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How do deep-diving sea creatures withstand huge pressure changes?
Paul J. Ponganis and Gerald L. Kooyman of the Center for Marine Biotechnology and Biomedicine at Scripps Institution of Oceanography provide the following answer.
A sperm whale can dive down more than 2,000 meters and can stay submerged for up to an hour.
(A) Some sea creatures exploit great depths. The biggest physiological challenges in adapting to pressure are probably faced by those animals that must routinely travel from the surface to great depth. Two such animals are the sperm whale and the bottlenose whale. From the days of whaling, these animals have been recognized as exceptional divers, with reports of dives lasting as long as two hours after they were harpooned. Today, with the use of sonar tracking and attached time-depth recorders, dives as deep as 6,000 feet (more than a mile below the surface of the ocean) have been measured. Routine dive depths are usually in the 1,500- to 3,000-foot range, and dives can last between 20 minutes and an hour.
(B) Loss of gas exchange at depth has another important implication: the lungs of the deep diver cannot serve as a source of oxygen during the dive. Instead deep-diving whales and seals rely on large oxygen stores in their blood and muscle. Several adaptations enable this. First, these animals have mass specific blood volumes that are three to four times those found in terrestrial mammals (i.e., 200 to 250 milliliters of blood per kilogram body mass, in contrast to a human value of 70 milliliters blood per kilogram). Second, the concentration of hemoglobin (the oxygen-transport protein in blood) is also elevated to a level about twice that found in humans. Third, the concentration of myoglobin, the oxygen storage protein in muscle, is extremely elevated in these animals, measuring about 10 times that in human muscle.
(C) In summary, the primary anatomical adaptations for pressure of a deep-diving mammal such as the sperm whale center on air-containing spaces and the prevention of tissue barotrauma. Air cavities, when present, are lined with venous plexuses, which are thought to fill at depth, obliterate the air space, and prevent "the squeeze." The lungs collapse, which prevents lung rupture and (important with regard to physiology) blocks gas exchange in the lung. Lack of nitrogen absorption at depth prevents the development of nitrogen narcosis and decompression sickness. In addition, because the lungs do not serve as a source of oxygen at depth, deep divers rely on enhanced oxygen stores in their blood and muscle.
(D) Diving to depth can result in mechanical distortion and tissue compression, especially in gas-filled spaces in the body. Such spaces include the middle ear cavity, air sinuses in the head, and the lungs. Development of even small pressure differentials between an air cavity and its surrounding tissue can result in tissue distortion and disruption—a condition in human divers known as "the squeeze." In some species of cetaceans, the middle ear cavity is lined with an extensive venous plexus, which is postulated to become engorged at depth and thus reduce or obliterate the air space and prevent development of the squeeze. Cetaceans also have large Eustachian tubes communicating with the tympanic cavity of the ear and the large pterygoid sinuses of the head. These air sinuses of the head have an extensive vasculature, which is thought to function in a manner similar to that of the middle ear and facilitate equilibration of air pressure within these spaces. Lastly, most marine mammals lack frontal cranial sinuses like those present in terrestrial mammals.
(E) Collapse of the lungs forces air away from the alveoli, where gas exchange between the lungs and blood occurs. This blunting of gas exchange is important in the deep diver because it prevents the absorption of nitrogen into the blood and the subsequent development of high blood nitrogen levels. High blood nitrogen pressures can exert a narcotic effect (so-called nitrogen narcosis) on the diver. It may also lead to nitrogen bubble formation during ascent—a phenomenon known as decompression sickness or "the bends." Collapse of the lungs in the deep diver avoids these two problems.
(F) Another organ susceptible to compression damage is the lung. In deep-diving whales and seals, the peripheral airways are reinforced, and it is postulated that this allows the lungs to collapse during travel to depth. Such collapse has been observed radiographically and confirmed with blood nitrogen analyses in the deep-diving Weddell seal.
Exercise 7. Make up a list of the 10 key terms used in the text, then agree with the whole group on the final list. Retell the article using these terms.
Exercise 8. Prepare your own report about adaptations of other species to their habitat and lifestyle.
Section 2. Recommended Report and Presentation Topics
Section 3.
Unit 11. Human Evolution
The species does not grow into perfection: the weak again and again get the upper hand of the strong,—their large number, and their greater cunning are the cause of it.
Friedrich Nietzsche
Exercise 1. What is evolution?
Exercise 2. Explain the meaning of the following key terms connected with evolution:
| natural selection selective breeding genotype phenotype adaptation | mutation heredity species population ecological niche | variation norm of reaction divergence convergence parallelism | atavism rudiment competition biological progress biological regression |
Exercise 3. Now read the article to check some of your answers in Exercise 1.
Culture Speeds Up Human Evolution
Analysis of common patterns of genetic variation reveals that humans have been evolving faster in recent history
By David Biello
Homo sapiens sapiens has spread across the globe and increased vastly in numbers over the past 50,000 years or so—from an estimated five million in 9000 B.C. to roughly 6.5 billion today. More people means more opportunity for mutations to creep into the basic human genome and new research confirms that in the past 10,000 years a host of changes to everything from digestion to bones has been taking place.
