TIME -- what exactly is it? If you were an early Homo sapiens, time meant little. You grew old; the sun rose and set; the seasons came, went and came again; but all this took place on a stage that had no temporal bounds (no apparent beginning, no expected end) and had always looked the same.

Today we know that our planet is 4.5 billion years old, with a life expectancy of roughly 4.5 billion more; that its land masses are nothing like those of, say, 230 million years ago; that it’s adrift in a universe three times its age. How, in a mere 130,000 years, did our species exchange its Edenic ignorance for this breathtaking awareness? How is it that we know just where Earth lies on the great nonspatial continuum?

This is the question geophysicist Pascal Richet, of the Institut de Physique du Globe de Paris, asks -- and, in well over 300 dense pages of text, exhaustively answers -- in “A Natural History of Time.”


Actually (for lack of evidence) he ignores the notions, if any, held by the prowlers of the savanna and begins with early myths, stories humans told themselves to make sense of their world. These myths were “outside of time,” he writes, “because nature, above all, is governed by cycles” and “neither beginning nor end can be discerned.” The Egyptians, for example, counted years in cycles, starting with each new reign. Speaking of the Egyptians, one of the entrancing nuggets in this nugget-studded book is the information that their hours “varied in duration according to the length of the day.” We owe the stable, 60-minute hour to the Greeks, via “the sexagesimal notation of the Mesopotamians.”

Linear time takes over from cyclical time, Richet argues, with the arrival of the “Mosaic chronology,” supplemented by the New Testament: “Within a universe that was bounded in time -- by the Creation and the Apocalypse -- a history that recounted unique events, one marked by majestic, divine interventions, could not be repeated.”

This divinity has posed difficulties, though diminishing ones, up to the present day. Not the least problem created by the Mosaic chronology was the list of “begats” in Genesis, which James Ussher (1581-1656), archbishop of Armagh, used to count back to the exact date of the world’s creation: Oct. 23, 4004 BC. What Richet calls a “golden age of chronology” ensued, advanced mostly by theologians, whose biblical interpretations did not always agree. Richet notes that “the Hebrew text and the Septuagint” -- a Greek translation of the Old Testament -- “differed by fifteen centuries in the amount of time that elapsed between the Creation and the birth of Abraham.”

The weightier discrepancy, of course, was between the begats and the physical evidence. Richet sifts it at great length, beginning with a discussion of how fossils were eventually recognized as organic in origin and ancient in age and moving on to a description of the rocks they were found in: the “stratigraphic column,” or layering of Earth’s sequential surfaces.

He peppers his account with biographical sketches of obscure pioneers of Earth science. My favorite is Benoît de Maillet (1656-1738), France’s consul general in Egypt, “on familiar terms with beys and pashas, and well read in the great Arabic authors.” Maillet patrolled the shores of the Mediterranean observing signs of erosion and sedimentation and concluded that “we have had a diminishing sea for over two billion years.” The manuscript of his book on the sea’s diminution and other unnerving time frames “really shocked people too much,” Richet observes -- so much that his time spans were left out of the first edition “and in the second, three or four zeros were removed from the figures of the billions and millions of years.”

By the late 19th century, the shock had worn off somewhat, but there were still enormous discrepancies. The revered British physicist Lord Kelvin insisted, based on his estimate of Earth’s internal heat, that the planet could be no older than 40 million years. The geologists, who read Earth’s age in its rocks and calculated the time these formations took to assemble and then erode, were less precise but just as insistent that the age was in the hundreds of millions of years. Likewise, Charles Darwin and Alfred Russel Wallace argued that geological time spans were necessary for the operations of natural selection. (Richet might have spared readers the digression -- in a book full of them -- into Wallace’s interest in spiritualism.)

At last, at last: Radioactivity is discovered and provides a surefire method of dating rocks, based on the abundance of naturally occurring isotopes and their decay rates. And this is where Richet winds up, in a curiously anticlimactic way -- with the work of another relatively obscure scientist, geochemist Clair Patterson, who in 1956, after studying lead and uranium isotopic data from the Canyon Diablo meteorite, declared that Earth, along with the rest of the solar system, formed 4.55 billion years ago, a date that has stood up ever since. Patterson, Richet writes, was happy with geological time but miserable about the planet’s future, fearing it would be (in Patterson’s words) “perverted by the dominant breed of utilitarian scientists” and “a new species of hominid, Homo sapiens android” with a 500-year life span. “But that would be another history of time,” Richet concludes, “one that Patterson would not be on hand to narrate.”