Chapter 17 The History of Life

Transcription

1 Chapter 17 The History of Life The fossil record provides evidence about the history of life on Earth. It also shows how different groups of organisms, including species, have changed over time. Paleontologists determine the age of fossils using two techniques: relative dating and radioactive dating. relative dating the age of a fossil is determined by comparing its placement with that of fossils in other layers of rock. The rock layers form in order by age the oldest layers on the bottom, with more recent layers on top. index fossils a species must have had a wide geographic range. It will be found in only a few layers of rock, but these specific layers will be found in different geographic locations. Radioactive Dating Scientists use radioactive decay to assign absolute ages to rocks. Radioactive elements decay, or break down, into nonradioactive elements at a steady rate, which is measured in a unit called a halflife. A halflife is the length of time required for half of the radioactive atoms in a sample to decay. In radioactive dating, scientists calculate the age of a sample based on the amount of remaining radioactive isotopes it contains. Carbon14, has a halflife of about 5730 years. Carbon14 is taken up by living things while they are alive. After an organism dies, the carbon14 in its body begins to decay to form nitrogen14 Carbon12, the most common isotope of carbon, is not radioactive and does not decay. By comparing the amounts of carbon14 and carbon12 in a fossil, researchers can determine when the organism lived. The more carbon12 there is in a sample compared to carbon14, the older the sample is. Geologic Time Scale Paleontologists use divisions of the geologic time scale to represent evolutionary time. The table below shows the most recent version of the geologic time scale. 1

2 2 These times were used to mark where one segment of geologic time ends and the next begins long before anyone knew how long these various segments actually were.

3 Geologists divide the time between the Precambrian and the present into three eras. They are the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era Eras are subdivided into periods, which range in length from tens of millions of years to less than two million years Formation of Earth Earth is about 4.5 billion years old pieces of cosmic debris were probably attracted to one another over the course of about 100 million years The most dense elements formed the planet's core. There, radioactive decay generated enough heat to convert Earth's interior into molten rock. Moderately dense elements floated to the surface These elements ultimately cooled to form a solid crust. The least dense elements including hydrogen and nitrogen formed the first atmosphere. Ideas About the origin of Life some scientists question whether life originated on Earth They suggest life on Earth had an extraterrestrial origin They hypothesize that life was carried here by an asteroid or by a meteorite Organic molecules make up about 2% by weight of some meteorites A. Divine Creation many cultures believe that life was put on Earth by divine (relating to a god or gods) forces the divine creation cannot be explained by science because a it can not be tested using a scientific hypothesis Because the idea that life originated through divine creation cannot be tested by scientific methods, it falls outside the realm of science. This is not to say that the belief is wrong, but rather that science can never test it. B. Spontaneous Origin Most scientists think that life on Earth had a spontaneous origin, developing by itself through natural chemical and physical processes They hypothesize that molecules of nonliving matter reacted chemically during the first 1 billion years of Earth's history, forming a variety of simple organic molecules Changes that increased the stability of certain molecules would have allowed (selected) those molecules to persist for a longer time Molecules that could be replicated would have become more common than those that could not be replicated 3

4 The Primordial Soup Model In the 1920s, the Russian scientist A. I. Oparin proposed a hypothesis that Earth's oceans were once a vast primordial soup containing large amounts of organic molecules Oparin envisioned these molecules forming spontaneously in chemical reactions activated by energy from solar radiation, volcanic eruptions, and lightning Oparin thought that over millions of years, these molecules had gradually come together to form living matter It was proposed that Earth's early atmosphere lacked oxygen the early atmosphere was instead rich in nitrogen (N2) and hydrogencontaining gases such as hydrogen (H2), water vapor (H2O), methane (CH4), and ammonia (NH3) Electrons in these gases would have been frequently pushed to higher energy levels by photons crashing into them from the sun or by electrical energy in lightning In 1953, Oparin's hypothesis was tested by Stanley Miller Miller placed the proposed gases into an apparatus and then to simulate lighting, he zapped the mixture with electrical sparks After a few days, Miller found a complex mixture that included some of life's basic building blocks: amino acids, fatty acids, and hydrocarbons These results demonstrated that some basic chemicals of life could have formed spontaneously on the early Earth under conditions like those in the experiment 4

5 Recently scientists have reevaluated Oparin's primordial soup model discoveries of 3.5billion yearold fossils indicate that the time for life to begin was much shorter than previously thought Another problem with the primordial soup model is the Earth had no free oxygen 4 billion years ago and would not have had a protective layer of ozone gas, 03. Scientists think that without an ozone layer, ultraviolet light would have destroyed any ammonia and methane present in the atmosphere When these gases are missing from experiments similar to Miller's, molecules such as amino acids are not produced The Bubble Model In 1986, the geophysicist Louis Lerman suggested that bubbles produced by wind, wave action, the impact of raindrops, and the eruption of volcanoes cover about 5 percent of the ocean's surface at any given time because water molecules are polar, water bubbles tend to attract other polar molecules chemical reactions would proceed much faster in bubbles (where reactants would be concentrated) than in Oparin's stagnant primordial soup Thus, life could have originated in a much shorter period of time than it could have according to Oparin's model Also, inside the water bubbles the methane and ammonia required to produce amino acids would have been protected from destruction by ultraviolet light. 5

