1. Discovering the Atmosphere

2. Where did Earth's Atmosphere come from?

2009 Dec 10. Our Atmosphere Came From Space Gases, Study Says. By Ker Than, for National Geographic News. Excerpt: The gases that make up Earth's atmosphere came from a swarm of comets, not from bubbling volcanoes as long thought, a new study says. The new theory came about after scientists discovered that pristine samples of the elements krypton and xenon, recently collected from deep within the Earth, have the same chemical makeup as ancient meteorites....
It's still true that volcanoes spewed out some gases, "but [that] contribution was insignificant" for the creation of Earth's atmosphere," Ballentine said.
... most of Earth's krypton has remained unchanged since its arrival on our planet—allowing scientists to precisely study the conditions of early Earth.
Based on their research, Ballentine and colleagues claim that our atmosphere likely formed when gas and water-rich comets bombarded Earth, shortly after its formation 4.54 billion years ago.
...Scientists have already discovered that the comet barrage likely formed Earth's oceans....

2003 September 17. 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?

2009 June 7. Early rocks to reveal their ages. By Jennifer Carpenter, BBC News. Excerpt: A new technique has been helping scientists piece together how the Earth's continents were arranged 2.5 billion years ago. The novel method allows scientists to recover rare minerals from rocks. By analysing the composition of these minerals, researchers can precisely date ancient volcanic rocks for the first time. By aligning rocks that have a similar age and orientation, the early landmasses can be pieced together.
...Analysis of rocks that formed when continents drifted apart can help geologists reconstruct early landmasses. Dr Richard Ernst, a geologist from the University of Ottawa, explained that molten magma fills the cracks formed by shifting continental plates. The magma cools to form long veins of basalt - a volcanic rock - that has a "distinct magnetic signature" revealing the rock's orientation and latitude when it formed. By combining this "magnetic signature" with the ages of these rocks, researchers can tell whether rocks on different continents were once part of the same volcanic up-welling.
But until now, researchers have been unable to determine the ages of many of these ancient rocks because of the difficulty in extracting the minerals used to date them.
"We are dealing with such small mineral crystals - typically much less than 100 microns long - we are talking about grains far smaller than the width of a human hair," explained Dr Michael Hamilton, a geologist and co-leader on the project.
But with the development of new techniques, minerals - such as baddeleyite - can now be successfully recovered. Baddeleyite is useful because it incorporates large amounts of uranium into its crystal-structure, and because uranium naturally decays to lead. Scientists also know the rate at which this happens.
"[They] can use these minerals as radioactive clocks," Dr Hamilton added. "All we need to do is measure the the amounts of uranium and lead very precisely."...

1998 September 22. 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

2009 August 19. NASA RELEASE: 09-192. NASA Research Reveals Major Insight Into Evolution Of Life On Earth. Excerpt: MOFFETT FIELD, Calif. -- Humans might not be walking on Earth today if not for the ancient fusing of two microscopic, single-celled organisms called prokaryotes, NASA-funded research has found.
By comparing proteins present in more than 3000 different prokaryotes - a type of single-celled organism without a nucleus - molecular biologist James A. Lake from the University of California at Los Angeles' Center for Astrobiology showed that two major classes of relatively simple microbes fused together more than 2.5 billion years ago. Lake's research reveals a new pathway for the evolution of life on Earth....
This endosymbiosis, or merging of two cells, enabled the evolution of a highly stable and successful organism with the capacity to use energy from sunlight via photosynthesis. Further evolution led to photosynthetic organisms producing oxygen as a byproduct. The resulting oxygenation of Earth's atmosphere profoundly affected the evolution of life, leading to more complex organisms that consumed oxygen, which were the ancestors of modern oxygen-breathing creatures including humans.
"Higher life would not have happened without this event," Lake said. "These are very important organisms. At the time these two early prokaryotes were evolving, there was no oxygen in the Earth's atmosphere. Humans could not live. No oxygen-breathing organisms could live."...

2009 June 15. New Glimpses of Life’s Puzzling Origins. By Nicholas Wade, The NY Times. Excerpt: Some 3.9 billion years ago, a shift in the orbit of the Sun’s outer planets sent a surge of large comets and asteroids careening into the inner solar system....
Yet rocks that formed on Earth 3.8 billion years ago, almost as soon as the bombardment had stopped, contain possible evidence of biological processes. If life can arise from inorganic matter so quickly and easily, why is it not abundant in the solar system and beyond? If biology is an inherent property of matter, why have chemists so far been unable to reconstruct life, or anything close to it, in the laboratory?
The origins of life on Earth bristle with puzzle and paradox. Which came first, the proteins of living cells or the genetic information that makes them? How could the metabolism of living things get started without an enclosing membrane to keep all the necessary chemicals together? But if life started inside a cell membrane, how did the necessary nutrients get in?
The questions may seem moot, since life did start somehow. But for the small group of researchers who insist on learning exactly how it started, frustration has abounded. Many once-promising leads have led only to years of wasted effort....
In the last few years, however, four surprising advances have renewed confidence that a terrestrial explanation for life’s origins will eventually emerge....

