The great extinctions

 Since life appeared on Earth 3,6 billion years ago, millions and millions of species have appeared, flourished, and then become extinct only to be replaced by new forms. Over the course of evolution, the extinction of species is a common, almost everyday event that is integral to the dynamics of biological evolution, its "background noise".

Aside from the natural death of species, paleontologists have noted that over geological time there have bee periods of crsis characgterized by the disappearance of many groups of plants and animals. Over the last 600 million years, marine and terrestrial ecosystems have been virtually wiped out in 27 different crises. Of these, five are called "great extinctions," because they were so much more vast than the others: during each of them at least 20 to 50% of the families of invertebrates and vertebrates living in the oceans disappeared. Each time, the overall biological diversity of the planet was greatly diminished. The causes and consequences of these "biological catastrophes" are still poorly understood and the subject of much debate. In the following, we list the principal groups of plants and animals affected by each of the five great extinctions, as well as what scientists think may be the main causes behind these events.

At the end of the Ordovician Period (438 million years ago), about 22% of the families of invertebrates living in the ocean disappeared. At that time there was one vast southern continent, known as Gondwana, located over the South Pole that included Africa, South America, the Indies, Antarctica, and Australia. A glacial cap covered northern Africa. This extinction event is primarily known from fossil remains found in Europe and North America, and it seems to have occurred in successive stages over a period of several million years. Its main cause was climatic: the glaciation at the end of the Ordovician lead to global cooling. Gradually, planktonic organisms such as certain conodonts and acritarchs disappeared, as did organisms living in reefs or on the continental shelves such as the graptolites, and certain corals, brachiopodos, nautiloids, and trilobites.

The second great extinction occurred eigthy-eight million years later when the concentration of oxygen in the world's ocean bodies diminished radically, literally suffocating a large number of species out of existence. This phenomenon was probably related to the general rise in the level of the oceans at that time. The extinction was brief, lasting only from 10.000 to several hundred thousand years -but afterward, the planet's biological diversity continued to be impoversished for more than one million years. This crisis has been well studied in North America, the Rhineland, and the southern portion of France's Massif Central. Research shows that the conodonts and certain planktonic foraminifers were affected, as were reef-dwellers and the invertebrate and vertebrate fauna on the continental plateau, including not only sessile animals such as coral, and deep bottom dwelling such as trilobites and brachiopods, but also free-swimming vertebrate species like the placoderms and the acanthodians.

The most devastating extinction of ll time occurred at the end of the Permian Period, 250 million years ago. Over half the families of marine invertebrates disappeared, which meant the extinction of 75 to 90% of species. This is one of the most poorly understood extinctions because deposits from this period are rare (Greenland, the Southern Alps, Armenia, Pakistan, southern China, and South Africa). At that time, all emerged land was part of one gigantic super-continent, Pangaea, and the oceans probably had little influence on the climate of this enormous land mass. Seasons were very clearly differentiated, and the presence of evaporites (such as gypsum) and red formations indicate there were long periods during which vast areas became increasingly arid. In addtion, layers of basalt discovered in Siberia show there was heavy volcanic activity at this period. The climate, volcanic activity, and the drop in the sea level were probably the major influences behind this extinction, which spanned several million years. The last trilobites disappeared from the seas. Reef communities were decimated: certain coral (Tabulata and Rugosa) and bryozoans, and the fusulinids vanished. Among the terrestrial species, certain insects and mammmalian reptiles (therapsids), as well as almost 80% of the families of amphibians disappeared. Numerous plant groups were also wiped out: Bothyropteridales, Marattiales, Cordaitales, Trigonocarpales.

The fourth gret extinction occurred several million years later, at the end of the Triassic Period (220 million years ago). But this crisis, too, es poorly understood because there are few fossil remains. It probably stretched over several million years, and it seems that terrestrial environments were affected before marine environments. Among the land vertebrates, the principal victims were the labyrinthodont amphibians and mammalian reptiles (dicynodonts and cynodonts). Among the marine reptiles, only the icthyosaurs survived. The super-continent Pangaea also began to fragment at this time.

The las great extinction occurred at the end of the Cretaceous Period (65 million years ago). Certain researchers think it was caused by a meteorite colliding into the Earth and that the crisis lasted only a few years. Others pinpoint hevay volcanic activity as the cause and theorize that the crisis lasted at least 500.000 years. The most well-known victims were the dinosaurs, but marine organisms were also affected: certain nannoplankton, and the ammonites and belemnites disappeared.

The study of geological crises shows that:

1) All major extinctions are selective and contingeut. On one hand, not all organisms are affected in the same manner, and on the other, nothing indicates that the forms which disappeared were more vulnerable than those which suvived. At any given time and regardless of their specie's age, all organisms have an equal chance of surviving.

2) All major extinctions are linked to the appearance of new species. The "vacuum" left by forms which disappear is quickly filled by adaptive radiations in other groups.

The presumed causes for these major crises continue to provoke intense debate. The causes may have been external, such as asteroids falling to earth or the passage of comets; or they may jave been internal to terrestrial systems, for example volcanic eruption or a drop in sea level; or they may have been due to a combinatino of both internal and external factors, as in the case of major climatic upheavals.

