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Timeline: The evolution of life | New Scientist
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The Origin of Humankind. Science Masters Series. New York: Basic Books. Margulis, Lynn San Francisco, CA: W. Freeman and Company. McKinney, Michael L. At the same time, bacterial mats were progressively destroyed and forced into more restricted habitats i. This change in the substrate is thought to be partly responsible for the demise of the Ediacaran biota. Other factors such as a change in water chemistry or an increase of predators may also have played important roles in their extinction see above.
The revolution turned the once-uniform sea floor into a heterogeneous patchwork, opening up a variety of new niches for animals - including those of the Burgess Shale - to exploit. Exceptionally well-preserved soft-bodied fossils of Cambrian age were first described from the Burgess Shale over years ago. Today, dozens of Burgess Shale-type deposits with comparable assemblages of fossils have been found around the world. These deposits are usually found in Lower and Middle Cambrian rock layers, but may extend as far as the early Ordovician.
These deposits are characterized by a similar mode of preservation called "Burgess Shale-type preservation". Acinocricus cricus , part of a lobopod from the Spence Shale. Images of landscapes and fossils from different Burgess Shale-type deposits in Utah. Photos: Jean-Bernard Caron fossil specimens. Photo: David Harper. Maotianshan Hill, China where the first Lower Cambrian Chengjiang fossils were discovered, including Naraoia spinosa see below. The name Chengjiang comes from a nearby village in Yunnan Province.
Photo: Fangchen Zhao. Photos: Maoyan Zhu. Photos: Jih-Pai Lin. Compared to conventional fossil deposits, in which only the remains of more durable body parts are typically preserved, Burgess Shale-type deposits provide a much more complete picture of a normal Cambrian marine community.
Evolution: Out Of The Sea
In modern marine settings, animals with mineralized body parts shells, carapaces, etc. This is also the case in most Burgess Shale-type deposits where the shelly assemblage usually represents a small percentage of specimens collected. Thus, without the fossilized remains of soft-bodied organisms, especially from the Burgess Shale, our knowledge of Cambrian ecosystems would be extremely limited.
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Similarities among various Burgess Shale-type deposits around the world suggest the deep marine ecosystem was geographically uniform and evolutionarily conservative from the Lower to at least the Middle Cambrian i. The characteristic assemblage of organisms is often referred as the Burgess Shale-type biota. Why did the Cambrian explosion happen when it did, and why was it such a unique event?
While there is no current consensus among scientists, most researchers agree the explosion cannot be ascribed to a single, simple causal mechanism. The potential triggers can be classified in three main categories: environmental, genetic, and ecological. Deciphering the impact of each of these factors remains one of the most important challenges faced by palaeontologists today.
Deriving physiologic first principles
Very few organisms ever enter the fossil record; after death, their remains are usually completely destroyed and recycled. Animals with hard body coverings, such as trilobites, are much more likely to be preserved than those with only soft body parts. So the evolutionary development of mineralized shelly parts by different groups would be marked in the rock record by a sudden jump in fossil numbers. Thus, preservation bias alone could create the appearance of an "explosion" of new life forms at the beginning of the Cambrian.
When he published On the Origin of Species in , Charles Darwin puzzled over the apparently sudden appearance of complicated organisms in the fossil record at the beginning of the Cambrian Period. He noted this could be used as an argument against his controversial new theory, which predicted a more gradual appearance of simpler organisms. At the time, Darwin pointed to the imperfection of the fossil record as his only defence, arguing complex animal life must have lived long before the Cambrian, but traces of that life had not yet been found.
The presence of large, soft-bodied, putative animals problematic as they may be in Ediacaran seas does indeed make the "explosion" appear less abrupt. But the fact remains that the Early Cambrian was a time of major change in marine animal communities and environments, with the rapid and unprecedented advent of disparate new body plans and novel ecological niches. By the end of the period, every major animal phylum was firmly established, and life after the Cambrian was radically different from what had gone before. So it is safe to call this event an "explosion" - it was crucial to the evolution of life on Earth as we know it.
Before complex animals could evolve on Earth, there had to be an environment favourable for their survival. Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a rise in oxygen levels and the end of extreme glacial conditions. Multicellular animals use oxygen to fuel their metabolism. At low oxygen levels, they don't function well … without it, they cannot survive. Photosynthesis could have caused a rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian, allowing the evolution of larger, more complex animals with respiratory and circulatory systems.