2009 May 20. NASA STUDY SHOWS ASTEROIDS MAY HAVE ACCELERATED LIFE ON EARTH. NASA RELEASE: 09-11. Excerpt: WASHINGTON -- A NASA-funded study indicates that an intense asteroid bombardment nearly 4 billion years ago may not have sterilized the early Earth as completely as previously thought. The asteroids, some the size of Kansas, possibly even provided a boost for early life. The study focused on a particularly cataclysmic occurrence known as the Late Heavy Bombardment, or LHB. This event occurred approximately 3.9 billion years ago and lasted 20 to 200 million years. ... while the Late Heavy Bombardment might have generated enough heat to sterilize Earth's surface, microbial life in subsurface and underwater environments almost certainly would have survived. "Exactly when life originated on Earth is a hotly debated topic," said Michael H. New, the astrobiology discipline scientist and manager of the Exobiology and Evolutionary Biology Program at NASA Headquarters in Washington. "These findings are significant because they indicate that if life had begun before the LHB or some time prior to 4 billion years ago, it could have survived in limited refuges and then expanded to fill our world."
"Our new results point to the possibility life could have emerged about the same time that evidence for our planet's oceans first appears," said Mojzsis, principal investigator of the project.
A growing scientific consensus is that during our solar system's formation, planetary bodies were pummeled by debris throughout the Late Heavy Bombardment. ...Surface habitats for microbial life on early Earth would have been destroyed repeatedly by the bombardment. However, at the same time, impacts could have created subsurface habitats for life, such as extensive networks of cracks or even hydrothermal vents. Any existing microbial life on Earth could have found refuge in these habitats.

2008 December 8. Meteor impacts may have sparked life on Earth. By Emma Young, New Scientist. Excerpt: While space rocks hurtling in from space threaten to deal modern life a mortal blow, meteorite impacts during Earth's early history may have played a pivotal role in kick-starting life on the planet.
Exactly how and when organic molecules appeared in abundance on the young Earth, leading to the origin of life about 4 billion years ago, has been unclear. But new research suggests that meteor impacts could have created amino acids, the building blocks of life.
Yoshihiro Furukawa at Tohoku University in Sendai, Japan, and colleagues used a high-velocity propellant gun to simulate the impacts of ordinary carbon-containing chondrite meteorites - the most common type of meteorite - into the early ocean. Afterwards, they recovered a variety of organic molecules, including fatty acids, amines, and an amino acid.
Oceans began to form about 4.3 billion years ago, when meteorites were hitting Earth about 1000 times more frequently than they do today, says Furukawa. "This study is the first to show that an amino acid can be synthesised by a naturally possible mechanism on the early Earth," he says....

2008 October 16. Volcanoes May Have Provided Sparks and Chemistry for First Life. NASA Earth Observatory. Excerpt: Lightning and gases from volcanic eruptions could have given rise to the first life on Earth, according to a new analysis of samples from a classic origin-of-life experiment by NASA and university researchers....
...From 1953 to 1954, Professor Stanley Miller, then at the University of Chicago, performed a series of experiments with a system of closed flasks containing water and a gas of simple molecules. At the time, the molecules used in the experiment (hydrogen, methane, and ammonia) were thought to be common in Earth's ancient atmosphere.
The gas was zapped with an electric spark. After running the experiment for a few weeks, the water turned brown. When Miller analyzed the water, he found it contained amino acids, which are the building blocks of proteins -- life's toolkit... The spark provided the energy for the molecules to recombine into amino acids, which rained out into the water....
...Professor Jeffrey Bada, a co-author of the paper, was [Miller's] graduate student in chemistry between 1965 and 1968....
"...When Adam and I found the samples from the original experiments, it was a great opportunity to reanalyze these historic samples using modern methods," said Bada. The team wanted to see if modern equipment could discover chemicals that could not be detected with the techniques of the 1950s. They analyzed the samples and turned to Daniel Glavin and Jason Dworkin of NASA's Goddard Space Flight Center in Greenbelt, Md....
..."We discovered 22 amino acids, 10 of which have never been found in any other experiment like this," said Glavin. This is significant because thinking on the composition of Earth's early atmosphere has changed. Instead of being heavily laden with hydrogen, methane, and ammonia, many scientists now believe Earth's ancient atmosphere was mostly carbon dioxide, carbon monoxide, and nitrogen....

Archive of Past Articles for Chapter 4

5. The Origin of Our Oxygen-Rich Atmosphere

2009 March 25. Deep-sea Rocks Point To Early Oxygen On Earth. Science Daily. Excerpt: Red jasper cored from layers 3.46 billion years old suggests that not only did the oceans contain abundant oxygen then, but that the atmosphere was as oxygen rich as it is today, according to geologists.
This jasper or hematite-rich chert formed in ways similar to the way this rock forms around hydrothermal vents in the deep oceans today.
...The researchers drilled diagonally into the base of a hill in the Pilbara Craton in northwest Western Australia to obtain samples of jasper that could not have been exposed to the atmosphere or water. These jaspers could be dated to 3.46 billion years ago.
"Everyone agrees that this jasper is 3.46 billion years old," said Hiroshi Ohmoto, professor of geochemistry, Penn State. "If hematite were formed by the oxidation of siderite at any time, the hematite would be found on the outside of the siderite, but it is found inside," he reported in a recent issue of Nature Geoscience.
...said Ohmoto, "It also means that there was oxygen in the atmosphere 3.46 billion years ago, because the only mechanism for oxygen to exist in the deep oceans is for there to be oxygen in the atmosphere."
In fact, the researchers suggest that to have sufficient oxygen at depth, there had to be as much oxygen in the atmosphere 3.46 billion years ago as there is in today's atmosphere. To have this amount of oxygen, the Earth must have had oxygen producing organisms like cyanobacteria actively producing it, placing these organisms much earlier in Earth's history than previously thought....

