Using a process known as radiometric dating , scientists can determine the age of a rock layer by examining how certain atoms in the rock have changed since the rock formed. As atoms change, they emit different levels of radioactivity. Changes in radioactivity are standard and can be accurately measured in units of time.
By measuring radioactive material in an ancient sample and comparing it to a current sample, scientists can calculate how much time has passed. Radiometric dating allows ages to be assigned to rock layers, which can then be used to determine the ages of fossils.
Paleontologists used radiometric dating to study the fossilized eggshells of Genyornis , an extinct bird from Australia.
They discovered that Genyornis became extinct between 40, and 50, years ago. Fossil evidence from plants and other organisms in the region shows that there was abundant food for the large, flightless bird at the time of its extinction.
Climate changes were too slow to explain the relatively quick extinction. By studying human fossils and ancient Australian cave paintings that were dated to the same time period, paleontologists hypothesized that human beings—the earliest people to inhabit Australia—may have contributed to the extinction of Genyornis. Paleontology Today Modern paleontologists have a variety of tools that help them discover, examine, and describe fossils.
Electron microscopes allow paleontologists to study the tiniest details of the smallest fossils. X-ray machines and CT scanners reveal fossils' internal structures. Advanced computer programs can analyze fossil data , reconstruct skeletons, and visualize the bodies and movements of extinct organisms. Paleontologists and biologists used a CT scan to study the preserved body of a baby mammoth discovered in Siberia in A CT scanner allows scientists to construct 3-D representations of the bones and tissue of the organism.
Using this technology , scientists were able to see that the baby mammoth had healthy teeth, bones, and muscle tissue. This suggested to scientists that the animal was healthy, but most likely suffocated in a muddy river or lake. Scientists can even extract genetic material from bones and tissues. Paleontologists made a remarkable genetic discovery when the bones of a Tyrannosaurus rex were broken during an excavation in the s. Soft tissue was discovered inside the bones. Soft tissue is the actual connective tissue of an organism, such as muscle, fat, and blood.
Soft tissue is rarely preserved during fossilization. Paleontologists usually must rely on fossilized remains—rocks. Paleontologists now hope to use this rare discovery of million-year-old tissue to study the biology and possibly even the DNA of the T.
Even with all these advancements, paleontologists still make important discoveries by using simple tools and basic techniques in the field. The National Geographic Society supports field work in paleontology throughout the world. There, Alemseged and his colleagues unearth and study fossils that contribute to the understanding of human evolution. Emerging Explorer Bolortsetseg Minjin is a paleontologist who has found fossils of dinosaurs, ancient mammals, and even corals in the Gobi Desert of Mongolia.
She also works to teach Mongolian students about the dinosaurs in their backyard, and is hoping to establish a paleontology museum in the country. Many dig sites offer visitors the chance to watch paleontologists work in the field, including the following U. Evolutionary Biology Many paleontologists are also evolutionary biologists. Evolutionary biology is the study of the origin, development, and changes evolution in species over time.
Other scientists that contribute to evolutionary biology are geologists and geneticists. Soaking Up History The oldest fossils ever discovered are stromatolites, the remains of ancient cyanobacteria, or blue-green algae.
The oldest animal fossils ever discovered are sponges. Prehistoric sponges have been discovered on the Arabian Peninsula and Australia. Fossils and Myths Ancient cultures did not always understand what fossils were, and adapted their discovery to fit with myths and stories.
China is rich in dinosaur fossils. Dinosaurs are ancient reptiles whose bones share characteristics with both reptiles and birds. Ancient Chinese people often interpreted dinosaur skeletons as the remains of flying dragons!
Fossilized remains of dwarf elephants have been found on several Mediterranean islands. Dwarf elephants grew to only 2 meters 6 feet tall. Their skulls are about the same size as a human skull, with a large hole in the middle where the living animal's trunk is. In the ancient Mediterranean cultures of Greece and Rome, the remains of dwarf elephants were often interpreted as the remains of cyclopes, a type of feared, one-eyed giant.
Mary Anning The 19th-century British fossil collector Mary Anning proved you don't have to be a paleontologist to contribute to science. Anning was one of the first people to collect, display, and correctly identify the fossils of ichthyosaurs, plesiosaurs, and pterosaurs.
Her contributions to the understanding of Jurassic life were so impressive that in , Anning was named among the ten British women who have most influenced the history of science.
Amber is sometimes considered a gemstone. Also called a CAT scanner. Also called blue-green algae even though it is not algae and in freshwater habitats pond scum. Also called an excavation. A hypothesis is tested to determine if it is accurate. The last ice age peaked about 20, years ago.
Also called glacial age. Female mammals produce milk to feed their offspring. The last mammoths became extinct about 5, years ago. Also called radioactive dating. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited. Jill Wertheim, National Geographic Society.
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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. Extinction is the complete disappearance of a species from Earth. Species go extinct every year, but historically the average rate of extinction has been very slow with a few exceptions.
