed to last forever–an absurdly long time given the shortness of the span during which our actions actually mattered–Borges’s immortals return to their mortal existence, where life at least had value based on its fleetingness and irrevocability.

Dawkins completes a fascinating trio of thoughts for me. Most of us are congenitally unable to take seriously the passage of time, not on a biological, cosmological, or moral scale.

To come back to the reason I prefer the term moral conception of time (as opposed to the “religious” one): For all of our extraordinarily brief lives, our actions have moral consequences. Everything we do matters. Living by this principle is one of the things that marks us off as human. Borges’ immortals abandon Heaven, not just because they didn’t want to live forever, but also because the could not imagine living lives completely devoid of moral significance. Nothing they would do in Heaven for the rest of their eternal lives would matter.

Mark Twain voiced his objections to this nightmare in a vulgar fashion. Hardly anyone on earth, he observed, enjoys singing Hosannas for more than 20 minutes, if that long. But, we profess we wish to do so forever. Twain has his doubts, and so do I.

Borges makes Twain’s point in a more devastating form. The timespan during which we picture being in Heaven would be unrecognizably human, and therefore unrecognizable as life. It would be missing the one thing that inspires all the triumph and tragedy, blood sweat and tears here during our mortal existence–meaning.

Science builds a scaffold from which we can picture vast timespans that are just barely imaginable. Billions of years. But even these would be a twinkling of the eye when measured against eternity. And we would propose to live out the nightmare of a meaningless Heaven over a timespan that would dwarf these eons? We can’t be serious.