27 September 2007 NASA RELEASE: 07-215 - NASA RESEARCH INDICATES OXYGEN ON EARTH 2.5 BILLION YEARS AGO. Excerpt: MOFFETT FIELD, Ca lif. - NASA-funded astrobiologists have found evidence of oxygen present in Earth's atmosphere earlier than previously known, pushing back the timeline for the rise of oxygen in the atmosphere. Two teams of researchers report that traces of oxygen appeared in Earth's atmosphere from 50 to 100 million years before what is known as the Great Oxidation Event. This event happened between 2.3 and 2.4 billion years ago, when many scientists think atmospheric oxygen increased significantly from the existing very low levels.
Scientists analyzed a kilometer-long drill core from Western Australia, representing the time just before the major rise of atmospheric oxygen. They found evidence that a small but significant amount of oxygen was present in Earth's oceans and atmosphere 2.5 billion years ago. The findings appear in a pair of research papers in the Sept. 28 issue of the journal Science.
"We seem to have captured a piece of time during which the amount of oxygen was actually changing -- caught in the act, as it were," said Ariel Anbar, an associate professor at Arizona State University, Tempe, and leader of one of the research teams.
...One possible explanation for the Great Oxidation Event is the ancient ancestors of today's plants first began to produce oxygen by photosynthesis. However, many geoscientists think organisms began to produce oxygen much earlier, but the oxygen was destroyed in reactions with volcanic gases and rocks.
..."What we have now is new evidence for some oxygen in the environment 50 to 100 million years before the big rise of oxygen," Anbar said. "Our findings strengthen the notion that organisms learned to produce oxygen long before the Great Oxidation Event, and that the rise of oxygen in the atmosphere ultimately was controlled by geological processes."
...For more information about the NASA Astrobiology Institute, visit:
http://nai.nasa.gov

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?

Understanding Evolution - a one-stop source for information on evolution.

2008 October 17. Evolution Primers. National Center for Science Education - articles on evolution.

7. Earth's Shifting Crust

2010 Feb 27. Underwater Plate Cuts 400-Mile Gash. By HENRY FOUNTAIN, NY Times. Excerpt: The magnitude 8.8 earthquake that struck off the coast of Chile early Saturday morning occurred along the same fault responsible for the biggest quake ever measured, a 1960 tremor that killed nearly 2,000 people in Chile and hundreds more across the Pacific.
Both earthquakes took place along a fault zone where the Nazca tectonic plate, the section of the earth’s crust that lies under the Eastern Pacific Ocean south of the Equator, is sliding beneath another section, the South American plate. The two are converging at a rate of about three and a half inches a year.
Earthquake experts said the strains built up by that movement, plus the stresses added along the fault zone by the 1960 quake, led to the rupture on Saturday along what is estimated to be about 400 miles of the zone, at a depth of about 22 miles under the sea floor. The quake generated a tsunami, with small surges hitting the West Coast of the United States and slightly larger ones in Hawaii and other parts of the Pacific. A 7.7-foot surge was recorded in Talcahuano, Chile.
Jian Lin, a geophysicist with the Woods Hole Oceanographic Institution, said the quake occurred just north of the site of the 1960 earthquake, with very little overlap. “Most of the rupture today picked up where the 1960 rupture stopped,” said Mr. Lin, who has studied the 1960 event, which occurred along about 600 miles of the fault zone and was measured at magnitude 9.5....

2008 May.The Living Story of Sulawesi. by Kathleen M. Wong, ScienceMatters@Berkeley. Excerpt: The Indonesian island of Sulawesi is a 12,000-square-mile jigsaw puzzle. During the past 25 million years, drifting tectonic plates tore four separate paleo-islands from the far corners of the South Pacific and smashed them together in a steamy corner of Southeast Asia.
This turbulent history has turned Sulawesi into a complex biological cipher. Today, it houses a mélange of species with confusing origins: some may have been passengers on the original islands, some may have arrived afterward, and some may have evolved from the mix.
...Jim McGuire, curator of herpetology at Berkeley's Museum of Vertebrate Zoology and a professor of integrative biology, is studying how these species evolved and came to be distributed on Sulawesi today.
.."It was as if they were cut off from each other at some point. But in many cases we don't know what the underlying mechanism would be," McGuire says.
...Based on these data, he uses computer simulations to reconstruct the evolutionary history of these animal groups. He then plans to go back and study contact zones between species more closely to try to identify any environmental or ecological barriers, such as past flooding or the presence of a predator, that are enforcing species isolation....

28 August 2007. A Daddy Longlegs Tells the Story of the Continents' Big Shifts. By CARL ZIMMER, NY Times. Excerpt: Few people have heard of the mite harvestman, .... The animal is a relative of the far more familiar daddy longlegs. But its legs are stubby rather than long, and its body is only as big as a sesame seed. ... "They look like grains of dirt," said Gonzalo Giribet, an invertebrate biologist at Harvard. ... Dr. Giribet and his colleagues have spent six years searching for them on five continents. The animals have an extraordinary story to tell: they carry a record of hundreds of millions of years of geological history, chronicling the journeys that continents have made around the Earth.
The Earth's land masses have slowly collided and broken apart again several times, carrying animals and plants with them. These species have provided clues to the continents' paths.
The notion of continent drift originally came from such clues. In 1911, the German scientist Alfred Wegner was struck by the fact that fossils of similar animals and plants could be found on either side of the Atlantic. The ocean was too far for the species to have traveled themselves. Wegner speculated - correctly, as it turned out - that the surrounding continents had originally been welded together in a single landmass, which he called Pangea.
Continental drift, or plate tectonics as it is scientifically known, helped move species around the world. Armadillos and their relatives are found in South America and Africa today because their ancestors evolved when the continents were joined. ...The 5,000 or so mite harvestmen species can be found on every continent except Antarctica. Unlike animals found around the world like cockroaches, mite harvestmen cannot disperse well. The typical harvestman species has a range of less than 50 miles. Harvestman are not found on young islands like Hawaii.
"It's really hard to find a group of species that is distributed all over the world but that also don't disperse very far," said Sarah Boyer, a former student of Dr. Giribet, now an assistant professor at Macalester College in St. Paul...