"We found very many human genes undergoing selection," says anthropologist Gregory Cochran of the University of Utah, a member of the team that analyzed the 3.9 million DNA sequences* showing the most variation. "Most are very recent, so much so that the rate of human evolution over the past few thousand years is far greater than it has been over the past few million years." "We believe that this can be explained by an increase in the strength of selection as people became agriculturalists—a major ecological change—and a vast increase in the number of favorable mutations as agriculture led to increased population size," he adds.
Roughly 10,000 years ago, humanity made the transition from living off the land to actively raising crops and domesticated animals. Because this concentrated populations, diseases such as malaria, smallpox and tuberculosis, among others, became more virulent. At the same time, the new agriculturally based diet offered its own challenges—including iron deficiency from lack of meat, cavities and, ultimately, shorter stature due to poor nutrition, says anthropologist John Hawks of the University of Wisconsin–Madison, another team member. "Their bodies and teeth shrank. Their brains shrank, too," he adds. "But they started to get new alleles that helped them digest the food more efficiently. New protective alleles allowed a fraction of people to survive the dread illnesses better."
By looking for wide swaths of genetic material that vary little from individual to individual within these sections of great variation, the researchers identified regions that both originated recently and conferred some kind of advantage (because they became common rapidly). For example, the gene known as LCT gave adults the ability to digest milk and G6PD offered some protection against the malaria caused by Plasmodium falciparum parasite. "Ten thousand years ago, no one on planet Earth had blue eyes," Hawks notes, because that gene— OCA2 —had not yet developed. "We are different from people who lived only 400 generations ago in ways that are very obvious; that you can see with your eyes."
Comparing the amount of genetic differentiation between humans and our closest relatives, chimpanzees, suggests that the pace of change has accelerated to 10 to 100 times the average long-term rate, the researchers write in Proceedings of the National Academy of Sciences USA. Not all populations show the same evolutionary speed. For example, Africans show a slightly lower mutation rate. "Africans haven't had to adapt to a fundamentally new climate," because modern humanity evolved where they live, Cochran says. "Europeans and East Asians, living in environments very different from those of their African ancestors and early adopters of agriculture, were more maladapted, less fitted to their environments."
And this speedy pace of evolution will not slow until every possible beneficial mutation starts to happen—the maximum rate of adaptation. This has already begun to occur in such areas as skin color in which different sets of genes are responsible for the paler shades of Europeans and East Asians, according to the researchers.
The finding raises many questions. Among them: "the medical applications of this kind of knowledge [as well as] exactly what most of the selected changes do and what drove their selection," Cochran says.
But the history of humanity is beginning to be read out from our genes, thanks to a detailed knowledge of the thousands of them that have evolved recently. "We're going to be classifying these by functional categories and looking for matches between genetic changes and historic and archaeological changes in diet, skeletal form, disease and many other things," Hawks says. "We think we will be able to find some of the genetic changes that drove human population growth and migrations—the broad causes of human history." (From Scientific American Online, December 10, 2007)
* This article wrongly characterized the HapMap genotype dataset used for this analysis as "genes" rather than "DNA sequences."
Exercise 4. Are the following statements true or false, according to the text? Explain your answer.
1. Homo Sapiens has demonstrated great biological progress over its history.
2. The rate of human evolution has slowed down due to vast numbers of people living.
3. Transition to agricultural lifestyle and domestication of animals provided a reliable source of food for people.
4. It also helped to improve human health condition.
5. Different populations demonstrate different rates of evolutionary changes.
6. Humans have developed numerous adaptations and thus have achieved the maximum rate of adaptation.
7. Genetic research sheds light on human evolution.
Exercise 5. Are humans still evolving? Discuss the following questions:
a. What changes have taken place since the emergence of Homo Sapiens? Give as many examples as you can.
b. What adaptations to their environment and lifestyle have people developed?
Exercise 6. Read the text which provides some information about the changes of human phenotype.
Why are we getting taller as a species?
Humans increased in stature dramatically during the last 150 years, but we have now likely reached the upper limit. The average height of a human man will probably never exceed that of basketball player Shaquille O'Neal, who stands 7 feet and 1 inch tall.
This answer comes from Michael J. Dougherty, assistant director and senior staff biologist at Biologic Image: SportsLine USA, Inc.
In fact, over the last 150 years the average height of people in industrialized nations has increased approximately 10 centimeters (about four inches). Why this relatively sudden growth? Are we evolving to greater heights? Before answering these questions, we need to remember that evolution requires two things: variation in physical and/or behavioral traits among the individuals in a population; and a way of selecting some of those traits as adaptations, or advantages to reproduction.
For example, finches that have large, powerful beaks also have an advantage cracking large, tough seeds during periods when small, soft seeds are scarce. As a consequence, large-beaked birds are more likely to eat better, survive longer and reproduce than small-beaked birds. Because beak shape is an inherited trait, more successful reproduction by large beaked birds means that the genes predisposing finches to large beaks are transmitted to the next generation in relatively larger numbers than those genes encoding small beaks. Thus, the population of finches in the next generation will tend to have larger beaks than finches in their parent's generation.
