1. Discovering the Atmosphere

2. Where did Earth's Atmosphere come from?

17 September 2003. Ancient Relatives of Algae Yield New Insights into Role of CO2 in Earth's Early Atmosphere. NASA's Earth Observatory. Greenhouse gas has been playing a critical role in warming our planet for billions of years, according to a new study that looks at the photosynthetic cycle by which plants convert light energy and CO2 into cellular tissue.

3. How do Scientists Play the Dating Game?

22 September 1998. How Old are the Rocks? Using Radioactivity to Find Out. When a volcanic magma cools down and solidifies, radioactive "clocks" in it can be set. Geologists can use these "clocks" to find out how long ago the rock formed.

4. The Beginning of Life on Earth

Archive of Past Articles for Chapter 4

6 June 2006. STUDY SHOWS OUR ANCESTORS SURVIVED 'SNOWBALL EARTH' - Earth Observatory. Excerpt: It has been 2.3 billion years since Earth's atmosphere became infused with enough oxygen to support life as we know it. About the same time, the planet became encased in ice that some scientists speculate was more than a half-mile deep. That raises questions about whether complex life could have existed before "Snowball Earth" and survived, or if it first evolved when the snowball began to melt. New research shows organisms called eukaryotes -- organisms of one or more complex cells that engage in sexual reproduction and are ancestors of the animal and plant species present today -- existed 50 million to 100 million years before that ice age and somehow did survive. The work also shows that the cyanobacteria, or blue-green bacteria, that put the oxygen in the atmosphere in the first place, apparently were pumping out oxygen for millions of years before that, and also survived Earth's glaciation. The findings call into question the direst models of just how deep the deep freeze was, said University of Washington astrobiologist Roger Buick, a professor of Earth and space sciences. While the ice likely was widespread, it probably was not consistently as thick as a half-mile, he said. "That kind of ice coverage chokes off photosynthesis, so there's no food for anything, particularly eukaryotes. They just couldn't survive," he said. "But this research shows they did survive."

Archive of Past Articles for Chapter 4

5. The Origin of Our Oxygen-Rich Atmosphere

3 February 2004. When Giants Had Wings and 6 Legs. By HENRY FOUNTAIN, New York Times. Before the dinosaurs, it was the insects that were huge. Why? It may have been the air.... There was an array of giant flightless insects, and a five-foot-long millipede-like creature, Arthropleura, that resembled a tire tread rolled out flat. But perhaps the most remarkable of all were the giant dragonflies, Meganeuropsis permiana and its cousins, with wingspans that reached two and a half feet. They were the largest insects that ever lived. These large species thrived about 300 million years ago, when much of the land was lush and tropical and there was an explosion of vascular plants (which later formed coal, which is why the period is called the Carboniferous). But the giant species were gone by the middle to late Permian, some 50 million years later. Scientists have long suspected that atmospheric oxygen played a central role in both the rise and fall of these organisms. Recent research on the ancient climate by Dr. Robert A. Berner, a Yale geologist, and others reinforces the idea of a rise in oxygen concentration - to about 35 percent, compared with 21 percent now - during the Carboniferous. Because of the way many arthropods get their oxygen, directly through tiny air tubes that branch through their tissues rather than indirectly through blood, higher levels of the gas might have allowed bigger bugs to evolve.... "It's been out there in the literature for a long time without a causal mechanism," said Dr. Robert Dudley, a professor at the University of California at Berkeley who has studied the effects of elevated oxygen pressures on modern insects. ...Dr. Jon F. Harrison, a professor at Arizona State ... said, "It's still in the realm of speculation."

6. How and When did Complex Life Begin?

7. Earth's Shifting Crust

8. Highs and Lows over the Past 750 Million Years

Archive of Past Articles for Chapter 8

14 November 2006. Paleoclimatology: Understanding the Past to Predict the Future. By Holli Riebeek. Scientists use complicated climate models to predict how Earth's climate might change in the future. One of the best ways to test the reliability of such models is to see how well they recreate climates of the past.