8. Highs and Lows over the Past 750 Million Years

Archive of Past Articles for Chapter 8

2010 March 4. NSF Release 10-037: Scientists Find Signs of "Snowball Earth" Amidst Early Animal Evolution. Excerpt: Geologists have found evidence that sea ice extended to the equator 716.5 million years ago, bringing new precision to a "snowball Earth" event long suspected to have taken place around that time.
...The new findings--based on an analysis of ancient tropical rocks that are now found in remote northwestern Canada--bolster the theory that our planet has, at times in the past, been ice-covered at all latitudes.
"This is the first time that the Sturtian glaciation has been shown to have occurred at tropical latitudes, providing direct evidence that this particular glaciation was a 'snowball Earth' event," says lead author Francis Macdonald, a geologist at Harvard University.
...According to Enriqueta Barrera, program director in NSF's Division of Earth Sciences, which supported the research, the Sturtian glaciation, along with the Marinoan glaciation right after it, are the greatest ice ages known to have taken place on Earth. "Ice may have covered the entire planet then," says Barrera, "turning it into a 'snowball Earth.'"
The survival of eukaryotes--life forms other than microbes such as bacteria--throughout this period suggests that sunlight and surface water remained available somewhere on Earth's surface. The earliest animals arose at roughly the same time.
..."The fossil record suggests that all of the major eukaryotic groups, with the possible exception of animals, existed before the Sturtian glaciation," Macdonald says. "The questions that arise from this are: If a snowball Earth existed, how did these eukaryotes survive? Did the Sturtian snowball Earth stimulate evolution and the origin of animals?"...

2009 June 21. Carbon Dioxide Higher Today Than Last 2.1 Million Years. ScienceDaily. Excerpt: Researchers have reconstructed atmospheric carbon dioxide levels over the past 2.1 million years in the sharpest detail yet, shedding new light on its role in the earth's cycles of cooling and warming.
The study...is the latest to rule out a drop in CO2 as the cause for earth's ice ages growing longer and more intense some 850,000 years ago. But it also confirms many researchers' suspicion that higher carbon dioxide levels coincided with warmer intervals during the study period.
The authors show that peak CO2 levels over the last 2.1 million years averaged only 280 parts per million; but today, CO2 is at 385 parts per million, or 38% higher. This finding means that researchers will need to look back further in time for an analog to modern day climate change.
In the study, Bärbel Hönisch, a geochemist at Lamont-Doherty Earth Observatory, and her colleagues reconstructed CO2 levels by analyzing the shells of single-celled plankton buried under the Atlantic Ocean, off the coast of Africa. By dating the shells and measuring their ratio of boron isotopes, they were able to estimate how much CO2 was in the air when the plankton were alive. This method allowed them to see further back than the precision records preserved in cores of polar ice, which go back only 800,000 years.
...The low carbon dioxide levels outlined by the study through the last 2.1 million years make modern day levels, caused by industrialization, seem even more anomalous, says Richard Alley, a glaciologist at Pennsylvania State University, who was not involved in the research.
"We know from looking at much older climate records that large and rapid increase in CO2 in the past, (about 55 million years ago) caused large extinction in bottom-dwelling ocean creatures, and dissolved a lot of shells as the ocean became acidic," he said. "We're heading in that direction now."...

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

2009 April 28. New Blow Against Dinosaur-killing Asteroid Theory, Geologists Find. ScienceDaily. Excerpt: The enduringly popular theory that the Chicxulub crater holds the clue to the demise of the dinosaurs, along with some 65 percent of all species 65 million years ago, is challenged in a paper to be published in the Journal of the Geological Society on April 27, 2009.
The crater, discovered in 1978 in northern Yucutan and measuring about 180 kilometers (112 miles) in diameter, records a massive extra-terrestrial impact.
When spherules from the impact were found just below the Cretaceous-Tertiary (K-T) boundary, it was quickly identified as the "smoking gun" responsible for the mass extinction event that took place 65 million years ago.
It was this event which saw the demise of dinosaurs, along with countless other plant and animal species.
However, a number of scientists have since disagreed with this interpretation.
The newest research, led by Gerta Keller of Princeton University in New Jersey, and Thierry Adatte of the University of Lausanne, Switzerland, uses evidence from Mexico to suggest that the Chicxulub impact predates the K-T boundary by as much as 300,000 years.
...From El Penon and other localities in Mexico, says Keller, "we know that between four and nine meters of sediments were deposited at about two to three centimeters per thousand years after the impact. The mass extinction level can be seen in the sediments above this interval."
...The scientists also found evidence that the Chicxulub impact didn't have the dramatic impact on species diversity that has been suggested.
..."We found that not a single species went extinct as a result of the Chicxulub impact," says Keller.
...Keller suggests that the massive volcanic eruptions at the Deccan Traps in India may be responsible for the extinction, releasing huge amounts of dust and gases that could have blocked out sunlight and brought about a significant greenhouse effect....