The fossil record reveals five uniquely large mass extinction events during which significant events such as asteroid strikes and volcanic eruptions caused widespread extinctions over relatively short periods of time. On a molecular level, this can happen due to random mutation unrelated to adaptive changes. Some organisms possess structures with no apparent function which appear to be residual parts from a past ancestor. For example, some snakes have pelvic bones despite having no legs because they descended from reptiles that did have legs.
Another example of a structure with no function is the human vermiform appendix. These unused structures without function are called vestigial structures. Other examples of vestigial structures are wings which may have other functions on flightless birds like the ostrich, leaves on some cacti, traces of pelvic bones in whales, and the sightless eyes of cave animals.
Vestigial appendix : In humans the vermiform appendix is a vestigial structure; it has lost much of its ancestral function. There are also several reflexes and behaviors that are considered to be vestigial. The arrector pili muscle, which is a band of smooth muscle that connects the hair follicle to connective tissue, contracts and creates the goose bumps on skin.
Vestigial structures are often homologous to structures that function normally in other species. Therefore, vestigial structures can be considered evidence for evolution, the process by which beneficial heritable traits arise in populations over an extended period of time. The existence of vestigial traits can be attributed to changes in the environment and behavior patterns of the organism in question. In some cases the structure becomes detrimental to the organism.
Whale Skeleton : The pelvic bones in whales are also a good example of vestigial evolution whales evolved from four-legged land mammals and secondarily lost their hind legs. Letter c in the picture indicates the undeveloped hind legs of a baleen whale. If there are no selection pressures actively lowering the fitness of the individual, the trait will persist in future generations unless the trait is eliminated through genetic drift or other random events. Although in many cases the vestigial structure is of no direct harm, all structures require extra energy in terms of development, maintenance, and weight and are also a risk in terms of disease e.
The vestigial versions of a structure can be compared to the original version of the structure in other species in order to determine the homology of the structure. Homologous structures indicate common ancestry with those organisms that have a functional version of the structure. Vestigial traits can still be considered adaptations because an adaptation is often defined as a trait that has been favored by natural selection.
Adaptations, therefore, need not be adaptive, as long as they were at some point. The biological distribution of species is based on the movement of tectonic plates over a period of time. Biogeography is the study of the geographic distribution of living things and the abiotic factors that affect their distribution. Abiotic factors, such as temperature and rainfall, vary based on latitude and elevation, primarily. As these abiotic factors change, the composition of plant and animal communities also changes.
Ecologists who study biogeography examine patterns of species distribution. No species exists everywhere; for example, the Venus flytrap is endemic to a small area in North and South Carolina. An endemic species is one which is naturally found only in a specific geographic area that is usually restricted in size.
Other species are generalists: species which live in a wide variety of geographic areas; the raccoon, for example, is native to most of North and Central America. Since species distribution patterns are based on biotic and abiotic factors and their influences during the very long periods of time required for species evolution, early studies of biogeography were closely linked to the emergence of evolutionary thinking in the eighteenth century. Some of the most distinctive assemblages of plants and animals occur in regions that have been physically separated for millions of years by geographic barriers.
Biologists estimate that Australia, for example, has between , and , species of plants and animals. Australia : Australia is home to many endemic species. The a wallaby Wallabia bicolor , a medium-sized member of the kangaroo family, is a pouched mammal, or marsupial. The b echidna Tachyglossus aculeatus is an egg-laying mammal. The geographic distribution of organisms on the planet follows patterns that are best explained by evolution in conjunction with the movement of tectonic plates over geological time.
Broad groups that evolved before the breakup of the supercontinent Pangaea about million years ago are distributed worldwide. Groups that evolved since the breakup appear uniquely in regions of the planet, such as the unique flora and fauna of northern continents that formed from the supercontinent Laurasia and of the southern continents that formed from the supercontinent Gondwana.
Biogeography : The Proteacea family of plants evolved before the supercontinent Gondwana broke up. Today, members of this plant family are found throughout the southern hemisphere shown in red. Privacy Policy. Skip to main content.
Evolution and the Origin of Species. Search for:. Evidence of Evolution. The Fossil Record as Evidence for Evolution Fossils tell us when organisms lived, as well as provide evidence for the progression and evolution of life on earth over millions of years.
Learning Objectives Synthesize the contributions of the fossil record to our understanding of evolution. Key Takeaways Key Points Fossils are the preserved remains or traces of animals, plants, and other organisms from the past.
Fossils are important evidence for evolution because they show that life on earth was once different from life found on earth today. Usually only a portion of an organism is preserved as a fossil, such as body fossils bones and exoskeletons , trace fossils feces and footprints , and chemofossils biochemical signals. Paleontologists can determine the age of fossils using methods like radiometric dating and categorize them to determine the evolutionary relationships between organisms.
Key Terms biomarker : A substance used as an indicator of a biological state, most commonly disease. Fossil Formation Fossils can form under ideal conditions by preservation, permineralization, molding casting , replacement, or compression. Learning Objectives Predict the conditions suitable to fossil formation. Key Takeaways Key Points Preservation of remains in amber or other substances is the rarest from of fossilization; this mechanism allows scientists to study the skin, hair, and organs of ancient creatures.
