National Geographic : 2001 Sep
from what fossil records show. He and his col leagues have compared genes from three animal phyla, and their molecular clocks point to an origin over a billion years ago-once more a doubling of evolutionary history. The conflict between fossils and genes will take a long time to sort out. Critics of molecu lar clocks suspect that evolution can make them speed up or slow down. But Hedges counters that he and his colleagues can guard against this sort of variability, and when they do, their dates still hold up. As for the lack of fossils to support his dates, Hedges argues that the earliest forms didn't leave fossils behind, or at least any that have yet been discovered. Only around the start of the Cambrian did they get big enough for us to find. Telling time is important not just to the history of life but to the history of the universe itself. Clocks that pin down the formation of the solar system can be found in meteorites that have fallen to Earth after wandering around the sun for billions of years. But for more ancient time telling, scientists cannot use any clock to be found on Earth. They have to look at the sky. The sky was cloudy on the evening I met George Djorgovski, an astronomer from Cal tech working in Hawaii, and rain was falling as How Old Is... ... the Grand Canyon? !Bonr Iyc: 2 bioli0l d years 0 Top kwcr:250 mllon yearsold Dating techniques: Bottom layer Radiometric. Top layer. Fossils Although the Grand Canyon's oldest known rocks are almost half the age of Earth, the Colorado River did not start carving the canyon itself until just five or six million years ago-a blink of the eye in geologic time. we walked quickly across a dark lawn. "Can you believe we can look at stars in this weather?" he asked. We entered a small building and slipped into a room filled with bright fluorescent light and eight giant computer screens. Even if the sky was clear, we couldn't have seen the stars through the drawn blinds. Djorgovski sat in front of three computer screens pushed next to each other. The com puters are hooked up to data cables that run 48 miles from this room to the 13,800-foot high summit of Mauna Kea-and to two of the finest telescopes in the world, at the W. M. Keck Observatory. As the sun set, Djorgovski sent coordinates to technicians at the top of Mauna Kea, and the telescope he was using swung across the sky. A disembodied voice from one of the computers in the room said, "Exposure complete," and a white field filled with black spots appeared. One giant blob dominated the center of the picture-a ferociously bright object known as a quasar, with the intensity of trillions of suns. "That's our guy," said Djorgovski. He touched the image of the quasar with one finger. "Just think," he said. "As the Earth formed, the light from this had already traveled two-thirds of its way here." Quasars and galaxies are hurtling away from us as the universe expands. As they speed off, the light they emit lowers in frequency and shifts toward the red end of the spectrum much as a train whistle drops in pitch as it passes ... this Pyramid? Dating technique: Star alignment The Pyramid of Khafre may be about 70 years younger than usually thought, says a new theory, which shows that the northern alignment of the Pyramids at Giza matches that of two polar stars visible during that era.