2008 December 15. Fight over dinosaur death flares anew in S.F. By David Perlman, San Francisco Chronicle. Excerpt: SAN FRANCISCO -- The age of the dinosaurs ended abruptly about 65 million years ago when some catastrophic event drove them to extinction, and now a vehement controversy over their disappearance is emerging anew.
...An international group of scientists is arguing that poisonous fumes from violent waves of volcanic eruptions in India millions of years ago killed off the beasts, not - as UC Berkeley scientists first proposed nearly 30 years ago - the impact of a giant meteorite that blasted a huge undersea crater in Mexico and touched off a kind of "nuclear winter" that darkened the skies with a pall of dust and debris that the creatures could not possibly have survived.
The origins of the big debate began nearly 30 years ago when the geologist Walter Alvarez at UC Berkeley and his father, Luis, a Nobel physics laureate, proposed that a cosmic collision by an object from space at least six miles wide crashed just off Mexico's Yucatan peninsula about 65 million years ago and created what is known as the Chicxulub crater.
...Gerta Keller of Princeton University..., joined by Vincent Courtillot of the University of Paris and Sunil Bajpai, of the Indian Institute of Technology, ...insisted that the impact crater was formed at least 300,000 years before the great extinction and "caused no species extinctions" - certainly not to the dinosaurs. She came to her conclusion, in part, by age-dating clusters of mineral spherules that presumably spewed out of the Chicxulub crater and landed in Texas, where Keller said she gathered and tested them.
Instead, she argued, the extinction coincided with three or four waves of volcanism in a region of northwest India known as the Deccan traps....
...Walter Alvarez...rejected the idea that volcanism in India was the sole cause of the mass extinction.
..."Few experts on the mass extinction would agree with Keller that the Chicxulub impact is older than the mass extinction," he said....

2008 Mar 25. Theory on Dinosaurs and Volcanic Activity 65 Million Years Ago. By HENRY FOUNTAIN, NY Times. Excerpt: An asteroid or comet impact gets most of the credit for the event that wiped out the dinosaurs 65 million years ago. But massive volcanic activity around the same time might have played a role, too, by pumping enormous amounts of gases containing sulfur and chlorine into the atmosphere. An analysis by Stephen Self of the Open University in Milton Keynes, England, and colleagues lends new support to that idea. By looking at tiny bits of glass that formed inside the lava flows, they've been able to reconstruct how much sulfur and chlorine were released. The volcanic activity over thousands of years produced a flood of lava, now known as the Deccan Trapps, that is thousands of feet thick over thousands of square miles of central India. The researchers' analysis, reported in Science, suggests the eruption could have produced ...on an annual basis, ...the amount of SO2 ... at least 10 times greater than the current amount released by worldwide volcanic activity. The environmental impact of that much gas, they add, was probably severe.

2007 November 6. Rethinking What Caused the Last Mass Extinction. By JOHN NOBLE WILFORD. NY Times. Excerpt: FREEHOLD, N.J. - Splashing through a shallow creek in suburban New Jersey, the paleontologists stepped back 65 million years to the time of the last mass extinction, the one notable for the demise of the dinosaurs. ...At the time, sea levels were higher and New Jersey was warmer. The proto-Atlantic waters reached the center of the current boundaries of New Jersey, standing more than 60 feet deep here, where on a recent day the paleontologists were up to their ankles in a creek. They had their eyes on the sediments in the bank just above the iridium clay. They call this the Pinna layer.
On previous visits, they had found in the Pinna rock and soil a surprising number of marine fossils, including small clams, crabs and sea urchins. There was an abundance of ammonites, considered index organisms of the uppermost Cretaceous environment. Somehow, here at least, life appeared to have not only persisted but also flourished for tens, perhaps hundreds, of years after the putative asteroid impact.
..."It is undeniable that the iridium spike at the base of the Pinna layer was produced by the impact," Dr. Landman said. "That's amazing and makes it hard to explain the ammonite abundances we find above the iridium anomaly."
Gerta Keller, a paleontologist and professor of geosciences at Princeton University, said the research by Dr. Landman's group "shows the complexity of this extinction event and the difficulty explaining it by the currently popular impact theory."....

Archive of Past Articles for Chapter 9

10. The Ice Ages

Archive of Past Articles for Chapter 10

2009 May 7. Rise Of Oxygen Caused Earth's Earliest Ice Age. ScienceDaily. Excerpt: Geologists may have uncovered the answer to an age-old question - an ice-age-old question, that is. It appears that Earth's earliest ice ages may have been due to the rise of oxygen in Earth's atmosphere, which consumed atmospheric greenhouse gases and chilled the earth.
Alan J. Kaufman, professor of geology at the University of Maryland, Maryland geology colleague James Farquhar, and a team of scientists from Germany, South Africa, Canada, and the U.S.A., uncovered evidence that the oxygenation of Earth's atmosphere - generally known as the Great Oxygenation Event - coincided with the first widespread ice age on the planet.
"We can now put our hands on the rock library that preserves evidence of irreversible atmospheric change," said Kaufman. "This singular event had a profound effect on the climate, and also on life."
Using sulfur isotopes to determine the oxygen content of ~2.3 billion year-old rocks in the Transvaal Supergroup in South Africa, they found evidence of a sudden increase in atmospheric oxygen that broadly coincided with physical evidence of glacial debris, and geochemical evidence of a new world-order for the carbon cycle.
...The result of the Great Oxidation Event, according to Kaufman and his colleagues, was a complete transformation of Earth's atmosphere, of its climate, and of the life that populated its surface....

