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COLUMN ONE : Time to Set the Human Clock Back : Lasers and volcanic ash are helping scientists date fossils more precisely. Pinpointing Lucy's age is only the beginning of the evolution revolution.

April 19, 1994|ROBERT LEE HOTZ | TIMES SCIENCE WRITER

BERKELEY — The path to humanity's origins leads through a side door in a divinity school, down a basement staircase and into the laboratory where, by a laser's fierce light, Robert C. Walter is prospecting for time.

From his swivel chair, Walter is combing samples of a prehistoric lake shore that vanished before our primitive ancestors learned to make tools or discovered fire. He hopes to pinpoint the moment that that coastline--and the scampering apelike pre-humans that inhabited it--flourished millennia ago.

When he is done, Walter will have helped to transform an arid hellhole of bedded silt, sand and volcanic ash in a desolate corner of Ethiopia into a calibrated yardstick of human evolution, marking off 400,000 years of early life.

At the private Institute of Human Origins in Berkeley, Walter and his colleagues are using their ability to read the hidden time in the layers of lava deposits around the world to re-date--and rewrite--the history of human evolution over the past 4 million years.

Scientists who can read the hands on the natural clock of Earth have become arbiters of the acrimonious wars that often rage over scientific theories of human development. In the process they have settled some heated academic disputes, while triggering new, even fiercer debates.

"We are truth-sayers," said Paul R. Renne, director of the institute's geochronology laboratory, which has become a leading center for fossil dating.

In recent weeks, the scientists' ability to date human fossils more precisely has spurred new questions about how quickly primitive hominids evolved into modern humans, when they began making tools, and when they first left their African homeland. They also ended a decades-long mystery by pinpointing the age of Lucy--the 3.18-million-year-old skeleton that may be our most famous early ancestor.

The institute's work this year also has triggered debate over whether Africa was the cradle of humanity, as many scientists believe, or whether our ancestors evolved in many places around the world at the same time.

All that these dating experts have to work with is the volcanic rock and ash that anthropologists might otherwise discard in their search for scraps of bones preserved in stone. By analyzing the chemical contents of rock crystals from fossil-bearing deposits, researchers can calculate their age.

Given the paucity of data on the ancient human record, the ability to date the rocks that yield fossils is especially crucial, experts said.

In the past, researchers could best gauge the age of a hominid fossil by comparing its anatomy to bones found at other sites or by evaluating more numerous animal fossils found in nearby sediments.

The results were often open to question. The hominid fossils were too old for some laboratory dating techniques, too young for others, and too precious to grind up as samples for any of them.

Today, scientists can calibrate time using the periodic swings of Earth's magnetic poles through the ages, or by the rate at which radioactive uranium naturally turns to lead. Sometimes they can calculate the passage of time by counting the minute tracks on a rock left by spontaneous fission.

Some researchers can measure time by recording the light emitted by residual electrons from sunlight trapped in buried sediments, or by assessing chemical changes in the composition of the fossil.

This is a boon to anthropologists because a hominid fossil without a reliable date is a foundling on the doorstep of time, with no sure place in the human family.

At best, it is a museum curiosity that conceals just how it fits precisely into the puzzle of evolution, scientists said. At worst, with a misleading date, it is a false clue.

Despite decades of dedicated searching, paleoanthropologists have discovered only a few thousand fragments of bone to cover the millions of years it took humanity to acquire upright posture, a large brain, language and tool-making ability. For some of the most crucial periods, there are no known human fossils.

An ancient blackened molar can be enough for ambitious theorists to conceive a new hominid species. Reputations can be built around a single skeleton or a fossilized footprint.

"Anthropologists love to develop theories and then treat the theories as fact," said dating expert Garniss Curtis, who founded the institute laboratory and developed much of the technology in it.

"In the human evolutionary story, it is very difficult to test these opposing models and hypotheses because the evidence is so fragmentary and the dating of it has been so poor," said Tim D. White of UC Berkeley's Laboratory for Human Evolutionary Studies. "You could accommodate the fossil record to whatever point of view you had."

New, more sensitive dating techniques allow researchers to reconcile the evidence of fossil hominids found around the world.

The new technology of dating also is giving scientists their first real glimpse into the speed and pace of human evolution.

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