Tipos de mutaciones involucradas en cáncer

An American geophysicist and the fi rst director of the Lamont Doherty Labora-tory of Columbia University. He was instrumental in the reexamination of Alfred Wegener’s theory of continental drift, and explained it by the idea of seafl oor spread, or creation of new bottom at the mid-ocean ridge. Ewing took the fi rst seismic recordings in open seas (1935) and the fi rst deep-sea photographs in 1939. He was one of several scientists who suggested that earthquakes and the undersea rifts associ-ated with oceanic ridges were linked. One of his major publications was The Floor of the Oceans: The North Atlantic (1977).

See continental drift; Heezen, Bruce Charles; Wegener, Alfred Lothar.

explorers and explorations

An explor-er is defi ned as one who travels in search of geographic or scientifi c information.

However, the Europeans of the 15th cen-tury and the explorers that followed, until the 18th century, were impelled to travel great distances for what some have called the causes of gold, God, or guns. We know their names, but they were not the fi rst ocean explorers, nor were they unique.

Ewing, William Maurice

Polynesians had travelled vast distances in open ocean, navigating by stars and return-ing to particular island groups. Their voy-ages were most certainly not unidirectional.

Chinese sailors navigated successfully as far as Africa, making regular trips. Arab mari-ners and traders had well-established routes along the African and Indian coasts.

The fi rst known name in connection with exploration is that of Necho, the Pharaoh who fi nanced a Phoenician cir-cumnavigation of Africa. Herodotus, in the 5th century b.c.e., produced a map of the world that stretched from Gibraltar to India. The astronomer-geographer Pytheus was also an explorer. He sailed north to Iceland in 325 b.c.e., using celestial navi-gation to arrive at a measure of latitude.

Eratosthenes measured the Earth and developed the concept of latitude and lon-gitude. Ptolemy, in the 2nd century c.e., continued and expanded on these works.

The Romans were users of geographic works, but were not particularly interested in exploration for the sake of science. After the fall of the Roman Empire, the center of exploration shifted to both the east and west. Brendan and the Norsemen in the west continued to travel and chart new lands; Arabs in the east did the same.

With the Renaissance and the renewed interest in the world in general came the potential for new trade. That and the tech-nological advances of better ships, better navigational instruments, and more accu-rate maps made long voyages possible.

These were undertaken both for national honor and for profi t. This was the age of da Gama and Columbus.

These early navigators were followed by traders and, in the 18th century, by the mainly scientifi c explorers such as Cook, LaPerouse, and Bougainville. Natural-ists accompanied the geographers, thus reviving a practice of the ancient Greeks.

Banks, a botanist, and Darwin, a natural scientist, were on voyages of exploration as “scientists on board.”

By the 19th century it was recognized by all maritime nations that maps and soundings of all parts of the world were

important. This led to a renewal in vari-ous arctic interests and the voyages of the Rosses. Surveying techniques were devel-oped by Maury and others.

Increasing information led to the need for consolidation and classifi cation of data. The need to know and to provide as much information as possible as a public service to all scientists led to the Chal-lenger expedition around the world in 1872–76 and then the polar explorations of Nansen in the early 20th century. The Meteor survey was the major work of the 1920s, although its report was not pub-lished until the 1950s.

Exploration in the second half of the 20th century has produced images sent to Earth from space, photography from the deeps, and the development of free div-ing. New genera unseen and unknown before 1950 are now classifi ed. The use of submersibles has greatly extended the researchers’ reach. Recent expeditions in 2003 have used Alvin in a number of trips to the sea fl oor in the Gulf of Alaska. The object of this search was the exploration of seamounts such as the Chirikof and the Warwick in an attempt to unravel the vol-canic history of these structures and the biota that live near or on them.

One of the more unusual recent fi nds is cold-water coral. Most corals live in shal-low, warm water, but deep-water speci-mens thrive in the waters of the polar ocean. Like their warm-water counter-parts, the slow-growing corals of colder seas create a habitat for other creatures.

Yet we still know more about the moon than about the Earth’s ocean deeps. There is much left to learn. See Alvin, bacteria, history, mucus, seamounts.

extinction

The natural end, or disap-pearance, of an organism. Throughout geologic time, many taxonomic fami-lies and groups have become extinct. At least four—some researchers say fi ve or more—distinct periods saw large-scale or mass extinctions. The disappearance of a taxonomic family was an ongoing process that lasted about a million years.

extinction

Under normal extinction patterns, fewer than eight families per million years disappeared. The major variations in this occurred around the ends of the Ordovician, Permian, Triassic, and Cre-taceous periods. At the end of the Creta-ceous up to 19.3 extinctions per million years were noted.

When calculating the number of extinctions, allowances have been made for those organisms, both plant and ani-mal, that are rarely preserved. Shells and skeletons become fossilized, but soft tissue usually disappears without leaving a trace.

There have been a number of theories to explain extinctions. It is no coincidence, however, that the major breaks occur at the ends of geologic periods. The origi-nal 19th-century geologic timescale was constructed on the evidence of large-scale change, and geologic periods ended with more or less abrupt (give or take a million years) changes.