During these geological catastrophes, the biological diversity of plant and animal communities is drastrically reduced. But after each such event, diversity is restored and groups that had previously lacked variety undergo some remarkable adaptive radiations.

Jean-Lous Hartenberger

Director of Research

University of Montpellier.

Les plantes, cheval de Troie des insectes pour la conquête du milieu terrestre

 Pendant très longtemps, la vie a été confinée aux milieux aquatiques. C'est dans la mer que se différencieut, il y a plus de 500 millions d'années les principaux groupes d'animaux pluricellulaires. C'est aussi dans la mer que se diversifient les algues vertes, ancètres prêsumés de toutes les plantes terrestres. Les premières traces fossiles incontestables d'occupation du milieu terrestre,datèes de 500 Ma environ sont des spores, semblables à celles des mousses actuelles. Ces élèments reproducteurs unicellulaires possèdent une paroi protectrice qui leur permer de résister à la dessiccation. C'est cette paroi qui est conservée à l'état fossile, alor que le reste de l'organisme est très rapidement décomposè.

Ce sont donc très probablement des proches parents des mousses qui ont èté les premiers organismes pluricellulaires vivant sur les terres émergées. Un peu plus tard sont apparues des formes apparentées à nous fougères actuelles, d'abord connues, elles aussi par leurs spores. Vers 420 Ma, leurs restes sont trouvés associés à des fossiles dànimaux arthropodes, manifestement prédateurs. Il existait donc dans les mêmes milieux des animaux qui leur servaient de proies. Ces espèces inconnues devaient se nourrir de végétaux vivants ou morts. Leurs présence était donc conditionnée par celle des végètaux: c'était donc conditionnée par celle des végètaux: c'ètait le début d'une longue histoire commune.

A l'époque carbonifère, la diversification explosive des plantes terrestres offre aux animaux une grande variété de lieux de vie et de sources de nourriture. Alors débute la conquète des continents par les vertébrés, tandis que les arthropodes terrestres, et tout particulièrement les insectes, acquièrent une imporessionnate diversité, montamment dans le domaine de l'alimentation. Les ons, apparentés aux blattes actuelles, se nourissent de débris. D'autres exploitent des productions végétales à haute valeur nutritive: le tube digestif de certains insectes fossiles est rempli de spores, et l'on connaît des graines fossiles portant des perforations probablement dues à des insectes dont le corps et les ailes ressemblent à ceux des blattes mais dont la tête porte au voisinage de la bouche des stylets piqueurs qui font plutît penser aux pièces buccales des cigales. D' autres insectes encore, très vraisemblablement issus de la même souche que les actuelles libellules sont, comme celles-ci, des prédateurs d'autres insectes.

La défense des plantes contre les insectes.

Ce qui prècède laisse penser que le développement des premiers peuplements végétaux en milieu terrestre a permis la prolifération de nombreux insectes herbivores, qui, en retour, ont ravagé les plantes. Si des plantes mutantes accumulaient dans leurs feuilles ou dans leurs graines des substances toxiques pour les insectes, elles étaient moins sujettes que leurs voisines à la destruction. De telles mutations avaient donc toutes les chances d`être retenus par la sélection naturelle. C'est certainement la dèfense contre les insectes, elles étaient moins sujettes que leurs voismes à la destruction. De telles mutations avaient donc toutes les chances d'être retenues par la sélection naturelle. C'est certainement la défense contre les insectes ou, plus généralement, contre les herbivores qui explique la toxicité de très nombreuses plantes. C'es défenses chimiques sont très variées: certaines plantes produisent du cyanure, par exemple dans les graines (pècher, amandier), d'autres produisent des alcaloides (la nicotine), d'autres encore des terpènes (Abondants dans la résine des conifères).

Ces défenses sont certes efficaces, mais le mécanisme mutation-sélection n'a pu manqué de faire apparaître des insectes résistants aux toxiques, ce à quoi les plantes ont dú répondre par un perfectionnement de leurs défenses. Une illustration des résultats de cette escalade est fournie par les relations entre conifères et scolytes (ceux-ci sont des insectes dont les larves creusent des galeries dans le bois). Les conifères possèdent dans leur résine non pas un, mais plusieurs terpènes différents, ainsi qu'éventuellement d'autres composés toxiques. Ces toxiques sont synthétisés de façon continue, et ils sont volatils. Les insectes en perçoivent la présence et, en général ne s'aprrovhent pas des arbres qui les émettent. Certains scolytes, toutefois, ne sont pas repoussés et attaquent tout de même le bois. Cette attaque stimule en général la production de terpènes au niveau de la blessure de l'arbre. Certaines espèces de scolytes, résistantes aux terpènes, n'en sont nullement incommodées et même réémettent ver l'extériur un des terpènes qu'elles avaient ingérés, ce qui constitue un signal attractif pour les insectes de la mème espèce. Tout ceci résulte d'adaptations réciproques qui se sont succédées au cours de l'évolution: une espèce d´terminée de conifères est protégée par ses défenses vis-à-vis de beaucoup d'insectes, mais parmi eux quelques espèces ont tourné les défenses et s'attaquent de façon quasi exclusive à elle.