2007 March 23. 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.

2006 June 8. 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

2010 March 1. Human Culture, an Evolutionary Force. By Nicolas Wade, NY Times. Excerpt: As with any other species, human populations are shaped by the usual forces of natural selection, like famine, disease or climate. A new force is now coming into focus. It is one with a surprising implication — that for the last 20,000 years or so, people have inadvertently been shaping their own evolution.
...Although it does shield people from other forces, culture itself seems to be a powerful force of natural selection. People adapt genetically to sustained cultural changes, like new diets. And this interaction works more quickly than other selective forces, “leading some practitioners to argue that gene-culture co-evolution could be the dominant mode of human evolution,” Kevin N. Laland and colleagues wrote in the February issue of Nature Reviews Genetics. Dr. Laland is an evolutionary biologist at the University of St. Andrews in Scotland....

2009 October 4. Ardipithecus: We Meet At Last. By Carl Zimmer, Discover. Excerpt: Meet Ardipithecus.
This introduction has been a long time coming. Some 4.4 million years ago, a hominid now known as Ardipithecus ramidus lived in what were then forests in Ethiopia. Fifteen years ago, Tim White of Berkeley and a team of Ethiopian and American scientists published the first account of Ardipithecus, which they had just discovered. But it was just a preliminary report, and White promised more details later, once he and his colleagues had carefully prepared and analyzed all the fossils they had unearthed. “Later,” it turned out, meant 15 years.
...Today, the journal Science has handed many of its pages over to White and his colleagues, who have filled them with lots of details about Ardipithecus.... Ardipithecus has gone from being an enigmatic collection of bones to a new touchstone for our early hominid ancestors....
C. Owen Lovejoy of Kent State University spearheaded the studies on how Ardipithecus moved. He and his colleagues argue that its pelvis could support its upper body during bipedal walking. It wasn’t a fabulous walker, and was probably a terrible runner. Nevertheless, it had some of the same anchors for muscles that we have on our pelvis, and which chimpanzees and other apes lack....
...Ardipithecus’s feet...were adapted for walking on the ground. Yet the big toe was still opposable, much like our thumbs. This sort of big toe helped Ardipithecus move through the trees much more adeptly.... Ardipithecus probably moved carefully through the trees, using its hands and feet all at once to grip branches....

2008 November 27. Did Neanderthal cells cook as the climate warmed? By Ewen Callaway, New Scientist. Excerpt: Neanderthals may have gone extinct because their cells couldn't cope with climate change, according to a new hypothesis...
Metabolic adaptations to Ice Age Europe may have proved costly to Neanderthals after the continent's climate started to change, says Patrick Chinnery, a molecular biologist at Newcastle University, UK.
He and colleague Gavin Hudson identified potentially harmful mutations in the newly sequenced Neanderthal mitochondrial genome. In particular, the researchers found genes that are associated with neurodegenerative diseases and deafness. "If they were found in modern humans they would be bad news," Chinnery says.
The extinction of Neanderthals, close relatives of modern humans, some 25,000 years ago remains unexplained.
...Chinnery and Hudson suggest that mutations in mitochondria helped Neanderthals cope with the cold weather, but that when the climate started fluctuating between warm and cold periods, they were at a disadvantage.
In all cells, from yeast to human, a mitochondrion's main job is to produce the energy that powers cells... Our mitochondria do this quite efficiently under ideal conditions...
Mutations that sap this efficiency would generate heat instead - a potentially useful trick for Neanderthals who are known to have had adaptations to cold weather, Chinnery says. However, a warmer and less climatically stable habitat could have spelled trouble for Neanderthals with such mutations....

2008 August 14. Graves Found From Sahara’s Green Period. By JOHN NOBLE WILFORD, The New York Times. Excerpt: When Paul C. Sereno went hunting for dinosaur bones in the Sahara, his career took a sharp turn from paleontology to archaeology. The expedition found what has proved to be the largest known graveyard of Stone Age people who lived there when the desert was green.
The first traces of pottery, stone tools and human skeletons were discovered eight years ago at a site in the southern Sahara, in Niger. After preliminary research, Dr. Sereno, a University of Chicago scientist who had previously uncovered remains of the dinosaur Nigersaurus there, organized an international team of archaeologists to investigate what had been a lakeside hunting and fishing settlement for the better part of 5,000 years, originating some 10,000 years ago.
...the team described finding about 200 graves belonging to two successive populations. Some burials were accompanied by pottery and ivory ornaments....
...The sun-baked dunes at the site, known as Gobero, preserve the earliest and largest Stone Age cemetery in the Sahara, Dr. Sereno’s group reported...
Other scientists said the discovery appeared to provide spectacular evidence that nothing, not even the arid expanse of the Sahara, was changeless. About 100 million years ago, this land was forested and occupied by dinosaurs and enormous crocodiles. Around 50,000 years ago, people moved in and left stone tools and mounds of shells, fish bones and other refuse. The lakes dried up in the last Ice Age.
Then the rains and lakes of a fecund Sahara returned about 12,000 years ago, and remained, except for one 1,000-year interval, until about 4,500 years ago. Geologists have long known that the region’s basins retained mineral residue of former lakes, and other explorers have found scatterings of human artifacts from that time, as Dr. Sereno did at Gobero in 2000.
“Everywhere you turned, there were bones belonging to animals that don’t live in the desert,” he said. “I realized we were in the green Sahara.”...