Salinity changes, the deepening of ocean basins, and meteor impact have all been proposed as causative agents of extinction. Changes in the salt content of oceans would kill off marine organisms if there were suddenly less salt and would poison freshwater species if the salt content increased. If ocean basins became deeper, marsh organisms would be drowned or littoral life-forms would become stranded on dry beaches. Meteor impacts might have showered the Earth with dust and changed the weather much as a large volcanic eruption does now. The anoma-lously large iridium layer corresponding to the late Cretaceous extinctions may be an example of this phenomenon.

Changes in the polarity of the Earth are still another possible cause of extinc-tions. Some researchers have also argued that the level of extinctions and emergence of new groups has slowed because the more adaptable—meaning more highly evolved—organisms are better able to withstand environmental shock. Therefore they are more long-lasting or durable than those in past ages that were felled by small but, for them, signifi cant changes. Plants

and animals have been driven to extinc-tion by human activities, either from over-use or a loss of habitat.

An attempt to study the ecosystem and establish the consequences of extinction involves the sampling of ocean mud—the top layer of the ocean bottom. The mud holds the remains of creatures that have lived in the ocean for thousands of years.

This sediment is disturbed and churned by a large number of animals, such as crabs, clams, sea urchins, brittle stars, and marine worms. As these animals seek their food, they churn up the mud, thus oxy-genating both water and mud and making life possible in the mud. Looking at the rate of sediment mixing produces a model of the biota that lives in it and leads to prediction of what would be the result of losing species.

As larger organisms create more dis-turbance in any biome, they are the fi rst to become extinct. The loss of the large organisms results in less oxygen in the sed-iment, and that will push smaller organ-isms to extinction. Thus, some organorgan-isms have more impact on the rate of sediment mixing than others, and the extinction of those is critical to the overall health of a particular biome. Since the food web in the sea is intricately linked to what hap-pens in the sediment and how nutrients are generated, the sediment affects every creature that lives in the ocean.

The effects of human activity have dis-rupted the ocean bottom, in some places beyond recovery. There are “dead zones”

in the ocean resulting from the dumping of untreated sewage in some places or the introduction into ocean water of agricul-tural runoff that contains a high concen-tration of fertilizers. An example of the latter is the dead zone in the Gulf of Mex-ico. It is the result of fertilizers delivered to the area by the Mississippi River, and almost nothing lives there. See eutrophi-cation, pfisteria.

extremophiles

Organisms that live in regions of temperature and/or pres-sure that until recently would have been

extremophiles

called lifeless. The deep sea vents are an example of an extreme environment; they are covered with living organisms that can withstand the incredible pressure of a water column thousands of meters high and temperatures of 200°C or higher. At that pressure, the water does not boil. The organisms range from bacteria to giant worms, clams, crabs, and shrimp.

An ongoing investigation of extremo-philes is being conducted at the Hawaii Undersea Research Laboratory (HURL), which has been studying Loihi—the under-water volcano that will produce the next Hawaiian island—since 1988. This project is attempting to sample the organisms pres-ent, retrieve them, and maintain them in laboratories at the surface. See Alvinella, ice worm, Pompeii worm, Rimicalis, seeps, tube worms, vent communities.

eye

A real eye has a distinct transparent cornea covering a fl exible, spherical lens that receives light images, concentrates them, and directs them to an image-receiv-ing retina. This structure is composed of rods that receive size and shape informa-tion and cones that are color receptors.

This visual information is then transmitted to the brain by way of the optic nerve.

Almost all animal phyla have some photoreceptor system. In vertebrates, the eye is a brain substructure. In inverte-brates, the eye is an epidermal structure.

The blue “eyes” of scallops are light-sensitive tissue, not real eyes. Jellyfi sh, fl atworms, and echinoderms have ocel-lipigmented photoreceptors on their sur-faces. Many cephalopods have both eyes and light-sensitive vesicles near them. The vesicles are thought to have physiological

functions such as timing of reproduction.

Some polychaete worms have image-form-ing eyes, and crustaceans as a group have well–developed eyes and optic networks.

Therefore, what an animal “sees” is dependent upon the array of photosensi-tive tissue—both its quantity and concen-tration—and whether or not a cornea and lens exist, which determines the extent of motion detection possible. The presence of a lens separates animals that can really see from those that can only detect differ-ences in ambient light. Arthropods tend to have lenses of fi xed focal length, and some have an increased visible fi eld because they have eyes on stalks. A compound eye is not a collection of little eyes; instead, it is arrayed so that a number of receptors form an image by concentrating light. It may have advantages in motion detection.

The Alciopidae (segmented worms) have the best developed polychaete eyes. They are hydrostatically focused. The lenses of octapod eyes are moved by muscles.

In general, predators’ eyes tend to be more complex and capable of image formation than those of related species.

Predators with modifi ed eyes include the octopus. The eyes of deep-sea animals, including chimeras and gulpers, are fur-ther modifi ed; they are set forward on the head, have temporal rods that have longer light-sensitive areas adapted for dim sur-roundings, and have a wide binocularity that increases spatial perception. Some animals have tubular eyes—with the usual round lenses—as a space-saving device.

Modifi cations notwithstanding, bottom dwellers and fi sh in the abyssal depths do not have good vision.

eye

In document Desarrollo de un multibiosesor de adn para el diagnostico temprano de cancer de mama (página 54-57)