Coopération entre plantes et insectes

L'exemple précédent montrait comment une adaptation réciproque entre plantes et insectes pouvait résulter d'un antagonisme entre agresseur et agressé. Il existe entre ces deux catégories d'ètres vivants d'autres types d'interactions. Ainsi, en volant d'une fleur à l'autre, des insectes transportent du pollen, facilitant ainsi la reproduction de la plante: c'est avantageux pour elle, même si l'insecte prélève une partie du pollen pour sa nourriture. De ce fait, la sélection naturelle a retenu chez les plantes des dispositifs attirant les insectes vers les fleurs et chez les insectes l'aptitude à répondre efficiacement à ces dispositifs. Ainsi, chez la sauge, la fleur, colorée et odorante, attire abeilles et bourdons qui viennent en particulier y chercher du nectar. Par comparaison avec d'autres espèces de la même famille (thym, menthe), on voit que la lignée évolutive qui a conduit à la sauge a perdu deux étamines sur les quatre que possédaient les espèces ancestrales et que de plus les étamines restantes ont perdu la moitié des organes dans lesquels se forme le pollen. En compensation, ces étamines ont développé des organes en orme de pédales que l'insecte abaisse obligatoirement quand il s'enfonce dans la fleur à la quéte du nectar. Cela entraîne un mouvement des étamines tel que l'insecte reçoit du pollen sur son dos, pollen qu'il pourra ultérieurement transporter sur une autre fleur. La sélection a ici retenu des mutations qui apparemment réduisaient la fertilité de la fleur (production de pollen divisée par quatre). Ces mutations étaient en réalité favorables. Elles multipliaient, par un facteur certainement supérieur à quatre, les chances d'être réellement fécondant pour le pollen restant, à condition, bien sûr, que des insectes (compétents" soient bien là!

Les diverses lignées évolutives ne sont donc pas indépendantes les unes des autres. Les caractères acquis par l'une d'entre elles conditionnent l'évolution des autres. On ne peut espérer comprendre l'évolution sans tenir compte des divers types de relations entre espèces.

Les flèches fines indiquent le déplacement des sacs à pollen
quand un insecte appuie sur la "pédale" en s'enfonçant dans la fleur de sauge.

Jean Génermont

Professeur, Paris XI

AIDS: evolution and viral strategies

 The virus which causes AIDS manages to survive in the very hostile environment of the human immune system. It does this by forming an extremely polymorphous population in which at least one individual is capable of resisting any given attack. The result is Darwinian evolution on a time-scale of about one week.

Viruses and lentiviruses

At the beginning of the 20th century, pioneer virologists believed all diseases of viral origin involved the inflammation and destruction of the infected organs. However, this proved to be true only for so-called "acute" viral infections which are short-lived and whose clinical symptoms disappear when the virus is destroyed by our immune defennses. This original virus theory had to be re-examined when it was discovered that certain viruses cause chronic infections and are capble of surviving in the organism for long periods of time without damaging tissue.

Thus, in the 1950s, the concept of slow viral infection was developed. Slow viral infection involves a very long and preliminary asymptomatic incubation period before clinical symptoms of the disease appear. Today, many "slow" viruses have been identified. HIV (Human Immunodeficiency Virus), the AIDS virus, belongs to one family of these persistent viruses, the lentiviruses (or retroviruses).

"Acute" and "chronic" viral strategies

Acute and persistent infections are, in fact, two different strategies used by viruses that live in the very hostile environment of the immune systems in higher animals.

Broadly speaking, the "acute" strategy takes advantage of the initial quiet period, the first weeks following the invasion while the body is preparing the immune response. The virus uses this time to reproduce and/or to be transmitted to another individual, before it is destroyed by the immune system. Therefore, for this kind of virus, rapid reproduction and maximal contagiousness are essential from the outset. "Acute" viruses are treated by shortening the time required for the immune response, for example by "alerting" the immune system in advance with a vaccine that provokes the needed antibodies.

The essential problem for lentiviruses, however, involves resisting immune system attacks from the organism in which it must live. This kind of virus does not need to be contagious to survive in the long-term. It only has to be transmitted before the death of its host. The virus's long-term survival is guaranteed when it is transmitted naturally through sexual contact or during childbirth, as is the case with HIV.

Lentiviruses represent a major challenge to immune response and render vaccination problematic.

Viral infection and immunity

There are two components to the body's immune response. The so-called "humoral" response primarily involves the production of antibodies (or immunoglobulins) which are capable of fixing themselves specifically to the invading microoorganism in the blood and lymph. The cellular response relies heavily on lethal white blood cells, called cytotoxic T lymphocytes, that destroy infected cells. Since lentiviruses spend a large part of their life cycle inside cells where antibodies cannot reach them, an organism infected by a lentivirus must depend on the second weapon in the immune response: the cellular response.

Cytotoxic T lymphocytes (commonly kown as "T-cells") can identify and destroy infected cells. But the organism pass a high price for this protection: in destroying the virus' ecosystem, the organism attacks its own cells. The situation is all the more dramatic because the virus sometimes escapes T-cell attack and the cells destroyed are a pure loss for the organism.

Viruses realted to HIV are found in numerous animal species. They are all derived from a single common ancestor, but do not necessarily produce diseases like AIDS. None of them except the primate varieties can infect humans.