2008 May 9. How the Sahara Became Dry & Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. Jonathan A. Holmes. Science 9 May 2008: Vol. 320. no. 5877, pp. 752 - 753 DOI: 10.1126/science.1158105. Excerpt: Around 14,800 years ago, a strengthening of the summer monsoons led to a dramatic increase in North African lakes and wetlands and an extension of grassland and shrubland into areas that are now desert, creating a "green Sahara" (see the first figure). ...a lake sediment record ... sheds light on how this "African Humid Period" came to an end.

2008 May 9. Climate-Driven Ecosystem Succession in the Sahara: The Past 6000 Years. S. Kropelin, et al. Science: Vol. 320. no. 5877, pp. 765 - 768 DOI: 10.1126/science.1154913. Excerpt: Desiccation of the Sahara since the middle Holocene has eradicated all but a few natural archives recording its transition from a "green Sahara" to the present hyperarid desert. Our continuous 6000-year paleoenvironmental reconstruction from northern Chad shows progressive drying of the regional terrestrial ecosystem in response to weakening insolation forcing of the African monsoon and abrupt hydrological change in the local aquatic ecosystem controlled by site-specific thresholds. Strong reductions in tropical trees and then Sahelian grassland cover allowed large-scale dust mobilization from 4300 calendar years before the present (cal yr B.P.). Today's desert ecosystem and regional wind regime were established around 2700 cal yr B.P. This gradual rather than abrupt termination of the African Humid Period in the eastern Sahara suggests a relatively weak biogeophysical feedback on climate. ...One of the most prominent environmental changes of the past 10,000 years is the transition of northern Africa from a "green Sahara" during the early Holocene "African Humid Period" to the world'slargest warm desert today. Detailed knowledge of the tempo and mode of this transition is crucial for understanding the interaction between tropical and mid-latitude weather systems and the multiple impacts of mineral aerosols exported from the Sahara on global climate and distant ecosystems....

Archive of Past Articles for Chapter 11

12. Climate and Culture

2009 November 2. In the Mediterranean, Killer Tsunamis From an Ancient Eruption. By William J. Broad, The NY Times. Excerpt: The massive eruption of the Thera volcano in the Aegean Sea more than 3,000 years ago produced killer waves that raced across hundreds of miles of the Eastern Mediterranean to inundate the area that is now Israel and probably other coastal sites, a team of scientists has found.
The team, writing in the October issue of Geology, said the new evidence suggested that giant tsunamis from the catastrophic eruption hit “coastal sites across the Eastern Mediterranean littoral.” Tsunamis are giant waves that can crash into shore, rearrange the seabed, inundate vast areas of land and carry terrestrial material out to sea.
The region at the time was home to rising civilizations in Crete, Cyprus, Egypt, Phoenicia and Turkey.
For decades, scholars have suggested that the giant eruption, just 70 miles from Crete, might have brought about the mysterious collapse of Minoan civilization at the peak of its glory....

2009 July 24. An Amazon Culture Withers as Food Dries Up. By Elisabeth Rosenthal, The NY Times. Excerpt: XINGU NATIONAL PARK, Brazil — As the naked, painted young men of the Kamayurá tribe prepare for the ritualized war games of a festival, they end their haunting fireside chant with a blowing sound — “whoosh, whoosh” — a symbolic attempt to eliminate the scent of fish so they will not be detected by enemies. For centuries, fish from jungle lakes and rivers have been a staple of the Kamayurá diet, the tribe’s primary source of protein.
But fish smells are not a problem for the warriors anymore. Deforestation and, some scientists contend, global climate change are making the Amazon region drier and hotter, decimating fish stocks in this area and imperiling the Kamayurá’s very existence. Like other small indigenous cultures around the world with little money or capacity to move, they are struggling to adapt to the changes.
...The Intergovernmental Panel on Climate Change says that up to 30 percent of animals and plants face an increased risk of extinction if global temperatures rise 2 degrees Celsius (3.6 degrees Fahrenheit) in coming decades. But anthropologists also fear a wave of cultural extinction for dozens of small indigenous groups — the loss of their traditions, their arts, their languages....

2008 November 7. Rise and Fall of Chinese Dynasties Tied to Changes in Rainfall. By David Biello, Scientific American. Excerpt: In the late ninth century a disastrous harvest precipitated by drought brought famine to China under the rule of the Tang dynasty. By A.D. 907—after nearly three centuries of rule—the dynasty fell when its emperor, Ai, was deposed, and the empire was divided. According to the atmospheric record contained in a stalagmite, one of the causes of that downfall may have been climate change.
"We think that climate played an important role in Chinese history," says paleoclimatologist Hai Cheng of the University of Minnesota, a member of the scientific team that harvested and analyzed the stalagmite from Wanxiang Cave in Gansu Province in northwest China. The stalagmite reveals, for example, that the vital rains of the Asian monsoon weakened at the time of the downfalls of the Tang, Yuan and Ming dynasties over the past 1,810 years.
...Composed of calcium carbonate leached from dripping water, the 4.6-inch- (11.7-centimeter-) long stalagmite preserves a record of rainfall in this region, which is on the edge of the area impacted by the Asian monsoon. The region gets less rainfall when the monsoon is mild and more when it is strong...
These periods of strong and weak rains, when compared with Chinese historical records, coincide with periods of imperial turmoil or prosperity....
In fact, the collapse of the Tang Dynasty coincides with that of the Mayan civilization—both due to extreme drought. "We have demonstrated that the cave record correlates well with many other records, including the Little Ice Age in Europe, temperature changes [across the] Northern Hemisphere, and major solar variability," Cheng notes....