7 November 2006 In Ancient Fossils, Seeds of a New Debate on Warming. By WILLIAM J. BROAD. NY Times. Excerpt: In recent years, scientists have learned about the changing makeup of the vanished gases by teasing subtle clues from fossilized soils, plants and sea creatures. They have also gained information from computer models that predict how phenomena like eroding rocks and erupting volcanoes have altered the planet's evolving air. "It's getting a lot more attention," Michael C. MacCracken, chief scientist of the Climate Institute, a research group in Washington, said of the growing field. For the first time, the Intergovernmental Panel on Climate Change, a United Nations group that analyzes global warming, plans to include a chapter on the reconstructions in its latest report, due early next year.The discoveries have stirred a little-known dispute that, if resolved, could have major implications. One side foresees a looming crisis of planetary heating; the other, temperature increases that would be more nuisance than catastrophe. Some argue that CO2 fluctuations over the Phanerozoic follow climate trends fairly well, supporting a causal relationship between high gas levels and high temperatures. Other experts say that the fluctuations in the gas levels often fall out of step with the planet's hot and cold cycles, undermining the claimed supremacy of carbon dioxide. Highlighting the gap, the two sides clash on how much the Earth would warm today if carbon dioxide concentrations double from preindustrial levels, as scientists expect. Many climatologists see an increase of as much as 8 degrees Fahrenheit. Carbon dioxide skeptics and others see the reconstructions of the last 15 years as increasingly reliable, posing fundamental questions about the claimed powers of carbon dioxide. "Some of the work has been quite meticulous," Thure E. Cerling, an expert at the University of Utah on Phanerozoic climates, said. "We are likely to learn something."

Archive of Past Articles for Chapter 8

9. What Happened to the Dinosaurs?

Archive of Past Articles for Chapter 9

28 November 2006. New York Times. Marine Life Leaped From Simple to Complex After Greatest Mass Extinction. By Andrew C. Revkin. Excerpt: At least five mass extinctions, most presumably caused by asteroids that struck the earth, have transformed global ecology in the half-billion years since the emergence of multicelled life, lopping entire branches from the evolutionary tree and causing others to flourish. The greatest "great dying," 251 million years ago, erased 95 percent of species in the oceans (and most vertebrates on land). But new research suggests that it was followed by an explosion of complexity in marine life, one that has persisted ever since. Moreover, it happened quite suddenly... The shift to complicated, interrelated ecosystems was more like a flip of a switch than a slow trend. The researchers detected the change by analyzing records of marine fossils from 1,176 sites around the world, which are part of a new international archive, the Paleobiology Database (pbdb.org).

23 September 2006. DINOSAURS' CLIMATE SHIFTED TOO, REPORT SHOWS. Ancient rocks suggest dramatic climate changes during the dinosaur-dominated Mesozoic Era, a time once thought to have been hot and humid. NASA Earth Observatory.

Archive of Past Articles for Chapter 9

10. The Ice Ages

Archive of Past Articles for Chapter 10

23 March 2007. MICROFOSSILS UNRAVEL CLIMATE HISTORY OF TROPICAL AFRICA. Earth Observatory News. Scientists from the Royal Netherlands Institute for Sea Research obtained for the first time a detailed temperature record for tropical central Africa over the past 25,000 years. ... a marine sediment core taken in the outflow of the Congo River... contained eroded land material and microfossils from marine algae. The results show that the land environment of tropical Africa was cooled more than the adjacent Atlantic Ocean during the last ice-age. This large temperature difference between land and ocean surface resulted in drier conditions compared to the current situation, which favors the growth of a lush rainforest. These findings provide further insight in natural variations in climate and the possible consequences of a warming earth on precipitation in central Africa. The results will be published in this week's issue of Science. ...ocean surface and land temperatures behaved differently during the past 25,000 years. During the last ice age, temperatures over tropical Africa were 21¡C, or about 4¡C lower than today, whereas the tropical Atlantic Ocean was only about 2.5¡C colder. By comparing this temperature difference with existing records of continental rainfall variability, lead author Johan Weijers and his colleagues concluded that the land-sea temperature difference has by far the largest influence on continental rainfall. This can be explained by the strong relationship of air pressure to temperature. When the temperature of the sea surface is higher than that of the continent, stronger offshore winds reduce the flow of moist sea air onto the African continent. This occurred during the last ice age and, as a consequence, the land climate in tropical Africa was drier than it is in today's world, where it favours the growth of a lush rainforest.

8 June 2006. NEW STUDY SHOWS MUCH OF THE WORLD EMERGED FROM LAST ICE AGE TOGETHER - Earth Observatory. Excerpt: The end of the recurring, 100,000-year glacial cycles is one of the most prominent and readily identifiable features in records of the Earth's recent climate history. Yet one of the most puzzling questions in climate science has been why different parts of the world, most notably Greenland, appear to have warmed at different times and at different rates after the end of the last Ice Age. However, a new study appearing in the upcoming issue of the journal Science suggests that, except for regions of the North Atlantic, most of the Earth did, in fact, begin warming at the same time roughly 17,500 years ago. In addition, scientists suggest that ice core records from Greenland, which show that average temperatures there did not warm appreciably until about 15,000 years ago, may have remained in a hyper-cold state largely as a result of events triggered by warming elsewhere....

