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Transitional Fossil
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group.[1] This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.[2] In 1859, when Charles Darwin's On the Origin of Species
On the Origin of Species
was first published, the fossil record was poorly known
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Extinction Event
An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It occurs when the rate of extinction increases with respect to the rate of speciation. Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as "major", and the data chosen to measure past diversity. Because most diversity and biomass on Earth
Earth
is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.[1] Extinction occurs at an uneven rate
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Geochronology
Geochronology
Geochronology
is the science of determining the age of rocks, fossils, and sediments using signatures inherent in the rocks themselves. Absolute geochronology can be accomplished through radioactive isotopes, whereas relative geochronology is provided by tools such as palaeomagnetism and stable isotope ratios. By combining multiple geochronological (and biostratigraphic) indicators the precision of the recovered age can be improved. Geochronology
Geochronology
is different in application from biostratigraphy, which is the science of assigning sedimentary rocks to a known geological period via describing, cataloguing and comparing fossil floral and faunal assemblages. Biostratigraphy
Biostratigraphy
does not directly provide an absolute age determination of a rock, but merely places it within an interval of time at which that fossil assemblage is known to have coexisted
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Abiogenesis
Abiogenesis, or informally the origin of life,[3][4][5][a] is the natural process by which life has arisen from non-living matter, such as simple organic compounds.[6][4][7][8] While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity that involved molecular self-replication, self-assembly, autocatalysis, and the emergence of cell membranes.[9][10][11] Although the occurrence of abiogenesis is uncontroversial among scientists, there is no single, generally accepted model for the origin of life, and this article presents several principles and hypotheses for how abiogenesis could have occurred. Researchers study abiogenesis through a combination of molecular biology, paleontology, astrobiology, oceanography, biophysics, geochemistry and bioch
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Geologic Record
The geologic record in stratigraphy, paleontology and other natural sciences refers to the entirety of the layers of rock strata — deposits laid down by volcanism or by deposition of sediment derived from weathering detritus (clays, sands etc.) including all its fossil content and the information it yields about the history of the Earth: its past climate, geography, geology and the evolution of life on its surface. According to the law of superposition, sedimentary and volcanic rock layers are deposited on top of each other
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Geologic Time Scale
The geologic time scale (GTS) is a system of chronological dating that relates geological strata (stratigraphy) to time. It is used by geologists, paleontologists, and other Earth
Earth
scientists to describe the timing and relationships of events that have occurred during Earth's history
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Evolution Of Hair
Hair
Hair
is a protein filament that grows from follicles found in the dermis. Hair
Hair
is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair
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Human Evolution
Human
Human
evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates—in particular genus Homo—and leading to the emergence of Homo sapiens
Homo sapiens
as a distinct species of the hominid family, the great apes
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Evolution Of Cephalopods
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian
Cambrian
strata,[1] and purported stem-group representatives present in the earliest Cambrian
Cambrian
lagerstätten.[2] The class developed during the middle Cambrian, and underwent pulses of diversification during the Ordovician
Ordovician
period[3] to become diverse and dominant in the Paleozoic
Paleozoic
and Mesozoic
Mesozoic
seas. Small shelly fossils such as Tommotia
Tommotia
were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals,[4] and the earliest accepted cephalopods date to the Middle Cambrian Period
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Evolution Of Cetaceans
The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent
Indian subcontinent
from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla, and branched off from other artiodactyls around 50 mya (million years ago). Cetaceans are thought to have evolved during the Eocene
Eocene
or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retained many skeletal features from their terrestrial ancestors
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Evolution Of Dinosaurs
Dinosaurs evolved within a single lineage of archosaurs 232-234 Ma (million years ago) in the Ladinian
Ladinian
age, the latter part of the middle Triassic. Dinosauria
Dinosauria
is a well-supported clade, present in 98% of bootstraps. It is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.[1] In March 2017, scientists reported a new way of classifying the dinosaur family tree, based on newer and more evidence than available earlier. According to the new classification, the original dinosaurs, arising 200 million years ago, were small, two-footed omnivorous animals with large grasping hands
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Evolution Of Fungi
The evolution of fungi has been going on since fungi diverged from other life around 1.5 billion years ago, (Wang et al., 1999)[1] with the glomaleans branching from the "higher fungi" at ~570 million years ago, according to DNA analysis. (Schüssler et al., 2001; Tehler et al., 2000)[1] Fungi probably colonized the land during the Cambrian, over 500 million years ago, (Taylor & Osborn, 1996)[1] but terrestrial fossils only become uncontroversial and common during the Devonian, 400 million years ago.[1] Early fungi[edit] A rich diversity of fungi is known from the lower Devonian
Devonian
Rhynie chert; an earlier record is absent. Since fungi do not biomineralise, they do not readily enter the fossil record; there are only three claims of early fungi
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Paleontology
Paleontology, sometimes spelled palaeontology (/ˌpeɪliɒnˈtɒlədʒi, ˌpæli-, -ən-/) is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene
Holocene
Epoch (roughly 11,700 years before present). It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments (their paleoecology). Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, "old, ancient", ὄν, on (gen
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Evolution Of Insects
The most recent understanding of the evolution of insects is based on studies of the following branches of science: molecular biology, insect morphology, paleontology, insect taxonomy, evolution, embryology, bioinformatics and scientific computing. It is estimated that the class of insects originated on Earth
Earth
about 480 million years ago, in the Ordovician, at about the same time terrestrial plants appeared.[1] Insects evolved from a group of crustaceans.[2] The first insects were land bound, but about 400 million years ago in the Devonian
Devonian
period one lineage of insects evolved flight, the first animals to do so.[1] The oldest definitive insect fossil, Rhyniognatha hirsti, is estimated to be 407 to 396 million years old. Global climate conditions changed several times during the history of Earth, and along with it the diversity of insects
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Evolution Of Mollusca
The evolution of the molluscs is the way in which the Mollusca, one of the largest groups of invertebrate animals, evolved. This phylum includes gastropods, bivalves, scaphopods, cephalopods, and several other groups
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Evolution Of Sirenians
Sirenia
Sirenia
is the order of placental mammals which comprises modern "sea cows" (manatees and the Dugong) and their extinct relatives. They are the only extant herbivorous marine mammals and the only group of herbivorous mammals to have become completely aquatic. Sirenians are thought to have a 50-million-year-old fossil record (early Eocene-recent)
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