Features of immune response

The immune response is provoked when a foreign protein, known as an antigen, is detected in the organism. Response is highly specific: a killer lymphocyte is "programmed" for a certain antigen and cannot recognize the enemy if it changes shpae. Therefore, a virus which constantly modifies its proteins can escape the immune response.

This kind of high genetic variability is one of the essntial features of HIV. This variability results from errors during virus reproduction: the enzymes that copy viral genes sometimes do not produce exact copics. Each error, or mutation, results in a structural variation in the virus. For HIV, there is about one mutation per copy. Every HIV virus is thus unique and, even though all members of the viral population that cause AIDS are related, they are all different. Within these populations we can sometimes even find important differences in biological properties between viruses, particularly in regard to the kinds of cells they infect (cellular tropism), their toxicity (cytopathogenic effect), and the speed with which they replicate.

This diversity of properties within the viral population gives the virus an extensive adaptive capacity. At least one individual virus will always manage to survive any attack to the population as a whole, and it will be the forefather of future populations.

Rapid evolution of the virus

The evolutionary schema of HIV -mutation followed by natural selection- is unique in two ways. The first is the viral ecosystem, which in this case is the organism of the infected host, where the primary predator threatening the survival of the virus is the immune response. The second is the rapidity of viral evolution, due to the high percentage os errors during viral replication, as well as to the "solidarity" among individuals infecting any single cell.

The high rate of reproductive errors produces a large number of so-called "defective" individuals that can eventually threaten the viral population's vital functions. To deal with this risk, the virus can maintain its population at a sufficient level, while individuals infecting the same cell can exchange certain proteins and even genes.

All these mechanisms enable HIV to form extremely polymporphous populations always maintaining its vital functions, and to rely on this variability to survive in the highly selective environment of the organism.-8

Antiviral strategies

Given the rate of replication errors, there is a probability of about 10 per reproductive cycle that a virus can survive any attack which can be avoided through a single mutation. The most effective means for reducing this probability is to use vaccines and drugs to attack multiple viral targets so that a combination of mutations are required before a resistant generation of mutants can arise. The virus' probability of surviving is estimated at 10^-6 per cycle when two mutations are required, at 10^-15 for three, at 10^-29 for four, and so on. Resistnace to AZT, the first antiviral drug used against HIV, requires two or three mutations, in other words a period of about six months.

It would be unreasonable to expect the antiviral drugs currently available to provide complete cures, but progress in handling and, above all, in combining these drugs should enable us to more effectively slow the infernal cycle that leads to AIDS.

Pierre Sonigo

Director of Research, CNRS

Cochin Institue of Molecular Genetics

Modification of environments and diversification of human societies

 Homo sapiens spread from the African continent and progressively colonized all terrestrial environments. Every human society gradually transformed these environments for its own benefit by adapting their techniques for expliting natural resources.

The impact of Paleolithic hunter-gatherers

The activities of Paleolithic societies had little impact ou plants and animals because human populations were so sparse and relied ou primitive stone tools. Nontheless, these groups undoubtedly contributed to the extinction of certain wild game, such as the mammoth and the European wooly rhinoceros, slow-reproducting species that had been weakened by changed ecological conditions due to climatic variations. Small, isolated populations of endemic mammals, including the hippopotamus and dwarf elephant on the islands in the eastern Mediterranean, met similar fates. Early man prized these animals not only for their meat, but also for their hides, bones, horns, and antlers which provided the raw materials for clothing and tools.

Hunting scene of a Wolly rhino

The effect of fire

Fire, used by early man to flush game from its hiding places, had a freater environmental impact than hunting. This burning encouraged plant species that were resistant to fire and aided the development of savvanas and prairies. The burn technique also may have been used to enhance regrowth of certain edible plantas or to attract grazing animals by regenerating dry prairies, a method used by ranchers today.

Nonetheless, the large quantities of wood coals found in certain prehistoric geological strata remain difficult to interpret. Were the fires that produced them provoked naturally by lighting, or set by man? And in the latter case, were hunters, shepherds, or farmers responsible?

The growth of farming economies

With the appearance of farming 12.000 years ago, the impact of human societies on the environment changed radically since farming involves replacing natural vegetation with plants seeded or bedded by man.

In some cases, the first farming practices preceded the domestication of plants and animals. This was true in the Near East where agriculture was invented by populations that had been sedentary for 2.000 years. Elsewhere, domestication of plants and animals did not necessarily mean the transformation to a farm economy. In Peru, for example, certain nomadic groups continued to live by hunting and gathering even though they farmed a few plants and had domesticated animals.

The hunter-gatherer societies were not all composed of small itinerant groups. Some moved only within a defined territory, while others established permanent homes. Still others had begun to diversfy socially and established hierarchical societies; this was true of the coastal Indians in the American northwest, famous for their "potlathc" art and rituals, who stored acorns in silos and conserved salmon by smoking them.

Generally speaking, the hunter-gatherers were not the societies we imagine, wandering perpetually in search of some pittance and always on the verge os starvation. On the contrary, they had a wealth of natural resources available in their very rich environments, which were then gradually occupied by farming peoples.

The impact of farm practices

The first forms of agriculture had limited impact on the environment. But there can be absolutely no doubt early farming launched a process of environmental change.

Initially, seeds were sacttered directly on the ground, with little soil preparation, particularly in the wooded steppes of the Near East and regions where plant cover was not abundant. But gradually, specialized tools and irrigation techinques were developed.