However, there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary. Earlier increases might have triggered the evolution of large Ediacaran metazoans prior to the explosion, and a subsequent post-explosion rise in oxygen levels may have allowed animals to adopt more active, energy-intensive lifestyles such as swimming and hunting. Hundreds of thousands of fossil organisms, found in well-dated rock sequences, represent successions of forms through time and manifest many evolutionary transitions.
As mentioned earlier, microbial life of the simplest type was already in existence 3. The oldest evidence of more complex organisms that is, eucaryotic cells, which are more complex than bacteria has been discovered in fossils sealed in rocks approximately 2 billion years old. Multicellular organisms, which are the familiar fungi, plants, and animals, have been found only in younger geological strata. The following list presents the order in which increasingly complex forms of life appeared:. So many intermediate forms have been discovered between fish and amphibians, between amphibians and reptiles, between reptiles and mammals, and along the primate lines of descent that it often is difficult to identify categorically when the transition occurs from one to another particular species.
Actually, nearly all fossils can be regarded as intermediates in some sense; they are life forms that come between the forms that preceded them and those that followed. The fossil record thus provides consistent evidence of systematic change through time—of descent with modification. From this huge body of evidence, it can be predicted that no reversals will be found in future paleontological studies. That is, amphibians will not appear before fishes, nor mammals before reptiles, and no complex life will occur in the geological record before the oldest eucaryotic cells.
This prediction has been upheld by the evidence that has accumulated until now: no reversals have been found. Inferences about common descent derived from paleontology are reinforced by comparative anatomy. For example, the skeletons of humans, mice, and bats are strikingly similar, despite the different ways of life of these animals and the diversity of environments in which they flourish. The correspondence of these animals, bone by bone, can be observed in every part of the body, including the limbs; yet a person writes, a mouse runs, and a bat flies with structures built of bones that are different in detail but similar in general structure and relation to each other.
Scientists call such structures homologies and have concluded that they are best explained by common descent. Comparative anatomists investigate such homologies, not only in bone structure but also in other parts of the body, working out relationships from degrees of similarity. Their conclusions provide important inferences about the details of evolutionary history, inferences that can be tested by comparisons with the sequence of ancestral forms in the paleontological record. A bat wing, a mouse forelimb, and a human arm serve very different purposes, but they have the same basic components The similarities arise because all three species share a common four-limbed vertebrate ancestor.
The mammalian ear and jaw are instances in which paleontology and comparative anatomy combine to show common ancestry through transitional stages. The lower jaws of mammals contain only one bone, whereas those of reptiles have several. The other bones in the reptile jaw are homologous with bones now found in the mammalian ear.
Paleontologists have discovered intermediate forms of mammal-like reptiles Therapsida with a double jaw joint—one composed of the bones that persist in mammalian jaws, the other consisting of bones that eventually became the hammer and anvil of the mammalian ear. Biogeography also has contributed evidence for descent from common ancestors. The diversity of life is stupendous. Approximately , species of living plants, , species of fungi, and one million species of animals have been described and named, each occupying its own peculiar ecological setting or niche; and the census is far from complete.
Some species, such as human beings and our companion the dog, can live under a wide range of environments. Others are amazingly specialized. One species of a fungus Laboulbenia grows exclusively on the rear portion of the covering wings of a single species of beetle Aphaenops cronei found only in some caves of southern France.
The larvae of the fly Drosophila carcinophila can develop only in specialized grooves beneath the flaps of the third pair of oral appendages of a land crab that is found only on certain Caribbean islands. How can we make intelligible the colossal diversity of living beings and the existence of such extraordinary, seemingly whimsical creatures as the fungus, beetle, and fly described above? Evolutionary theory explains that biological diversity results from the descendants of local or migrant predecessors becoming adapted to their diverse environments.
This explanation can be tested by examining present species and local fossils to see whether they have similar structures, which would indicate how one is derived from the other. Also, there should be evidence that species without an established local ancestry had migrated into the locality.
Wherever such tests have been carried out, these conditions have been confirmed. A good example is provided by the mammalian populations of North and South America, where strikingly different native organisms evolved in isolation until the emergence of the isthmus of Panama approximately 3 million years ago. Thereafter, the armadillo, porcupine, and opossum—mammals of South American origin—migrated north, along with many other species of plants and animals, while the mountain lion and other North American species made their way across the isthmus to the south.
