Earth Science and Evolution Study Guide (page 2)
Earth today is the product of 4.5 billion years of changes. Life is an important player in connecting the earth's surface reservoirs of atmosphere, ocean, and soil. Here we look at questions that surround the history of the oceans, continents, and, in particular, the atmosphere. Did life play a role in the story of the air? Geology certainly played a role in the history of life, especially so when we consider mass extinctions such as the demise of the dinosaurs 65 million years ago.
History of the Continents, Oceans, and Atmosphere
Earth formed about 4.5 billion years ago. At that time, the planet was hot, too hot for liquid water, but at some point, Earth was cool enough to stabilize water, presumably in the form of oceans. Direct evidence for those oceans vanished long ago, but geologists search for indirect evidence in the form of sedimentary rock, which must form under water.
When were the first sedimentary rocks formed? Rocks on Earth are always being destroyed (and newly created) by the processes of plate tectonics. So as we look back at earlier and earlier rocks, we find fewer and fewer of them. However, most geologists would say that adequate evidence exists for liquid water by 4 billion years ago and probably earlier. Thus, Earth has had an ocean for at least 4 billion years.
What about the continents? The earliest continents are thought to have been something like the country of Iceland today. Iceland sits along the volcanic midocean ridge of the Atlantic Ocean. In fact, Iceland can be thought of as part of the mid-Atlantic ridge that simply sticks up above water. Iceland, despite its chilly latitude, is full of volcanic activity and steam vents everywhere in the country. Most of the country's energy is hydrothermal, meaning from hot water, supplied by the volcanic magic of plate tectonics.
Thus, the earliest continents were probably small. Earth's more appropriate name of "ocean" was even more true way back then, but continents, as noted earlier, are made of lighter material and tend to float on the heavier asthenosphere. Continental crust is even lighter than the oceanic crust. Continents, once formed, will stay. They might split and merge, driven by the dynamics of plate tectonics, but the continental crust rarely goes very far back down into the deep earth before melting and floating back to the surface. This means that continents have grown over time, and if the continents grew, the total area of the ocean must have shrunk.
Most continents' growth occurred during the first quarter or so of Earth history, when the interior of the earth was hotter and the dynamics of plate tectonics were more active. Geologists today are still debating the exact history of the continents, but all agree that the total area of all the continents probably began much smaller than the area today.
Probably the best understood of the histories (and most interesting for our purposes) is that of the atmosphere. The chemicals of the atmosphere react with the minerals of the soil and ocean sediments. By deciphering the chemical reactions that must have taken place long ago to create certain chemical signatures in sedimentary rocks, geologists and geochemists have been able to make some fairly definite statements about the history of Earth's atmosphere. How did the atmosphere come to be what it is today?
Evidence shows that the early earth's atmosphere contained virtually no oxygen. If you were to step out of a time machine on to an ancient shoreline, you would choke and die in a few breaths. Miniscule amounts of free oxygen were made by cosmic rays that could split molecules of water vapor in the atmosphere into hydrogen and oxygen, but these amounts were millions of times smaller than today's amount of oxygen. Recall that oxygen gas, O2, is the second most abundant gas in the atmosphere, at about 21%.
Some sedimentary rocks are known to contain layers that are ancient fossil soils. Once real soils, these zones were covered and protected by water and overlying sediments and were eventually squeezed into sedimentary rock. By analyzing the chemicals in these fossil soils, it has been discovered that a great change happened to Earth's atmosphere about 2 billion years ago, roughly at the halfway point in Earth's total history.
Two billion years ago, a dramatic increase in the level of atmospheric oxygen took place. This step did not create today's 21% oxygen level, but it did create perhaps 2% oxygen. This was a major change in the chemistry of the atmosphere and a change for the organisms that lived in the biosphere. Then, some where in the timeframe between 1 billion and 600 million years ago, a second rise in oxygen brought the atmosphere's oxygen level to a value close to that of today's oxygen level.
What could have caused these rises in oxygen? Oxygen is made as a waste product of photosynthesis. Plants get rid of the oxygen they make, just as we humans get rid of the waste carbon dioxide our cells make. So Earth's oxygen must come from photosynthesizers. At 2 billion years ago, no plants existed on land. So the oxygen for the rise at that time must have been generated by photosynthesizers in the ocean—by phytoplankton.
But remember: Other creatures in the ocean use up oxygen, just like we do, just like cats and dogs and butterflies and frogs do. In other words, just like all respiring organisms do. In particular, the bacteria in the deep ocean that feed upon and thus recycle the elements in dead organic detritus require oxygen for their recycling. So if the phytoplankton make oxygen but the bacteria consume the oxygen, then presumably there should be no oxygen left to accumulate in the atmosphere.
The only way for oxygen to accumulate in the atmosphere is if more oxygen is produced by photosynthesizers than is consumed by the oxygen users such as bacteria. In addition, certain gases given off by volcanoes (such as sulfur gases) also combine with oxygen in the atmosphere; these chemical reactions are essentially consuming oxygen as well. Therefore, for oxygen to accumulate in the atmosphere, more oxygen had to be generated by photosynthesis than consumed by the sum of oxygen-consuming organisms and natural chemical reactions.