The fist farmers selected plots that were easy to work, land that could be made fertile through irrigation or which was naturally fertile due to flloding, as in the Nile Valley.

In forested regions, the land had to be cleared withs axes (first made os stone) and fire for planting. This agriculture is often called "itinerant" because these farmers had to abandon their fields regularly and clear others. This is because, after a year or two, the wood ash no longer had any fertilizing effect, and menawhile weeds and natural vegetation invaded and regained control.

But changing fields did not necessarily mean that these farmers moved their homes. After a certain time, the original vegetation flourished again on the abandoned field and it could be recleared. The organic matter produced when the field was left fallow served to regenerate the soil. According to prehistorians, this technique was practiced by the first European farmers, and it is still used today in tropical forests.

However, as populations grew so did their need for additional territory. They were thus forced to clear their old fields before the forest was able to fegenerate itself. Certain species of trees became rare and others simply disappeared. Eventually, only those species belonging to the secondary vegetation, particularly those known as "pioneer" species which require sunlinght to sprout, survived.

Confronted with the need to produce crops regularly, farmers invented techniques making it possible to cultivate the same plots over and over again. Weeding and use of organic fertilizers, often supllied by animal waste, became common practices.

Resource management in early societies.

Each society was thus obliged to organize the agricultural use of its land, to provide for crop rotation and protection of useful tree species. When land was cleared, certain trees were merely pruned. Animal breeders also had to select pasture land where their grazing herds would not damage crops.

Numerous societies utilized live hedges to enclose herds and protect useful tree species. Elsewhere, protection measured led to the development of areas dominated by a single species, such as the karité or palm oil "parks" in Africa. In temperate climes where needs for firewood grew very rapidly, forestry was "rationalized". Oaks, for example, were conserved and the wood reserved for building because the acorns served as food for pigs.

The shaping of contemporary landscapes.

Over time, man's influence spread to increasingly greater areas. This led to a radical transformation of the vegetational cover, which was dominated by cultivated domestic species and self-propagating plants, both adventitious species on farmland and ruderal species around dwellings.

Most contemporary landascapes are influenced in this way by man, and the structure of plant populations is entirely dependant on human activities.

Herds play an important role in this transformation, particularly because they crush and selectively consume new shoots. Overgrazing can have dramatic consequences, because without vegetational cover, the bare soil erodes rapidly, particularly in hilly regions with heavy, seasonal rains.

Development of inhabited areas

Areas reserved for homes increase with the population; population growth in turn increases the needs for natural plant and animal resources. Certain animal species can only are attracted to and subsist on farmlands. Man either relies on this phenomenon to increase his source of wild game, or has to combat the invasion of destructive animals.

In regions of growing population density, certain techniques are used to increase the amount of arable land: in mountainous areas, terraces are built to control water run-off and limit erosion; syampy regions are drained; dry areas are irrigated, etc.

Man eliminates and creates environments that are favorable to certain species, or to certain varieties within a species, as it suits him. In his own territory, he maintains biological diversity if it furnishes things he needs. But elsewhere, in regions far from his own habitat, he does not hesitate to destroy this diversity, exploiting the resources for his own profit, despite the fact other human beings may depend on them.

Storks in a garbage can

Prof. Claudine Friedberg

National Museum of Natural History

Selection of cultivated plants: the case of the cabbage

 Man has cultivated wild plants to meet his many needs, and through selective breeding has modified these plants profundly. The cabbage (Brassica oleracea) provides a good illustration of this process.

Energy reserves and growth cycle

The wild cabbage is a biennial plant: its reproductive cycle normally stretches over a two-year period. In the first growing season, a very short stem with a compact head of leaves appears. These leaves are an active center of photosynthesis; the resulting substances are stockpiled in either the leaves themselves or the base of the stem. When winter comes, this activity ceases almost entirley. In the second spring, photosynthesis resumes, but the winter cold has completely modified the cabbage's growth pattern: the stem lenghtens quickly, bearing fewer and smaller leaves than it did at the base, and culminates in a flowered tip which will enable the plant to reproduce. Finally, when the cabbage reaches a height of few dozen centimeters, it produces several hundred seeds and dies. Thanks to the abundant store of energy reserves accumulated in the first year, this second-year phase of rapid growth, flowering, and seed productino lasts only a few weeks.

Selection: kohlrabi, broccoli, caulifrower, and more.

The cabbage's energy reserves make it a perfect food for herbivorous and onmivorous animals. Man cultivated it for its nutritional value. Later, growers selected those plants that seemed to offer what they considered the finest in terms of food quality. From generation to generation, they have increased the cabbage's nutritive value -in others words its reserves, and improved the distribution of these reserves in the plant. By selecting plants with the most abundant reserves possible, regardless of their distribution, growers produced the fodder cabbage. Its heavy stem lengthens markedly even during the first year and bears thick leaves. Although both the stems and leaves of the fodder cabbage are extremely high in nutrientes, they are not very tender, so this type of cabbage is used exclusively for animal feed. The common garden cabbage was developed by selecting plants which produce a head of numerous tight leaves in the first year and which store their reserves in the leaves rather than the stem. The leaves are especially tender and are consumed primariyly in autumn or winter, but in any case before the growth phase in the second year.

