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Earth Science and Evolution Study Guide (page 2)

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Updated on Sep 26, 2011

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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