We can conclude from this analysis that the rise in oxygen in Earth's atmosphere did not have to coincide with the beginning of photosynthesis. Indeed, most scientists who study the history of life think that photosynthesis began at least a billion or more years before the first great rise in oxygen. What happened 2 billion years ago is not yet certain. But the rise in oxygen could have been driven by an increase in the amount of photosynthesis and thus burial of organic carbon.
If the amount of photosynthesis (which creates oxygen) and the amount of respiration (which consumes oxygen) are equal, then the organic matter created by photosynthesis is all consumed by organisms that feed on the organic matter and derive their energy by performing respiration. But if some of the organic matter slips through the biosphere's recycling systems, then there is excess oxygen created that is not consumed by organisms feeding on the organic matter. Perhaps at 2 billion year ago, more organic matter started being buried, which meant more free oxygen entered the atmosphere. Whatever the answer, we do know that Earth's atmosphere has undergone a dramatic change in its level of oxygen, and furthermore, that this change involved life. Life has been a geologial force on the chemistry of the atmosphere.
Life's involvement with the carbon cycle means that life could also exert an influence on another important gas in the atmosphere, carbon dioxide (CO2). Living things, of course, create organic molecules made of carbon. When these molecules slip through the biosphere's recycling systems, as already noted, carbon is lost from the biosphere. It had been shown that 0.5 billion tons per year of carbon is actually lost from the biosphere by burial in the ocean's sediments. This amount is made up by the release of new carbon from volcanoes and the chemical dissolution of rocks.
Most of the burial of the 0.5 billion tons of carbon per year is, however, not in the form of organic matter. It is calcium carbonate, the shells of marine organisms, which eventually become carbonate sedimentary rock. The amount of the calcium carbonate that is buried is related to the atmosphere's level of carbon dioxide and to the activities of organisms in the soil, which can increase the amount of chemical weathering of soil minerals.
When earth scientists put together the story of how these influences determine the level of atmospheric CO2, it appears that the CO2 level has been declining gradually over long geological time periods. (Note: This is just the opposite from the rapid rise in CO2 that is happening today.) Apparently, over billions of years, the greenhouse effect of carbon dioxide has been decreasing. This is interesting for another reason—because at the beginning of the earth, the sun was weaker by 30%. A weaker sun meant less solar energy hitting the earth. Were a cosmic dial-turner able to turn down the sun by 30% today, the biosphere would become a solid ball of ice because the oceans would freeze. So a larger amount of carbon dioxide in the atmosphere several billion years ago is probably the reason that liquid water existed back then.
Evolution and Impacts from Space
Organisms not only affect the earth; the earth affects organisms. As continents collide and separate, as sea level rises and falls, as mountains uplift and erode, habitats for organisms are changed. In this section, we first review how evolution works. Then we will concentrate on one of the major findings in earth science over the last 20 years. This finding exemplifies how geological processes can affect life. The focus will be on the mass extinction of the dinosaurs due to an impact from space.
No one knows exactly how life began, but good evidence for the presence of life 3.5 billion years ago exists in the records offered to us by rock. Then, once life started, it was modified over the eons by the process of evolution.
We can consider evolution as a recipe for change. Its steps, which occur in a repeating sequence, are inheritance, variation, and selection.
The first step is inheritance. Organisms in each generation share many of the same features of their predecessors, because the genetic code of DNA is copied from parent to offspring.
Next, we consider variation. Often, offspring are not exactly like their parents. Variation is key because it serves as the raw material that can be molded by evolution into new types of creatures.
Finally comes the all-important step of selection (or natural selection). Not all offspring live long enough themselves to put forth the next generation. Statistically, the most fit survive: Those survive that can withstand draught, seek out food most efficiently, or run the swiftest. The filtering process selects certain types of creatures to carry on. In summary, evolution is modification by natural selection.
The process repeats: inheritance, variation, selection. It operates over and over, as generations roll along, and it has been doing so for nearly 4 billion years. The scientist and master writer of evolution, Englishman Richard Dawkins, coined the phrase the blind watchmaker. Evolution creates wondrous organisms, even though there is no maker, because the process is "blind"; it doesn't know where it is going. The recipe of inheritance-variation-selection is a creative process that has generated new forms of life, taking life from its earliest simple start as bacteria to today's giant redwood trees and 10 million total species, including us.
Here are a few of the major transitions, with dates, taken by the process of evolution on Earth:
- 3.5 billion years ago. Single-celled, bacteria-type creatures definitely exist.
- 2 billion years ago. Evolution of complex cells from simpler, bacteria-type cells began. The complex cells have enclosed nuclei for their DNA and are the kinds of cells that eventually led to fungi, plants, and animals.
- 1 billion years ago. First evidence for multicelled creatures, such as worms.