By taking the opposite approach and selecting plants which accumulate their reserves in the base of the stem rather than the leaves, growers obtained the kohlrabi, or turmp cabbage.

As for the brussels sprout, it is characterized by important lenghening of the lower portion of the stem, which bears large tender buds rich in nutrientes, during the first year. The stem is tipped by a cluster of leaves without much food value.

Broccoli was obtained by exploiting the fact that early in the second year, the growing stem is tender and pleasant tasting. Therefore, breeders selected cabbages with particularly fleshy stems that can be eaten just as the first flowers are forming.

The cauliflower is the result of selecting cabbages in which the stem grows very slowly in length, but thickens considerably because flower formatino is particularly late. There are numerous varieties of cauliflower. Somo of these, like the wild cabbage, require cold winter temperatures to stimulate the development of the so-called flower bearing stem. These varieties are seeded in summer and not harvested until the following spring. However, other varieties have also been selected in which the cauliflower-like stem develops spontaneously when the plant reaches a certain age. These varieties are seeded in spring and harvested in summer or autumn, depending on their growth rate. These examples illustrate how growers have modified characteristics of the reproductive cycle, allowing us to farm cauliflower at almost any period of the year by selecting the appropriate varieties and planting schedules.

All these vegetables are very different from the wild cabbage, not only in their shapes and sizes, but also in their reproductive cycle. They have been obtained by manipulating the wild cabbage's spontaneous variability. But if this is true, why aren't cabbages resembling cauliflowers or kohlrabi ever found in nature? The answer is simple: these types are less well-adapted than wild cabbage to surveve and reproduce in natural conditions. They only survive on farms because of fertilizers, weeding, and other care. It is remarkable to note that all these extremely different members of the cabbage family can crossbreed, producing vigorous and fertile hybrids (resembling wild cabbage more than either of their parents, and so useless to man). They all belong to the same species, Brassica oleracea, and probably only differ from wild forms in a relatively small number of genes-nonetheless, these genetic differences yield spectacular effects.

Similarly, many plants cultivated by man are nothing more than particular, grower-selected varieties of species we are familiar with in nature. This statement is even more true for animals, since it clearly seems that efforts to breed new races of domesticated animals have never produced a new species.

Prof. Jean Génermont

University of Paris XI

Petites causes, grands effets

 La ressemblance entre l'homme et les grands singes africains (gorilles, chimpazés) est frappante. Les donées de la biologie moléculaire confirment que ces espèces son très fortement apparentées. Leur dernier ancêtre commun pourrait avoir vécu il y a environ dix millions d'années. Si frappantes que soient les ressemblances, il est aisé de reconnaître, dans le domaine anatomique, de nombreux traits caractéristiques de l'espèce humaine.

L´homme est un marcheur bipède

Le mode de déplacement ordinaire de l'homme est la marche, utilisant exclusivement les deux membres postériurs. Gorilles et chimpazés son certes capables eux aussi de marcher sur leurs pattes postérieures, mais ce mode de locomotion est, chez eux, transitoire. Ils se déplacent fréquemment dans les arbres en utilisant leurs quatre membres; en outre, lorsqu' ils sont à terre, ils prennent appui sur leurs mains. A ces différences de comportement locomoteur correspondent de profondes différences de fonction du bassin. Le bassin est une formation osseuse qui, d'une part est soidaire de la partie de la colonne vertébrale appelée sacrum, d'autre part est articulée avec le fémur. Comme on le voit bien en comparant les silhouettes d´un gorille et d'un homme, la particularité du bassin humain est de suppoerter la totalité du poids du tronc et d´assurer le maintien de celui-ci en position verticale stable.

Cést possible grâce à une forme très diffèrente: le bassin de l'homme est à la fois plus large et bien moins long, tout particulièrement dans sa partie supérieure, l'ilion, que celui du gorille ou d'un autre singe.

FOTO

L'acquisition d'un bassin court et large, à patir du bassin ancestral de type singe, a été un des événements majeurs de l'évolution de la lignée humaine. Les australopithèques qui vivaient il y a quelque quatre millions dánnées, et dont on sait par des empreintes de pas quíls étaient bipèdes, possédaient un bassin de type humain.

Une autre caractéristique humaine liée à la bipédie est la longueur des membres postérieurs. La comparaison de foetus et d'adultes de groilles et d'hommes, à l'aide de dessins exécutés en choisissant une échelle telle que la "taille assis" paraisse constante, montre des proportions relatives très similaires chez les foetus. Chez l'homme, la croissance en longueur de la cuisse est par la suite très fortement accélérée par rapport à celle du gorille. On voit ici l'importance des écarts de croissances relatives des différents organes dans la divergence entre lignées évolutives.

Après les jambes, la tête

La comparaison des crânes de l'homme et du gorille révèle de pronfodes différences de forme. La face et les mâchoires du gorille sont très développèes. Ces parties sont bien plus réduites chez l'homme qui possède en revanche une boîte crânienne proportionnellement très voluminuese. Les différences de proportions entre les deux espèces sont nettement moins importantes chez les foetus que chez les adultes. Les caractères propres aux deux expèces sont donc pour une large part dus à des différences de croissances relatives lors de phases tardives du développement.