The evidence that Darwin found for the influence of geographical distribution on the evolution of organisms has become stronger with advancing knowledge. For example, approximately 2, species of flies belonging to the genus Drosophila are now found throughout the world.
About one-quarter of them live only in Hawaii. Until about 3 million years ago, North and South America were separated by a wide expanse of water, so mammals on the two continents evolved separately. After the isthmus of Panama formed, armadillos and opossums migrated north, and mountain lions migrated south. These movements are documented in the fossil record. More than a thousand species of snails and other land mollusks also are found only in Hawaii.
The biological explanation for the multiplicity of related species in remote localities is that such great diversity is a consequence of their evolution from a few common ancestors that colonized an isolated environment. The Hawaiian Islands are far from any mainland or other islands, and on the basis of geological evidence they never have been attached to other lands. Thus, the few colonizers that reached the Hawaiian Islands found many available ecological niches, where they could, over numerous generations, undergo evolutionary change and diversification.
No mammals other than one bat species lived in the Hawaiian Islands when the first human settlers arrived; similarly, many other kinds of plants and animals were absent. The Hawaiian Islands are not less hospitable than other parts of the world for the absent species. For example, pigs and goats have multiplied in the wild in Hawaii, and other domestic animals also thrive there. The scientific explanation for the absence of many kinds of organisms, and the great multiplication of a few kinds, is that many sorts of organisms never reached the islands, because of their geographic isolation.
Those that did reach the islands diversified over time because of the absence of related organisms that would compete for resources. Embryology, the study of biological development from the time of conception, is another source of independent evidence for common descent. Barnacles, for instance, are sedentary crustaceans with little apparent similarity to such other crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through a free-swimming larval stage in which they look like other crustacean larvae. The similarity of larval stages supports the conclusion that all crustaceans have homologous parts and a common ancestry.
Similarly, a wide variety of organisms from fruit flies to worms to mice to humans have very similar sequences of genes that are active early in development. These genes influence body segmentation or orientation in all these diverse groups. The presence of such similar genes doing similar things across such a wide range of organisms is best explained by their having been present in a very early common ancestor of all of these groups.
The unifying principle of common descent that emerges from all the foregoing lines of evidence is being reinforced by the discoveries of modern biochemistry and molecular biology. The code used to translate nucleotide sequences into amino acid sequences is essentially the same in all organisms. Moreover, proteins in all organisms are invariably composed of the same set of 20 amino acids. This unity of composition. In , scientists at Cambridge University in the United Kingdom determined the three-dimensional structures of two proteins that are found in almost every multicelled animal: hemoglobin and myoglobin.
Hemoglobin is the protein that carries oxygen in the blood. Myoglobin receives oxygen from hemoglobin and stores it in the tissues until needed. These were the first three-dimensional protein structures to be solved, and they yielded some key insights. Myoglobin has a single chain of amino acids wrapped around a group of iron and other atoms called "heme" to which oxygen binds. Hemoglobin, in contrast, is made of up four chains: two identical chains consisting of amino acids, and two other identical chains consisting of amino acids.
However, each chain has a heme exactly like that of myoglobin, and each of the four chains in the hemoglobin molecule is folded exactly like myoglobin. It was immediately obvious in that the two molecules are very closely related. During the next two decades, myoglobin and hemoglobin sequences were determined for dozens of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. All of these sequences were so obviously related that they could be compared with confidence with the three-dimensional structures of two selected standards—whale myoglobin and horse hemoglobin.
Even more significantly, the differences between sequences from different organisms could be used to construct a family tree of hemoglobin and myoglobin variation among organisms. This tree agreed completely with observations derived from paleontology and anatomy about the common descent of the corresponding organisms. Myoglobin, which stores oxygen in muscles, consists of a chain of amino acids wrapped around an oxygen-binding molecule.
The sequence of amino acids in myoglobin vanes from species to species, revealing the evolutionary relationships among organisms. Similar family histories have been obtained from the three-dimensional structures and amino acid sequences of other proteins, such as cytochrome c a protein engaged in energy transfer and the digestive proteins trypsin and chymotrypsin.
The examination of molecular structure offers a new and extremely powerful tool for studying evolutionary relationships. The quantity of information is potentially huge—as large as the thousands of different proteins contained in living organisms, and limited only by the time and resources of molecular biologists. As the ability to sequence the nucleotides making up DNA has improved, it also has become possible to use genes to reconstruct the evolutionary history of organisms.
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