Paleontologists today generally recognize that while the fossil record is imperfect , it is still adequate to assess questions about evolution. A Pattern of Explosions The eventual realization that the fossil record is not entirely incomplete has forced evolutionary biologists to accept that the record shows a pattern of explosions, not gradual evolution of living organisms.
One biology textbook explains:. Many species remain virtually unchanged for millions of years, then suddenly disappear to be replaced by a quite different, but related, form. Moreover, most major groups of animals appear abruptly in the fossil record, fully formed, and with no fossils yet discovered that form a transition from their parent group.
Probably the most famous instance of abrupt appearance is the Cambrian explosion, in which nearly all of the major living animal phyla appear for the first time. An invertebrate biology textbook explains:. These include such anatomically complex and distinctive types as trilobites, echinoderms, brachiopods, molluscs, and chordates.
Evolutionary scientists acknowledge that they cannot explain this rapid appearance of diverse animal body plans by classical Darwinian processes, or other known material mechanisms. But the Cambrian explosion is by no means the only explosion of life recorded in the fossil record.
As one paper states:. In spite of much research and analyses of different sources of data e. Angiosperms appear rather suddenly in the fossil record… with no obvious ancestors for a period of million years before their appearance. In a similar way, many orders of mammals appear in an explosive manner. A literal reading of the fossil record indicates that the early Cambrian c. Of course there are a handful of examples where evolutionary scientists believe they have found transitional fossils documenting gradual Darwinian evolution.
In particular, it is claimed there are fossil land-mammals with ear-bones similar to whales, and fossil whale-like mammals that retain their hindlimbs. Is this enough to demonstrate that this transition occurred? Even if there are fossils that look like potential intermediate forms, if the overall evolutionary story does not make sense, then the fossils cannot be transitional. 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. Learning Objectives Discuss the connection between evolution and the existence of vestigial structures. Key Takeaways Key Points Structures that have no apparent function and appear to be residual parts from a past ancestor are called vestigial structures. Examples of vestigial structures include the human appendix, the pelvic bone of a snake, and the wings of flightless birds. Vestigial structures can become detrimental, but in most cases these structures are harmless; however, these structures, like any other structure, require extra energy and are at risk for disease.
Vestigial structures, especially non-harmful ones, take a long time to be phased out since eliminating them would require major alterations that could result in negative side effects. Key Terms vestigial structure : Genetically determined structures or attributes that have lost most or all of their ancestral function in a given species.
Biogeography and the Distribution of Species The biological distribution of species is based on the movement of tectonic plates over a period of time. Learning Objectives Relate biogeography and the distribution of species.
Key Takeaways Key Points Biogeography is the study of geological species distribution, which is influenced by both biotic and abiotic factors. Some species are endemic and are only found in a particular region, while others are generalists and are distributed worldwide. Species that evolved before the breakup of continents are distributed worldwide.
Species that evolved after the breakup of continents are found in only certain regions of the planet. Key Terms endemic : unique to a particular area or region; not found in other places generalist : species which can thrive in a wide variety of environmental conditions Pangaea : supercontinent that included all the landmasses of the earth before the Triassic period and that broke up into Laurasia and Gondwana.
The origin of land plants: a matter of mycotrophism. Biosystems 6 , — Wang, B. Phylogenetic distribution and evolution of mycorrhizas in land plants. Mycorrhiza 16 , — Hawksworth, D. The magnitude of fungal diversity: the 1. Article Google Scholar. Bartnicki-Garcia, S. Evolutionary Biology of the Fungi eds Rayner, A. Google Scholar.
Detritus-dependent development of the microbial community in an experimental system: qualitative analysis by denaturing gradient gel electrophoresis. Unexpected importance of potential parasites in the composition of freshwater small-eukaryote community. Unveiling fungal zooflagellates as members of freshwater picoeukaryotes: evidence from a molecular diversity study in a deep meromictic lake. Amaral Zettler, L. The nucleariid amoebae: more protists at the animal-fungal boundary.
Lara, E. Protist , — Massana, R. Unveiling new microbial eukaryotes in the surface ocean. Bass, D. DNA evidence for global dispersal and probable endemicity of protozoa.
BMC Evol. Fuchs, B. Unlabeled helper oligonucleotides increase the in situ accessibility to 16S rRNA of fluorescently labeled oligonucleotide probes. Kim, E. Newly identified and diverse plastid-bearing branch on the eukaryotic tree of life. Natl Acad. USA , — Mangot, J. Community structure and dynamics of small eukaryotes targeted by new oligonucleotide probes: new insight into the lacustrine microbial food web.
Woods, A. Definition of individual components within the cytoskeleton of Trypanosoma brucei by a library of monoclonal antibodies.
Cell Sci. PubMed Google Scholar. Webster, J. Introduction to Fungi 3rd edn Cambridge University Press, Book Google Scholar.
Held, A. The zoospore of Rozella allomycis : ultrastructure. Rozella and Rozellopsis : naked endoparasitic fungi which dress up as their hosts. In essence, the argument about intermediate forms runs as follows. In fact, we might expect to find a succession of such animals, each a little bit more whale-like and a little bit less well adapted to life on land than its predecessor. To make things more complicated, each of these intermediate forms must have been a fully working animal: it must have been able to breathe, to move about its environment, to feed itself and to reproduce.
For a long time, biologists speculated about what these animals might have looked like. Surely at some point there must have been a creature that was at home on neither land nor sea, and so unable to compete with the animals already fully adapted to either habitat. The problem of intermediate forms is by no means a new one.
In The Origin of Species , published in , Charles Darwin included a whole chapter on some of the difficulties facing his new theory. He started with the problem of intermediate forms:. Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? It has been asked by the opponents of such views as I hold, how, for instance, a land carnivorous animal could have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?
First, the scarcity of intermediate forms in the fossil record. Otters are wonderful swimmers and can chase and catch fish, but they also move about well enough to hunt on land. Seals and sea lions are clearly most at home in the water, but they can still chase each other about on the breeding beaches.
So it is possible to imagine an early ancestor of the whales leading a similarly amphibious existence. Studies of the chemical make-up of proteins particularly antibodies in present-day species have shown that modern whales are closely related to the artiodactyls, a group of hoofed mammals that includes cows, sheep, deer and hippos.
For some years after the publication of The Origin of Species , all the known fossils of early cetaceans were either clearly terrestrial animals or primitive whales like those examined by Simpson. But recent finds have included the remains of creatures that might have had a more amphibious existence. These animals also lived in the Eocene 55 to 34 million years ago , in or around the warm, shallow waters of the Tethys Sea, an ancient ocean that stretched from modern Spain to Indonesia.
Figure 18 shows three representative animals in a postulated evolutionary sequence from an Eocene terrestrial ancestor to modern odontocetes and mysticetes. Pakicetus is approximately 50 million years old.
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