Ces différences son liées à des différences de voume cérébral. Le volume du cerveau d'un gorille mâle n'atteint qu'exceptionnellement 600 cm3, et les cerveaux des autres grandes singes sont encore plus petits, alors que le cerveau d'un homme actuel dépasse en général 1000 cm3. Les autralopithèques bipèdes, ancètres de l'homme moderne, qui vivaient il y a quelque quatre millions d'années, avaient un cerveau de 400 à 500 cm3, soit à peine plus qu'un grand singe actuel de même stature (ces autralopithèques mesurainent à peu près 1,20 métre). Ce n'est que progressivement que sèst réalisé dans la lignée humaine làccroissement du volume cérébral. Un grand pas semble avoir été franchi il y a environ deux millions d'années: des fossiles datant de cette époque, rangés dans le genre Homo habilis, sont remarquables par un volume cérébral atteignant 700 cm3, et surtout par la présence sur la boîte crânienne de traces attestant de l'existence dans le cerveau d'une zone particulière dite aire de Broca. Cette région joue chez l'homme actuel un rôle crucial dans l'aptitude au langage articulé. Il est tr'es vraisemblable que cette aptitude est apparue chez Homo habilis il y a quelque deux millions d'années, puisque l'aire de Broca semble absente chez les autralopithèques de la même époque.

Jean Génermont

Professeur, Paris XI

La extraña herida de bala que cambió la historia de la medicina

 Aunque los médicos del siglo XIX tenían un buen conocimiento de la anatomía humana a partir de la disección de cadáveres, su incapacidad de ver el interior de los cuerpos vivos obstaculizaba su comprensión del funcionamiento de los órganos internos. La espantosa cirugía de esa época no era una opción para la observación y experimentación in vivo. Pero un accidente con herida de bala dejó a la vista una extraña secuela y, gracias a ello y a la aún más inédita relación de colaboración que se estableció entre el paciente (el aventurero canadiense Alexis St. Martin) y el médico (el cirujano estadounidense William Beaumont, nacido un 21 de noviembre), la comprensión del funcionamiento del cuerpo humano cambió para siempre. 

Los médicos que querían entender lo que pasaba “bajo el capó” se limitaban a mirar por la garganta, sentir con los dedos o escuchar atentamente con un estetoscopio. Pero eso cambiaría en el verano de 1822. El 6 de junio de ese año, en el puesto de comercio de pieles de la isla de Mackinac en el lago Huron en el territorio de Michigan, un mosquete sostenido por descuido se disparó accidentalmente, disparando a un joven y saludable voyageur y trampero canadiense-francés llamado Alexis St. Martin en el pecho a corta distancia. La herida era tan grave que nadie esperaba que sobreviviera, pero sin embargo fue atendido por el médico más cercano, William Beaumont, un cirujano del Ejército de los Estados Unidos destinado en el cercano Fort Mackinac. El médico escribió más tarde que “encontró una porción del pulmón del tamaño de un huevo de pavo que sobresalía por la herida externa, lacerada y quemada; e inmediatamente debajo de ésta, otra protuberancia, que […] resultó ser una porción del estómago […] vertiendo la comida de su desayuno por un orificio lo suficientemente grande para admitir el dedo índice”.

William Beaumont.

Estos dos hombres, Beaumont y St. Martin, cirujano y paciente, difícilmente podrían haber sido más diferentes. Beaumont, descendiente de puritanos de Nueva Inglaterra, era un hombre ambicioso y capaz, eminentemente práctico, que buscaba posición y riqueza a través del trabajo duro y el ahorro. En cambio, St. Martín, nueve años menor que el médico, era un analfabeto católico francófono de los bosques de Quebec, flaco y fuerte, con gusto por el licor y una notoria reputación de comportamiento escandaloso.

Durante semanas después del accidente, el trampero se aferró tenazmente a la vida, luchando contra las infecciones y la fiebre alta. Todo lo que comía se le pasaba por el agujero de su estómago y se le mantenía vivo “por medio de nutritivos enemas” administrados por su atento cuidador. Pero contra todo pronóstico y para sorpresa de todos, el robusto joven logró sobrevivir y comenzar su lenta recuperación.

UN SUPERVIVIENTE POR SORPRESA Y UNA MENTE CURIOSA

Después de muchos meses de intentar sin éxito cerrar el orificio mediante la aplicación de presión, Beaumont quiso suturar los labios de la herida, pero St. Martín ya había soportado demasiadas operaciones dolorosas y se negó a someterse al procedimiento. Sin embargo, la herida se curó por sí misma, con el borde del orificio del estómago fusionándose con la abertura de la piel y el revestimiento del estómago formando una especie de válvula y “pareciéndose a un ano natural con un ligero prolapso”. Esta fístula gástrica permanente, como se conoce médicamente, impedía que la comida se escapara, pero cedía bajo la presión de un dedo, permitiendo el acceso directo al estómago. “Al presionar cuando el estómago está lleno, el contenido fluye abundantemente”, señaló Beaumont.

Retrato de Alexis St. Martin, a la edad de 81 años.

