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.
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.
“The mode of evolutionary change is often very obvious,” said institute President Donald C. Johanson. “The teeth get bigger. The skull gets larger. The brain case increases. What people would like to know is the tempo of evolutionary change.”
“The techniques of dating are increasing in accuracy in leaps and bounds,” said Elisabeth Vrba, an expert on paleobiology and geology at Yale University. “All kinds of new techniques are coming to bear.”
All the dating methods have drawbacks. Some work only if the sample is relatively new; others are effective only with the most ancient samples. Uranium-lead dating is only useful for rocks that are more than 25 million years old. Others have margins of error so broad that they are useless in calibrating the precise increments of human evolution.
Even when scientists can date layers of sediments and ash, they still must prove whether the fossil belongs in the layer where it was discovered.
If a fossil is young enough--usually less than 50,000 years old--researchers can determine its age precisely by measuring the amount of carbon-14 it may have absorbed from the atmosphere while it was alive. The technique has been a mainstay for decades, but if the fossil is too much older--as most early hominid fossils are--radio-carbon dating is not reliable.
Scientists owe their confidence in the newest dating results to refinements in potassium-argon dating, which derives information from analysis of volcanic deposits. It picks up where carbon-dating leaves off--50,000 years ago and earlier.
Computerized argon laser fusion, a variant of potassium-dating developed by University of Toronto researchers and the Berkeley group, is so sensitive that a scientist can use one crystal of volcanic ash to determine the geologic instant when our ancestors walked the Earth.
It has become the tool of choice for those attempting to date early human evolution, Renne said.
“Dating has gotten a lot better in the last few years, much more reliable, with much fewer errors,” said Andrew Hill, curator for anthropology at Yale’s Peabody Museum of Natural History. “And in connection with human evolution, it is largely because of that institute lab.”
In his basement laboratory, Walter methodically steers the laser beam over an irradiated grain of volcanic ash mined from an ancient lakeside in Ethiopia where the remains of some of mankind’s earliest known ancestors have been found. The laboratory’s computerized laser fusion setup can handle 221 samples at a time on a platform the size of an Oreo.
The grain is no larger than a crystal from a saltshaker.
He blasts it into oblivion.
In the instant the grain glows white hot, it gives up a gas like a last breath that is exhaled into a maze of heated steel tubes and wheezing vacuum pumps, purified, and charged by an electron beam.
A powerful magnet accelerates the charged gas and hurls it against a device that counts its constituent argon atoms.
Each speeding argon atom is a measure of time on a clock set when the universe was made.
By measuring the relative amounts of two isotopes of the element, researchers can tell how much time has elapsed since the lava cooled and crystals formed.
“Argon accumulates over time. The older the sample, the more argon there is in it and the easier it is to measure,” Walter said.
Earlier this year, Walter came up with the first reliable date for the fossil skeleton known as Lucy. Last month, he put a date on the first nearly complete skull from her species, making it the youngest known example of the earliest recognized human ancestor.
He was able to establish the date by bracketing the bones in time. He dated crystals of volcanic feldspar taken from layers of ash around where the fossils were found, establishing a narrow range when the diminutive hominids were alive.
“Lucy was discovered about 20 years ago and we have been trying to figure out how old this fossil was ever since,” Walter said. “Lucy has become an icon for her species, a fashion model. Everybody knows who she is, but we never understood how old she was.”
Her remains are now believed to be more than 3.18 million years old, making her one of the youngest known members of her species.
Johanson, who discovered the Lucy skeleton in 1974 and later established the nonprofit institute in the building that houses UC Berkeley’s divinity school, was relieved to see the age question finally settled.
“Now that, at long last, we have nailed down what was a slippery date for Lucy, it adds integrity to the whole notion of the family trees we put together,” he said.
Combined with other new fossil evidence discovered in different parts of Africa by White and Johanson, scientists are convinced that her fragmented remains represent a single species-- Australopithecus afarensis --that thrived for more than 900,000 years without significant physical changes, until something jolted them out of their evolutionary rut and onto the path to modern humanity.
The laboratory’s dating work also is eliminating the uncertainty clouding all the fossils discovered in the Hadar, the area of Ethiopia where Lucy was found. It is one of the world’s richest sources of hominid fossils.
“The Hadar, which was a remarkably rich source of fossils, was handicapped by the inability to date anything reliably,” said William H. Kimbel, institute director of paleontology.
Part of the problem was technical. When the first hominid fossils were discovered there in the mid-1970s, potassium dating was so crude that 500 pounds of samples might yield a few ounces of material pure enough for testing. The results often raised as many questions as they tried to answer.
But the problems were also political. For a decade, civil unrest in Ethiopia and changes in government policies effectively barred foreign researchers from the Hadar. So even when the technology improved, Johanson, Walter and their colleagues could not collect new samples to date the fossils. They finally were allowed to return in 1990.