2008 August 31. For the first time in human history, the North Pole can be circumnavigated. By Geoffrey Lean, The Independent. Excerpt: Open water now stretches all the way round the Arctic, making it possible for the first time in human history to circumnavigate the North Pole... New satellite images, taken only two days ago, show that melting ice last week opened up both the fabled North-west and North-east passages, in the most important geographical landmark to date to signal the unexpectedly rapid progress of global warming.
Last night Professor Mark Serreze, a sea ice specialist at the official US National Snow and Ice Data Center (NSIDC), hailed the publication of the images...as "a historic event", and said that it provided further evidence that the Arctic icecap may now have entered a "death spiral". Some scientists predict that it could vanish altogether in summer within five years, a process that would, in itself, greatly accelerate.
...scientists...have long regarded the disappearance of the icecap as inevitable as global warming takes hold, though until recently it was not expected until around 2070.
Many scientists now predict that the Arctic ocean will be ice-free in summer by 2030 – and a landmark study this year by Professor Wieslaw Maslowski at the Naval Postgraduate School in Monterey, California, concluded that there will be no ice between mid-July and mid-September as early as 2013....

Summer 2007. Forest Magazine. Thirsting for Water. By Allen Best. Excerpt: ...The dust traveled far, even to New York City. In Kansas, Oklahoma, Texas and Colorado, where the Dust Bowl was most severe, the roiling clouds were deadly. The young and old, even the formerly robust, succumbed to pneumonia. The luckier ones, the quitters, abandoned the dryland farms ... and migrated westward, ....
Several decades of wet weather had supported the widespread plowing of grasslands in a semi-arid climate. Then came drought, lasting the better part of the decade. In all, about a third of a million people left the Great Plains. It was, until Hurricane Katrina, the greatest population displacement in the United States caused by an environmental event.
The Dust Bowl, say climatologists, is unlikely to occur again.
Farmers and government scientists learned much from the experience about how to farm the land-and where not to. But drought most certainly will return, perhaps even more harshly. And turning to the American Southwest, ...experts say new evidence reveals a clearer picture of extended and sometimes severe droughts in the past 1,100 years that very well may reappear-this time with an overlay of hotter temperatures caused by increased levels of greenhouse gases. What effect these human-caused emissions will have on precipitation is still uncertain. On the matter of temperature, however, nearly all the computer models reach one conclusion: It will get hotter, much hotter, in places like Tucson, Colorado Springs and Reno. And hotter-even if precipitation stays the same-means drier. In other words, the "average" of the future will resemble what in the past we called drought.
...WHAT THE TREES SAY
...Climates of the past can be documented in various ways, but one of the most important methods is by studying tree rings, a scientific discipline called dendrochronology. ...
What these tree rings say is that the Southwest was far more arid in the past. ... A period from 800 to 1300 A.D. was generally more arid and punctuated by what paleoclimatologists call megadroughts. Some lasted thirty years. Archaeologists think that one of the final megadroughts, from about 1270 to 1300, may have partly caused the Ancestral Pueblo (also called the Anasazi) to vacate their cliff-dwelling communities at Mesa Verde in Colorado and Chaco Canyon in Arizona.....

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

Archive of Past Articles for Chapter 13

2009 June 19. Sudden Collapse In Ancient Biodiversity: Was Global Warming The Culprit? ScienceDaily. Excerpt: Scientists have unearthed striking evidence for a sudden ancient collapse in plant biodiversity. A trove of 200 million-year-old fossil leaves collected in East Greenland tells the story, carrying its message across time to us today.
...The researchers were surprised to find that a likely candidate responsible for the loss of plant life was a small rise in the greenhouse gas carbon dioxide, which caused Earth's temperature to rise.
Global warming has long been considered as the culprit for extinctions--the surprise is that much less carbon dioxide gas in the atmosphere may be needed to drive an ecosystem beyond its tipping point than previously thought.
...Until this research, the pace of the extinctions was thought to have been gradual, taking place over millions of years.
It has been notoriously difficult to tease out details about the pace of extinction using fossils, scientists say, because fossils can provide only snap-shots or glimpses of organisms that once lived.
Using a technique developed by scientist Peter Wagner of the Smithsonian Institution National Museum of Natural History in Washington, D.C., the researchers were able to detect, for the first time, very early signs that these ancient ecosystems were already deteriorating--before plants started going extinct.
The method reveals early warning signs that an ecosystem is in trouble in terms of extinction risk.
...By the year 2100, it's expected that the level of carbon dioxide in the modern atmosphere may reach as high as two and a half times today's level.
"This is of course a 'worst case scenario,'" says Jennifer McElwain of University College Dublin, the paper's lead author. "But it's at exactly this level [900 parts per million] at which we detected the ancient biodiversity crash.
"We must take heed of the early warning signs of deterioration in modern ecosystems. We've learned from the past that high levels of species extinctions--as high as 80 percent--can occur very suddenly, but they are preceded by long interval of ecological change."...

2006 February. 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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