Permineralization, where minerals like silica fill the empty spaces of shells, is the most common form of fossilization. Molds form when shells or bones dissolve, leaving behind an empty depression; a cast is then formed when the depression is filled by sediment.
Replacement occurs when the original shell or bone dissolves away and is replaced by a different mineral; when this occurs with permineralization, it is called petrification. In compression, the most common form of fossilization of leaves and ferns, a dark imprint of the fossil remains. Decay, chemical weathering, erosion, and predators are factors that deter fossilization. Fossilization of soft body parts is rare, and hard parts are better preserved when buried.
Key Terms amber : a hard, generally yellow to brown translucent fossil resin permineralization : form of fossilization in which minerals are deposited in the pores of bone and similar hard animal parts petrification : process by which organic material is converted into stone through the replacement of the original material and the filling of the original pore spaces with minerals. Gaps in the Fossil Record Because not all animals have bodies which fossilize easily, the fossil record is considered incomplete.
Learning Objectives Explain the gap in the fossil record. Because hard body parts are more easily preserved than soft body parts, there are more fossils of animals with hard body parts, such as vertebrates, echinoderms, brachiopods, and some groups of arthropods. Key Terms transitional fossil : Fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. Carbon Dating and Estimating Fossil Age The age of fossils can be determined using stratigraphy, biostratigraphy, and radiocarbon dating.
Learning Objectives Summarize the available methods for dating fossils. Key Takeaways Key Points Determining the ages of fossils is an important step in mapping out how life evolved across geologic time. The study of stratigraphy enables scientists to determine the age of a fossil if they know the age of layers of rock that surround it.
Biostratigraphy enables scientists to match rocks with particular fossils to other rocks with those fossils to determine age. Scientists use carbon dating when determining the age of fossils that are less than 60, years old, and that are composed of organic materials such as wood or leather. Key Terms half-life : The time required for half of the nuclei in a sample of a specific isotope to undergo radioactive decay.
The Fossil Record and the Evolution of the Modern Horse The detailed fossil record of horses has provided insight into their evolutionary progress. Learning Objectives Analyze the fossil record to understand the evolution of horses. Key Takeaways Key Points A dog-like organism gave rise to the first horse ancestors million years ago. The fossil record shows modern horses moved from tropical forests to prairie habitats, developed teeth, and grew in size.
The first equid fossil was a tooth from the extinct species Equus curvidens found in Paris in the s. Thomas Huxley popularized the evolutionary sequence of horses, which became one of the most common examples of clear evolutionary progression. Horse evolution was previously believed to be a linear progress, but after more fossils were discovered, it was determined the evolution of horses was more complex and multi-branched.
Horses have evolved from gradual change anagenesis as well as abrupt progression and division cladogenesis.
Key Terms cladogenesis : An evolutionary splitting event in which each branch and its smaller branches forms a clade. Homologous Structures Homologous structures are similar structures that evolved from a common ancestor.
Learning Objectives Describe the connection between evolution and the appearance of homologous structures. Key Takeaways Key Points Homology is a relationship defined between structures or DNA derived from a common ancestor and illustrates descent from a common ancestor.
Analogous structures are physically but not genetically similar structures that were not present the last common ancestor. Homology can also be partial; new structures can evolve through the combination or parts of developmental pathways.
Analogy may also be referred to as homoplasy, which is further divided into parallelism, reversal, and convergence. Key Terms homology : A correspondence of structures in two life forms with a common evolutionary origin, such as flippers and hands.
Convergent Evolution Convergent evolution occurs in different species that have evolved similar traits independently of each other. Learning Objectives Predict the circumstances supporting convergent evolution of two species.
Key Takeaways Key Points Examples of convergent evolution include the relationship between bat and insect wings, shark and dolphin bodies, and vertebrate and cephalopod eyes. Analogous structures arise from convergent evolution, but homologous structures do not. Convergent evolution is the opposite of divergent evolution, in which related species evolve different traits.
Convergent evolution is similar to parallel evolution, in which two similar but independent species evolve in the same direction and independently acquire similar characteristics.
Key Terms parallel evolution : the development of a similar trait in related, but distinct, species descending from the same ancestor, but from different clades convergent evolution : a trait of evolution in which species not of similar recent origin acquire similar properties due to natural selection divergent evolution : the process by which a species with similar traits become groups that are tremendously different from each other over many generations morphology : the form and structure of an organism.
Vestigial Structures Vestigial structures have no function but may still be inherited to maintain fitness. What are transitional fossils and why are they important to the fossil record? What are trace fossils and what are some examples? What is permineralization and how is it important for the preservation of fossils? What is a trace fossil and what can be learned from them? Question 6b3d1. Why are fossils rare in precambrian rocks? Why are fossils formed in sedimentary rocks? See all questions in Fossils.
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