Habiendo perdido su empleo en la American Fur Company debido a su discapacidad, y ahora sin un duro, St. Martín estaba en grave peligro de ser enviado de vuelta a su Quebec natal, un viaje de 2.500 kilómetros en barco. Beaumont, temiendo que su paciente no sobreviviera al largo viaje, llevó al joven a su casa para cuidarlo. En la primavera de 1824, St. Martín había recuperado totalmente su salud, para entonces ya trabajaba como sirviente y manitas en la casa de Beaumont; y el cirujano estadounidense, que reconocía la oportunidad científica que el destino le había deparado, estaría libre por la tarde para realizar experimentos con su huésped, el dueño del único estómago de la tierra al que se podía acceder directamente desde el exterior.

EXPERIMENTAR Y OBSERVAR SIN PREJUICIOS

Poco se sabía en ese momento sobre el funcionamiento del estómago humano y el sistema digestivo, y se debatió en la comunidad médica sobre el método por el cual el estómago digería los alimentos. Algunas teorías sugerían que el proceso de trituración o mecánico era dominante, mientras que otras argumentaban a favor de los procesos químicos, la fermentación, la putrefacción o la desintegración por calor. 

Al no tener ninguna teoría en particular para avanzar, Beaumont era libre de experimentar y observar. En un artículo publicado en The American Medical Recorder en 1825, escribió: “Este caso ofrece una excelente oportunidad de experimentar con los fluidos gástricos y el proceso de digestión. […] Uno podría introducir varias sustancias digeribles en el estómago, y examinarlas fácilmente durante todo el proceso de digestión.” 

Aunque el médico no era un investigador experimentado, era diligente, observador y mantenía apuntes meticulosos. Sus experimentos, sin duda desagradables y a veces humillantes para St. Martín, consistieron inicialmente en atar diferentes trozos de comida a una cuerda e insertarlos a través del agujero del estómago para medir las tasas de digestión. En experimentos posteriores, Beaumont abría el agujero y miraba dentro, observando el proceso digestivo en una amplia variedad de situaciones. Se lanzó a sus experimentos y llegó a probar los fluidos gástricos y la mucosa del estómago para tratar de determinar su contenido. También extrajo grandes cantidades de jugo gástrico para experimentar y envió muestras a Europa para su análisis.

Seno y costado izquierdo, con la abertura llena con la válvula.

Estos experimentos continuarían a trompicones mientras Beaumont era transferido de un estado fronterizo a otro antes de establecerse finalmente en St. Louis, Missouri. En una ocasión, St. Martin volvió a Canadá, donde volvió a entrar en el comercio de pieles, se casó y tuvo hijos, pero su situación de pobreza junto con las incesantes súplicas de Beaumont lo convencieron de volver a EE.UU., viajando más de 2.000 millas con su creciente familia a su lado, para someterse a más experimentos intrusivos del doctor a cambio de un sueldo.Con el paso del tiempo, St. Martín se cansó de ser un conejillo de indias y exigió más dinero. En 1832, con el fin de aportar cierta seguridad jurídica a la situación, Beaumont hizo que St. Martín (que no sabía leer) firmara un contrato en el que se comprometía a “someterse […] a los experimentos fisiológicos o médicos que el mencionado William dirigirá o hará realizar sobre o en su estómago, el mencionado Alexis […] y obedecerá […] a la exhibición y exposición de su mencionado estómago”.

LA DIGESTIÓN ES PRINCIPALMENTE UN PROCESO QUÍMICO 

Finalmente, después de más de 200 experimentos llevados a cabo durante ocho años, esta extraña colaboración llegó a su fin, y en 1833 los dos hombres se separaron para siempre. St. Martín y su familia regresaron a Quebec y a una existencia de pobreza, mientras que Beaumont publicó su relato de los experimentos, Experiments and Observations on the Gastric Juice, and the Physiology of Digestion, obra que le aportó el reconocimiento y el prestigio que buscaba.

Beaumont sacó muchas conclusiones sobre el proceso digestivo a partir de sus experimentos, la principal de las cuales fue que la digestión es principalmente un proceso químico, asistido por contracciones musculares, poniendo así fin al debate médico. Descubrió que el jugo gástrico contenía ácido muriático (clorhídrico) y otras enzimas y que era secretado por el revestimiento del estómago en presencia de alimentos. Descubrió que la carne o los alimentos con almidón se digerían más rápidamente que las verduras y señaló la importancia de la fibra en la dieta. El amor de St. Martín por la comida y la bebida mostró que el alcohol, así como comer en exceso, eran causas de irritación gástrica. 

El “Padre de la Fisiología Gástrica” construyó su reputación en su habilidad de acceder al estómago singular del trampero canadiense y hasta el final de su vida persiguió a St. Martín para que se mudara a St. Louis, Missouri, para continuar los experimentos. Sin embargo, el comerciante de pieles nunca regresó. St. Martin finalmente tuvo seis hijos y murió a la edad de 86 años en 1880, sobreviviendo a Beaumont, quien expiró en 1853 a la edad de 67 años después de resbalar en unos escalones cubiertos de hielo.

Neil Larsen

Fuente: https://www.bbvaopenmind.com/ciencia/apuntes-cientificos/la-extrana-herida-bala-cambio-la-historia-la-medicina/#:~:text=Aunque%20los%20m%C3%A9dicos%20del%20siglo,funcionamiento%20de%20los%20%C3%B3rganos%20internos.