6 The Puzzle of Life's Origin organic molecules is a long way from a living cell, and the leap from nonlife to life is the greatest gap in scientific hypotheses of Earth's early history. Geological evidence suggests that about 200 to 300 million years after Earth cooled enough to carry liquid water, cells similar to modern bacteria were common. How might these cells have originated? Formation of Microspheres Under certain conditions, large organic molecules can form tiny bubbles called proteinoid microspheres. Microspheres are not cells, but they have some characteristics of living systems. the basic molecules of plasma membranesproteins and lipidstend to aggregate (gather together) in water Phospholipids, which form the bilayer of a plasma membrane and short chains of amino acids produced aggregate into tiny vesicles called microspheres Scientists think that microspheres might have been the first step toward cellular organization Once the basic molecules were present the early oceans would have contained untold numbers of microspheresbillions in each spoonful of sea water Over millions of years, those micro spheres that could survive longer by more efficiently incorporating molecules and energy would have become more common Like cells, they have selectively permeable membranes through which water molecules can pass. Microspheres also have a simple means of storing and releasing energy. Several hypotheses suggest that structures similar to proteinoid microspheres might have acquired more and more characteristics of living cells. Still, microspheres could not be considered alive unless they had acquired the capacity to transfer their abilities to offspring Evolution of RNA and DNA How did amino acids link together to form proteins and how did nucleotides join to form long chains of DNA short chains of RNA, can (with difficulty) be made to form spontaneously in water Some scientist speculate that early life could have developed on a solid surface rather than in water In the 1980s, Thomas Cech found that certain RNA molecules can act like enzymes RNA's threedimensional structure provides surfaces with specific shapes for catalyzing reactions, much as protein shapes do Like DNA, RNA acts as an informationstoring molecule. Cech's work and experiments demonstrating that perhaps RNA was the first selfreplicating informationstorage molecule. After it had formed, such a molecule could also have catalyzed the assembly of the first proteins But more important, such a molecule would have been capable of evolving through natural selection. From this relatively simple RNAbased form of life, several steps could have led to the system of DNAdirected protein synthesis that exists now Origin of Heredity Remains a Mystery Most researchers suspect that RNA was the first information storing molecule to form and that RNA "enzymes" catalyzed the assembly of the earliest proteins Scientists think that doublestranded DNA probably evolved later But because researchers do not yet understand how DNA, RNA, and hereditary mechanisms first developed, science is currently unable to resolve disputes concerning the origin of life 6

7 How life might have originated naturally and spontaneously remains a subject of intense interest, research, and discussion among scientists Free Oxygen Microscopic fossils, or microfossils, of singlecelled prokaryotic organisms that resemble modern bacteria have been found in rocks more than 3.5 billion years old Those first life forms must have evolved in the absence of oxygen, because Earth's first atmosphere contained very little of that highly reactive gas. Over time, as indicated by fossil evidence, photosynthetic bacteria became common in the shallow seas of the Precambrian. By 2.2 billion years ago at the latest, these organisms were steadily churning out oxygen, an end product of photosynthesis. One of the first things oxygen did was to combine with iron in the oceans. In other words, it caused the oceans to rust! When iron oxide was formed, it fell from the sea water to the ocean floor. There, it formed great bands of iron that are the source of most of the iron ore mined today. Without iron, the oceans changed color from brown to bluegreen. Next, oxygen gas started accumulating in the atmosphere. As atmospheric oxygen concentrations rose, concentrations of methane and hydrogen sulfide began to decrease, the ozone layer began to form, and the skies turned their present shade of blue. Over the course of several hundred million years, oxygen concentrations rose until they reached today's levels. Biologists hypothesize that the increase in this highly reactive gas created the first global pollution crisis. To the first cells, oxygen was a deadly poison! The rise of oxygen in the atmosphere drove some life forms to extinction, while other life forms evolved new, more efficient metabolic pathways that used oxygen for respiration. Organisms that had evolved in an oxygenfree atmosphere were forced into a few airless habitats, where their anaerobic descendants remain today. Some organisms, however, evolved ways of using oxygen for respiration and protecting themselves from oxygen's powerful reactive abilities. The stage was set for the evolution of modern life. Origin of Eukaryotic Cells About 2 billion years ago, prokaryotic cells cells without nuclei began evolving internal cell membranes. The result was the ancestor of all eukaryotic cells. The Endosymbiotic Theory smaller prokaryotes began living inside larger cells Over time, a symbiotic, or interdependent, relationship evolved. According to the endosymbiotic theory, eukaryotic cells formed from a symbiosis among several different prokaryotic organisms. One group of prokaryotes had the ability to use oxygen to generate energyrich molecules of ATP. These evolved into the mitochondria that are now in the cells of all multicellular organisms. Other prokaryotes that carried out photosynthesis evolved into the chloroplasts of plants and algae. The endosymbiotic theory proposes that eukaryotic cells arose from living communities formed by prokaryotic organisms. 7

8 The Evidence 1. mitochondria and chloroplasts contain DNA similar to bacterial DNA. 2. mitochondria and chloroplasts have ribosomes whose size and structure closely resemble those of bacteria. 3. like bacteria, mitochondria and chloroplasts reproduce by binary fission when the cells containing them divide by mitosis. Thus, mitochondria and chloroplasts have many of the features of freeliving bacteria. These similarities provide strong evidence of a common ancestry between freeliving bacteria and the organelles of living eukaryotic cells. 8

9 Chapter 11 Vocabulary 1. Extraterrestrial 2. Meteorite 3. Asteroid 4. Spontaneous Origin 5. Divine Creation 6. Primordial Soup 7. A.I. Oparin 8. Stanley Miller 9. Bubble Model 10. Ozone 11. Microspheres 12. Radiometric dating 13 Radioisotope 14. Halflife 15. Fossil 9