Archive of Past Articles for Chapter 10

11. Climate and Human Evolution

Archive of Past Articles for Chapter 11

26 June 2007. Humans Have Spread Globally, and Evolved Locally. The New York Times. By NICHOLAS WADE. Excerpt: Historians often assume that they need pay no attention to human evolution because the process ground to a halt in the distant past. That assumption is looking less and less secure in light of new findings based on decoding human DNA. People have continued to evolve since leaving the ancestral homeland in northeastern Africa some 50,000 years ago, both through the random process known as genetic drift and through natural selection. A striking feature of many of these changes is that they are local. The genes under selective pressure found in one continent-based population or race are mostly different from those that occur in the others. These genes so far make up a small fraction of all human genes. The new scans for selection show so far that the populations on each continent have evolved independently in some ways as they responded to local climates, diseases and, perhaps, behavioral situations. The concept of race as having a biological basis is controversial, and most geneticists are reluctant to describe it that way. But some say the genetic clustering into continent-based groups does correspond roughly to the popular conception of racial groups.

21 September 2006. Little Girl, 3 Million Years Old, Offers New Hints on Evolution. By JOHN NOBLE WILFORD. NY Times. Excerpt: If the fossil Lucy, the most famous woman from out of the deep human past, had a child, it might have looked a lot like the bundle of skull and bones uncovered by scientists digging in the badlands of Ethiopia. The paleontologists who are announcing the discovery in the journal Nature today said the 3.3-million-year-old fossils were of the earliest well-preserved child ever found in the human lineage. It was ... a member of the Australopithecus afarensis species, the same as Lucy's.
An analysis of the skeleton revealed evidence of a species in transition, ...afarensis walked upright, like modern humans. But gorillalike arms and shoulders suggested that it possibly retained an ancestral ability to climb and swing through the trees. ...The Dikika girl's brain size ...was about the same as that of a similarly aged chimpanzee, but a comparison with adult afarensis skulls indicates a relatively slow brain growth slightly closer to that of humans. ...hyoid bone ...a rarely preserved bone in the larynx, or voice box, that supports muscles of the throat and tongue. ... appeared to be primitive and more similar to those found in apes than in humans, the scientists said, but is the first hyoid found in such an early human-related species and thus important in research about the origins of human speech.
The first relatively complete shoulder blades to be found in an australopithecine individual was one of the most puzzling aspects of the discovery, several scientists said. The lower body appeared to be adapted for upright walking by afarensis. But the shoulders and long arms were more apelike.

Archive of Past Articles for Chapter 11

12. Climate and Culture

13. What does Earth's Past Tell us about Our Future?

Archive of Past Articles for Chapter 13

February 2006. Affecting Evolution and Extinction. By David Pescovitz. ScienceMatters@Berkeley, Volume 3, Issue 18. Every so often, a huge number of species on Earth are wiped out relatively quickly. The last time a large extinction event occurred, between 50,000 and 10,000 years ago, two-thirds of large mammals were swept into the dustbin of history. Why? UC Berkeley paleontologist Anthony Barnosky sifts through the fossil record to understand how environmental changes can cause mammals to move, evolve, and sometimes die off. His research could even help reveal whether we're headed for another mass extinction. ...The aim... is to differentiate between effects of climate change that are natural, and those that could be harbingers of a bigger problem.... "Is part of being a species the fact that you move around in response to climate change and it's no big deal?" Barnosky says. "I'm trying to establish a natural baseline of how much communities change in response to climate change in the past." ... Barnosky ... investigate[d] the cause of large mammal extinctions in the late Pleistocene period, 50,000 to 10,000 years ago. Historically, scientists have thought that human populations of the time over-hunted, killing off animals such as mammoths, ground sloths, native American horses, and camels. However, Barnosky and his colleagues discovered that human impact wasn't the sole cause of the extinctions. Rather, climate change combined with the over-hunting was a "one-two punch" leading to the extinction, he says. The big concern, Barnosky says, is that the state of the planet then is not so different from today. "We've ramped everything up," he says. "Global warming has never been faster and human populations are exploding exponentially. Realistically, I think the ecosystem will change pretty dramatically.

Archive of Past Articles for Chapter 13

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