Nailing Down the Dates

Everyone knew that Longgushan and its fossils were old. But how old? Weidenreich thought that the Chinese Homo erectus were more evolved and more humanlike than Javan Homo erectus, and he speculated that they were more recent. Absolute dates could help resolve the time relationship between the hominids from China and those from Java. In addition, as more and more hominid fossils turned up in China in later years, the date of the cave near Zhoukoudian also became an issue. There were early Homo sapiens (more likely Homo heidelbergensis) fossils from China that rivaled the presumed ages of fossils at Longgushan. Could the two species have been contemporaries? And as earlier and earlier Homo erectus fossils were discovered in Africa, back more than a million and a half years, the dates for Chinese Homo erectus at Longgushan began to look more and more anomalous. Chinese scientists began an earnest quest for methods to date the cave sediments at Longgushan.

The basic premise of most absolute-dating methods is assessing the state of decay of a chemical element. That element can be a part of a fossil itself or it can be a component of the geological context of a fossil. Geochro-nologists—those scientists who research the age of the earth—have been very resourceful in finding elements to date. Many of the dating techniques use different forms of an element. These different forms of an element are called isotopes and they differ from one another in the number of neutrons in the nucleus (isotopes of the same element by definition share the same number of protons or "atomic mass" in the nucleus).

Carbon dating is the granddaddy of absolute-dating methods. Invented by Willard Libby in 1955 in Chicago, it measures an isotope of carbon, carbon 14 ("14" referring to the number of protons and neutrons in the atomic nucleus). The carbon 14 isotope is formed in the atmosphere from nitrogen 14 by ultraviolet radiation from the sun. A neutron collides with nitrogen, turning it into radioactive carbon 14 by releasing a proton as hydrogen and adding a neutron to the nucleus. Carbon 14, with six protons and eight neutrons, is found in the same amounts in all living organisms—just so long as they remain alive. When they die, no more carbon 14 is breathed in or ingested, and the amount of the isotope begins to disappear. It decays at a standard rate, losing its extra neutrons as the atom returns to more stable, lower-energy states. What is critically important for absolute dating is that elements decay at very predictable rates. Carbon 14 decays at a rate that removes one-half of the original amount in 5,700 years (a period of time known as its half-life).

Carbon 14 decays at such a rapid rate, in geological terms, that too little of the isotope is left after about fifty thousand years to yield a very accurate date with conventional carbon 14 dating. A newer method, potassium-argon dating, pioneered by Garniss Curtis and Jack Evernden at Berkeley in the 1960s, extended the time scale of absolute dates to millions of years. The era of ever-earlier African hominid discoveries began, calibrated by potassium-argon dates on the volcanic (and potassium-rich) rocks that bracketed the fossils. But back in China, and at other hominid fossil—bearing cave sites the world over, no revolutionary new dating methods existed. As the early hominids ofAfrica assumed the starring roles, the proverbial "cavemen" were pushed to the side in the new and seemingly ever-older narrative of human evolution.

Uranium-series dating has now allowed absolute dating of cave sediments in the time span of approximately three hundred thousand to more than a million years ago. The theory of uranium dating has been known since the late nineteenth century. But accurate determination of uranium's isotopes and the possibility of loss of decayed products to the enclosing rock have made application of the theory difficult. New advances have now made uranium dating reliable. A number of isotopes of uranium occur in nature, and each decays at a known rate. Uranium 238 decays to uranium 234 slowly (its half-life is 451 million years) whereas uranium 234 decays to thorium 230 much more rapidly (with a half-life of 245,000 years). Uranium dissolves easily in water and is also deposited in minerals precipitated from ground water, such as cave flowstone, also known as travertine, which is composed of calcium carbonate. Because bone and teeth are of similar mineral composition, it is also possible to determine the ages of fossil bones and teeth by uranium dating.

In 1985 Chinese scientists led by Shusen Zhao were able to measure the amounts of uranium 238, uranium 234, and thorium 230 in a sample of fossils from Longgushan using a mass spectrometer.2 This is an apparatus that "catches" and counts electrons of specific energies and thereby accurately determines the quantities of isotopes in a sample. Knowing the numbers of isotopes and their decay rates, one can calculate the age of the sample. Scientists determined two ages for Layers 1 through 3 in the Longgushan cave—230,000 years old and 256,000 years old—but there was more variation in the individual determinations than was expected. This implied that some specimens had lost some of their decay isotopes, probably due to weathering of the bone or some other "diagenetic" change, and thus appeared too young. In a study six years later, S. Yuan and his colleagues reported on a more intensive study of carefully selected and prepared bone from Layer 2 that gave an older and apparently more accurate date (with less scatter of the determinations) of 290,000 years.3 A mean age with a tighter cluster of measurements around the mean, they hoped, meant a more accurate age, but it could also simply mean that all samples sitting buried in the same sediments for hundreds of thousands of years had lost some of their isotopic-decay products to the same extent, not an unlikely proposition. Then in 1996 G. Shen and his colleagues used a new and more accurate measuring technique, thermal ionization mass spectrometry (TIMS)—a method of "step-heating" the samples to release their isotopes.4 They could thereby determine the oldest uranium isotopic-decay products from Layer 2 travertine. They determined the true age of the rock to be much older than the previous determinations on fossilized bones— 410,000 years. The effect of the new dates was to push back the age of Peking Man some two hundred thousand years.

The new dates were substantially older, and from the top of the Longgushan sediments. Other absolute-dating techniques were tried in order to confirm the old dates and to attempt to ascertain the age of the lowest sediments of the deposit. Paleomagnetism, a dating method based on the surprising phenomenon of Earth's flip-flopping of magnetic north and south during its geological history, proved important in defining the lower age limit of the cave deposit at Longgushan. Sediments record the orientation of magnetic north microscopically in the orientation of their sedimentary particles. Study of the sediments at Longgushan by F. Qian and his colleagues in 1985 revealed that the boundary between the Brunhes Epoch ("normal" polarity, during which a magnetic needle points north) and the older Matuyama Epoch ("reversed" polarity, during which a magnetic needle points south) occurs in Layer 14 at Longgushan.5 This boundary is dated as a worldwide event at 780,000 years ago, so the first fossiliferous strata at Longgushan, in Layer 13, are almost this old. The oldest hominid fossils occur in Layer 10, where the first complete skull (Skull III) was found at Locus E, and are probably about 110,000 years younger. This estimate is based on the rate of sediment accumulation in the cave. It establishes the oldest Homo erectus at Longgushan to be about 670,000 years old.

The accurate dating of Longgushan has made it possible to piece together the climatic history and ecology of Homo erectus in China. Homo erectus occupied Longgushan Cave intermittently from about 670,000 to 410,000 years ago.6 This was a span of time that we can now confirm as the middle Pleistocene—a period within the Ice Age when climates fluctuated between being very cold and being as warm as today.

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  • leo
    How to pronounce longgushan?
    3 years ago

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