Now experts must consider the idea that bands of these prehistoric pre-humans may have begun their journey from their evolutionary homeland more than half a million years earlier than believed. Curtis speculates that the primitive ancestors may have left Africa even earlier.
Until Curtis and Swisher rearranged the history of this prolific species, most anthropologists believed that its success was based on mastery of prehistoric high-technology. Armed with sharper stone axes, Homo erectus carved out a place for itself wherever it went, spreading to the Middle East, Europe and across Asia to the Pacific, experts thought.
Now they are not so sure. The new date means that the hominids may have been well-established around the world long before the development of the more sophisticated stone tools.
It complicates an entire pattern of human evolution, experts said. No one now can be certain whether Homo erectus developed in Asia rather than in Africa, as most evidence indicates, or whether the development of tools had anything to do with the species’ success, or even if the two are members of the same species.
Now no one can be sure which group gave rise to modern humanity. So far, most experts are treating the new date very cautiously.
“It shows that people got out of Africa earlier than we thought,” Hill said. “I don’t think it shows that people evolved in Asia. . . . It doesn’t shake the whole family tree,” he said.
In all likelihood, the learned arguments over the Java fossils will last out the decade.
But scientists on both sides acknowledge that the revolution in dating technology puts the debate over human origins in proper focus, with the emphasis on the meaning hidden in the rock-encrusted fragments of human ancestors, not the hunt for their proper place in time.
When scientists can find a fossil’s niche in time, they also discover its rightful place in the story of human evolution.
And when scientists refine the age of museum specimens, it can force other researchers to reconsider the meaning of even the well-studied human fossils, calling into question some of the most cherished conceptions of evolution.
In February, Curtis and institute researcher Charles Swisher III revealed that another well-known fossil of a more recent human ancestor--a prehistoric child whose skull was discovered in Java in 1936--was a million years older than believed. This upset what had been thought to be one of the few straightforward stories of human origins.
For decades, the tortoise-shell-shaped fossil--an example of a species called Homo erectus that followed Lucy and her kind--was thought to be 1 million years old.
Using state-of-the-art argon-dating, Curtis and Swisher calculated the skull’s age at 1.8 million years. That makes it the oldest hominid fossil found outside Africa, and as old as the earliest examples of Homo erectus found in East Africa.
The Evoluntionary Yardstick
Sophisticated dating techniques are turning simple sediments into high-technology timelines.
Scientists recently unearthed the first almost complete skull of humanity’s earliest recognized ancestor, Australopithecus afarensis .
The sparse fossil record suggests that there were several species of primitive hominids, sometimes co-existing, before modern humans emerged.
Many experts think that Neandarthals and modern humans are so different anatomically that they could not belong to the same species.
Homo neanderthalensis (Neanderthal)
Homo sapiens (Modern man)
120,000 years ago: Dating helps overturn a theory that Neanderthals evolved into early modern humans about 50,000 years ago, by identifying primitive Neandarthal and human remains between 100,000 years and 120,000 years old.
3.18 million years ago: More precise argon dating for humanity’s earliest ancestors helps to show that these ape-like hominds flourished for at least 900,000 years without change.
200,000 years ago: New dating of a fossil skull suggests that the oldest forms of Homo sapiens in Asia might be more than 200,000 years old.
1.8 million years ago: More precise dating this year revealed that fossils in Java of Homo erectus-- the first hominid to leave Africa--are twice as old as previously believed.
Evolving Theories
OUT OF AFRICA THEORY
This theory is based on dating evidence that showed oldest Homo erectus fossils in Africa to be 2 million years old, with similar fossils in Java only 800,00 years old.
The spread of Homo erectus began as much as 1.8 million years ago.
Homo sapiens’ expansion began around 100,000 years ago.
MULTIREGIONAL HYPOTHESIS
Better dating shows that the fossils in both places are about the same age, suggesting these hominds either left Africa a million years earlier or evolved elsewhere.
Regional populations of Homo erectus may have evolved into Homo sapiens while intermingling with one another.
The Fossil Record
The first almost complete afarensis skull was found in the Hadar region of Ethiopia, left, about a mile from the famous “Lucy” skeleton. Researchers use precise argon dating to read the age of volcanic deposits, and thus determine the place in time of fossils found there.
2.92 million years ago
New afarensis male skull.
3.18 million years ago
Lucy skull
“First Family” remains of 13-hominid individuals
3.22 million years ago
Female cranium
Male humerus
3.40 million years ago
Bedrock
Researched by ROBERT LEE HOTZ / Los Angeles Times
Graphics by MICHAEL HALL / Los Angeles Times
Sources: The Institute of Human Origins; Peking University; Nature; Science; The Origins of Modern Humans, Scientific American Library.
