With every passing day, events of the past become more distant from us. This is such a platitude that it hardly bears stating. However, at the same time those same events become more accessible. That apparent paradox forms the subject of this week's column.

When I was taught history in school, the word itself never was defined. However, it soon became obvious that we were dealing mostly with past knowledge of humans, plus the occasional mention of such interesting species as saber-toothed tigers and cave bears.

Our plan of historical study also was undefined, but after a term or two it became clear. We were given a quick description of early humans, more picturesque than factual, including the Old Stone Age, the New Stone Age, the Bronze Age, and the Iron Age. Actual dates for these were conspicuously absent. Then we had, somewhat sketchily, ancient Egypt and the Fertile Crescent. At last, with an almost audible sigh of relief, the teachers came to Greece and Rome and to history in which the record was continuous and in which dates, even if wrong, could be firmly stated (e.g. 753 BC, Founding of Rome).

Successive years of my history class struggled toward the present, but by the time I dropped the subject we had reached only 1850. Thus I am hazy concerning, say, the Franco-Prussian war, but I can tell you when Julius Caesar was born.

Chances are, the date I have in my head for the latter (100 BC) is officially right but actually wrong. Dates, particularly dates that precede the continuous written record, are the bane of historians. You can tell from the evidence that an event - say, a great battle - took place. But how do you know when?

Until the twentieth century, relative dating by strata was all we had for pre-history. In other words, evidence (fossil bones, arrow heads, charred sticks) of different events lay at different depths in the earth, and although there could be exceptions, the general rule was that deeper discoveries preceded more shallow ones. Of course, if the layers you were studying were widely separated geographically, the method had problems.

Geologists working in different parts of the world therefore gave their own names to the strata they were exploring, with little reference to anything found elsewhere. This led to the confusing situation in which a distant age could be known as the Vallesian, Sarmatian, and Pannonian in Europe, the Mohnian and the Clarendonian in North America, the Waiaun and Bairnsdalian in New Zealand and Australia - and all these referred to the same time of about ten million years ago.

The big question, which we have only in the past century been able to answer, is: When, in absolute rather than relative terms, did something happen?

The big breakthrough came with the method known as radioactive carbon dating. In the 1940's, Dr. Willard Libby pointed out that all living things contain a tiny percentage of a radioactive form of carbon known as carbon-14 (It is heavier than the common form, carbon-12.) Carbon-14 is formed in the atmosphere as a result of cosmic ray bombardment, and when plants or animals die they cease to absorb it. However, carbon-14 changes to a non-radioactive form of nitrogen. The half-life for this change is 5,730 years, which means that after 5,730 years half of the carbon-14 has gone, after 2 x 5,730 = 11,460 years three-quarters of it has gone, and so on. By comparing the amount of carbon-14 in a fossil remain with the amount of carbon-12, the age since death can be estimated. Small corrections are needed, and other techniques such as counting tree rings are used for calibration of results, but the method works well for times up to about 40,000 years ago.

Here it was at last, an absolute method for deciding how long ago something happened. Today, radiocarbon dating is used routinely on everything from the Ice Man to Egyptian mummies and frozen mammoths. The snag, of course, is the 40,000-year limit. That, in geological terms, is just an eyeblink ago. Fortunately, other radioactive materials with much longer half-lives are available and suitable for geologic time scales. Thus, radioactive potassium-40 has a half-life of 1.31 billion years, while uranium-238 (the common form, not the one used in bombs but still radioactive) has a half-life of 4.47 billion years. Using these "geological clocks" together with information on the Earth's magnetic field, ice ages, and fossil strata, an overall time scale for events on the planet has been pieced together. Thus, we are able to assert with some confidence that the dinosaurs became extinct 65 million years ago. Our grandfathers would not have been able to offer more than the blindest of guesses for such a major change.

When it comes to living organisms, we have other and still newer ways of probing history. Every living animal contains within it small energy-producing bodies known as mitochondria. The mitochondria are believed to be bacteria that long ago formed a symbiosis with animals, which could not now survive without them, but that is not our concern here. The important thing is that mitochondria have their own DNA and reproduce separately. Like all DNA, this mitochondrial DNA is subject to natural mutations. If we are willing to accept that the rate of such mutations is constant over time, then by measuring current rates of mutation we have available a "mitochondrial clock" which can tell us how long ago biological events occurred. For example, we can date the time when humans and chimps had a common ancestor. They separated from each other about five million years ago, about the same time as the human and gorilla lines diverged.

The biological dating techniques still have a long way to go. The mitochondrial clock will need some fine-tuning, just as radioactive carbon dating required calibration. The DNA in the cell nuclei provides an alternative biological clock. These two cell clocks need to be reconciled, and they must be combined with radioactive element decay methods to form a unified tool. When that is done, the dating of past events will become more accurate than ever before.

How far can we go? The seventeenth century writer, Sir Thomas Browne, in a famous passage in his book, "Urn Burial," stated: "What song the Sirens sang, or what name Achilles assumed when he hid among women, though puzzling questions, are not beyond all conjecture."

I will not go so far as to say that his questions will ever be answered. On the other hand, I feel certain that we will in the near future provide new information on our own long past, information that just yesterday it seemed quite impossible we would ever learn.

"Borderlands of Science" is syndicated by: