362
edits
Changes
Jump to navigation
Jump to search
User:TonySidaway/Evolution (view source)
Revision as of 14:25, September 7, 2012
, 14:25, September 7, 2012Created page with "'''Evolution''' is the common name for the emergence, life, and eventual extinction, of different species on earth over time. We know that species become extinct because this has..."
'''Evolution''' is the common name for the emergence, life, and eventual extinction, of different species on earth over time. We know that species become extinct because this has been observed directly and the proximate cause of a particular extinction is usually evident. We can also observed directly that living organism are not randomly distributed across the Earth but are adapted to their surroundings. But we now also know that new species come into existence over great expanses of time, and that all individual organisms are connected by a chain of ancestry. We also know much about the mechanisms that drive this evolution. This essay outlines the chain of evidence and reasoning that has led us to make this deduction.
==The fossil record==
Fossils were known in ancient times, and various explanations were offered to explain their existence. Our modern appreciation of how fossils form is based on the key observation that fossils tend to be confined to certain strata of rock. When European explorers started to examine strata outside Europe, they quickly began to see that rock strata are laid down in a predictable order and that the order represents the chronology of Earth's long history. Thus fossils represent the forms of life that existed at the time the strata were laid down. This chronology has come to be known as the fossil record.
==Geological time==
Early scientific estimates for the age of the Earth differed widely because although the geological strata indicate a chronology, there was no way to assign an age to different strata. Similar problems plagued attempts to measure the age of other astronomical bodies such as the sun and the other stars. In the past century this problem has been solved by the use of multiple radiometric markers. The principle is that radioactive decay follows an inverse exponential relationship, with a predictable half-life. While any one marker may produce a misleading result as a side-effect of contamination, by measuring multiple radioactive markers in the same rock sample we can reduce the uncertainty. Modern mass spectrometry can produce remarkably accurate assays of very small samples. The results obtained for the ages of rock samples are consistent with those from meteorites which indicate that the solar system is some 4 billion years old.
Thus we can now estimate that the rocks in which the earliest fossil microbes are found are about 3.5 billion years old, that the earliest known widespread fossil fauna of solid-bodied animals were laid down in the Cambrian sediments over 500 million years ago, the first bony fish appear some 440 million years ago, and so on. We can also date the appearance, long histories, and eventual extinction, of whole groups of species, most notably the trilobites.
''Work in progress''
==The fossil record==
Fossils were known in ancient times, and various explanations were offered to explain their existence. Our modern appreciation of how fossils form is based on the key observation that fossils tend to be confined to certain strata of rock. When European explorers started to examine strata outside Europe, they quickly began to see that rock strata are laid down in a predictable order and that the order represents the chronology of Earth's long history. Thus fossils represent the forms of life that existed at the time the strata were laid down. This chronology has come to be known as the fossil record.
==Geological time==
Early scientific estimates for the age of the Earth differed widely because although the geological strata indicate a chronology, there was no way to assign an age to different strata. Similar problems plagued attempts to measure the age of other astronomical bodies such as the sun and the other stars. In the past century this problem has been solved by the use of multiple radiometric markers. The principle is that radioactive decay follows an inverse exponential relationship, with a predictable half-life. While any one marker may produce a misleading result as a side-effect of contamination, by measuring multiple radioactive markers in the same rock sample we can reduce the uncertainty. Modern mass spectrometry can produce remarkably accurate assays of very small samples. The results obtained for the ages of rock samples are consistent with those from meteorites which indicate that the solar system is some 4 billion years old.
Thus we can now estimate that the rocks in which the earliest fossil microbes are found are about 3.5 billion years old, that the earliest known widespread fossil fauna of solid-bodied animals were laid down in the Cambrian sediments over 500 million years ago, the first bony fish appear some 440 million years ago, and so on. We can also date the appearance, long histories, and eventual extinction, of whole groups of species, most notably the trilobites.
''Work in progress''