- 540 million years ago. The so-called Cambrian Explosion took place. Over a brief period of about 10 million years, quite suddenly all kinds of marine animals with hard parts (which is why they were preserved) explode into the fossil record.
- 350 million years ago. Evolution of land plants took place. The fossil record shows that plants evolved from tiny, moss-sized beings into tall trees over a period that was only about 20 million years long.
- 220 million years ago. First dinosaurs diverged from early reptiles. A key invention was a new kind of hip joint. This allowed many early (and late) dinosaurs to run bipedally.
- 200 million years ago. The first mammals evolved from previous mammal-like reptiles, which had split off as a branch of reptiles about 260 million years ago.
In the fossil record, the last of the dinosaurs are found in rocks formed at about 65 million years ago. From these dates, you can see that dinosaurs lived for more than 200 million years. This did not mean that all species of dinosaurs were present for that entire time. No. Some species of dinosaurs went extinct and new species evolved. Tyrannosaurus Rex, for example, was a relatively late species in the evolution of dinosaurs and was around when the dinosaurs went extinct at 65 million years ago. Paleontologists have long known that something dramatic must have occurred 65 million years ago. But it took discoveries from geology, about 20 years ago, to determine the cause of the mass extinction. The answer has given new understanding to what factors contributed to the story of life.
Pinhead-size particles enter Earth's atmosphere every night and burn up—these are shooting stars. Larger objects can make it through the atmosphere and hit the ground as meteorites. Occasionally, the earth is struck by quite large rocks from space. For example, in the United States, a meteor crater can be seen in northern Arizona, evidence of an impact within the time of human beings. A very much larger event about 2 billion years ago created the Sudbury crater in Canada. The longer the time period, the greater the chance for a truly devastating impact to hit the earth.
We see evidence of enormous impacts on the moon and Mars. These planetary bodies show their craters because they have no or little geological change. On Earth, as wind and water shift sediments, as continents merge and split, most ancient craters have been buried or completely erased from the face of Earth.
In the 1980s, an unusually large amount of a rare element called iridium (chemical symbol: Ir) was discovered in a centimeter-thick clay layer in rocks in Italy, dating from the time of the dinosaur extinction. This anomaly of iridium was subsequently found all over the world.
Iridium occurs at such high concentrations only in meteorites. This discovery pointed to a large impactor (a comet or asteroid) as the cause of the iridium and the mass extinction. Such an object would have smashed into the earth at a speed of 20 kilometers per second and is estimated to have been about the size of Manhattan (say 10 kilometers, or 6 miles, in diameter).
A few years later, evidence from gravity patterns revealed a large crater buried under sediments in the Yucatan Peninsula of Mexico. Variations in gravity—tiny changes in Earth's gravitational field—exist because the rocks at Earth's surface can vary enough in density to be measured using sensitive equipment. Oil companies are interested in such gravity patterns because the presence of a reduced gravitational field can be evidence for a low-density reservoir of oil, which would otherwise be invisible beneath the surface. A circular pattern of variation in gravity was discovered in the Yucatan from data taken by a Mexican oil company.
The crater, buried under sediments in the Yucatan, is about 200 kilometers in diameter (about the estimated size of the crater made by a 10 kilometer object). It dates to exactly 65 million years ago, the end of what geologists call the Cretaceous (K) period and the beginning of the Tertiary (T) period. A wealth of other types of evidence for this K-T impact has been found, including material ejected close to the impact, shocked minerals, as well as chemical evidence for worldwide fires and other environmental disruptions.
At the K-T boundary, 65 million years ago, many other types of life also went extinct, on all scales, all the way down to the plankton. One group of creatures survived that had been alive at the time of the K-T extinction and were directly descended from the dinosaurs. These are birds. And, fortunately for us, mammals survived, too. This probably happened because the mammals back then were only the size of rats and could weather out the catastrophe underground in burrows.
Species are always going extinct. But once in a while, a mass extinction happened; we know this from the fossil record. To explain these extinctions, in some cases, scientists invoke climate change as the culprit. Others suspect that large impacts will be discovered as the general cause.
Though the stories of individual mass extinctions are still being assembled from field data, the discovery of the K-T impact and the mass extinction of the dinosaurs have given us new insight into how precarious life on Earth has been and how evolution has been subjected to random shocks from space. What if the impact had been larger? And what if it had not taken place?
Note from the previous numbers, that mammals lived during the time of the dinosaurs. But before the K-T mass extinction, the fossil record shows that mammals had remained small for over 100 million years. In the millions of years after the demise of the dinosaurs, mammals evolved into a huge variety of species, some of them as big as hippopotamuses and elephants. In the term of evolutionary biology, the mammals radiated. It is virtually certain that without such an extinction, this radiation would not have occurred. Without the impactor from space 65 million years ago, evolution would have taken a different course. We almost certainly would not be here.
Practice problems for this concept can be found at: Earth Science and Evolution Practice Questions
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