Let's use this basic operating principle of evolution to predict, retrospectively, the direction of change in human height if evolution were the cause of the change. We know from studies conducted in industrial England that children born into lower socioeconomic classes were shorter, on average, than children born into wealthy families. We also know that poorer families had larger numbers of children. Given those initial conditions, what would evolution predict? The average population should have become shorter because the shorter individuals in the population were, from an evolutionary fitness perspective, more successful in passing on their genes. But this did not happen. Instead, all segments of the population--rich and poor, from small and large families--increased in height. Thus, natural selection, the process whereby differences in reproductive success account for changes in the traits of a population, does not explain why we are taller.
If evolution doesn't explain height increases, what does? Most geneticists believe that the improvement in childhood nutrition has been the most important factor in allowing humans to increase so dramatically in stature. The evidence for this argument is threefold:
First, the observed increase in height has not been continuous since the dawn of man; it began sometime around the middle of the nineteenth century. In fact, examinations of skeletons show no significant differences in height from the stone age through the early 1800s. Also, during World Wars I and II, when hunger was a frequent companion of the German civilian population, the heights of the children actually declined. They only recovered during the post-war years. Such data are consistent with recent research indicating that slow growth induced by temporary malnourishment can usually be reversed. Chronic underfeeding during childhood, however, permanently affects stature and other traits, including intelligence.
Second, the trend toward increasing height has largely leveled off, suggesting that there is an upper limit to height beyond which our genes are not equipped to take us, regardless of environmental improvements. Interestingly, the age of menarche, which is also influenced by nutrition, has shown a corresponding decrease over this same time period. Some scientists believe that the increase in teenage and out-of-wedlock pregnancies in the developed world may be an unanticipated consequence of improved nutrition.
Third, conditions of poor nutrition are well correlated to smaller stature. For example, the heights of all classes of people, from factory workers to the rich, increased as food quality, production and distribution became more reliable, although class differences still remain. Even more dramatic, the heights of vagrant London boys declined from 1780 to1800 and then rose three inches in just 30 years--an increase that paralleled improving conditions for the poor. Even today, height is used in some countries as an indicator of socioeconomic division, and differences can reveal discrimination within social, ethnic, economic, occupational and geographic groups.
For those hoping that humans might someday shoot basketballs through 15-foot high hoops, the fact that the increase in human height is leveling off no doubt will be disappointing. For those who understand, however, that our genes are merely a blueprint that specifies what is possible given an optimal environment, a limit on height is just one of many limitations in life, and certainly not the most constraining.
With environmental variables perhaps near their optimum, what are the prospects for evolutionary increases in height as a consequence of changes to our genetic blueprints? Apply the methods of the thought experiment above and see.
Exercise 7. Using the information from the text prove that:
1. Evolution doesn’t explain the increased height of people.
2. Improvement in childhood nutrition has been the most important factor in increasing the stature.
3. People will hardly increase their height any more.
Exercise 8. In the following text the paragraphs are mixed. Put them in the correct logical order. The first and the last paragraphs are in their right places.
African Adaptation to Digesting Milk Is "Strongest Signal of Selection Ever"
East African cattle herding communities rapidly and independently evolved ability to digest lactose
By Nikhil Swaminathan
(A) For many adults in the world, the phrase "got milk?" is quickly followed by "got a nearby toilet?" Lactose, the primary sugar in milk, is a universal favorite in infancy but into adulthood the level of lactase-phlorizin hydrolase, the enzyme that metabolizes lactose in the small intestine, decreases and digestion of dairy products becomes difficult. In some populations, however, such as those located in northern Europe, the ability to digest milk remains most likely as a result of lifestyles based around cattle domestication. In 2002 Finnish scientists localized the genetic mutation that conferred this trait in northern Europeans to two regions on chromosome 2.
(B) Tishkoff and her students tested 470 people representing over 43 ethnic fractions in the Sudan, Kenya and northern Tanzania for lactose intolerance using glucose-monitoring kits, familiar to most diabetics. The team then selected the 40 most lactose intolerant participants and the 69 most tolerant and sequenced parts of their genomes around the two markers identified in the Finnish lactase persistence study. The researchers determined there were 123 single nucleotide polymorphisms--SNPs, or changes to one base in the genetic code--associated with digestibility. Of these, three SNPs were more promising than the others and one of them was very common among Tanzanians and Kenyans, showing up in 40 to 50 percent of the sequences.
(C) Tishkoff believes that because she found so many markers associated with lactose tolerance in the sequencing of her 109 subjects, evolution clearly develops multiple solutions when there is a strong selective force. "There are some populations that can digest milk, and they don't have any of these mutations," she says. "There are more out there." Dallas Swallow, a human geneticist at University College London, agrees with that assessment. She released a study on a small Sudanese tribe in Human Genetics this past November, finding three markers, two of which Tishkoff had isolated in her study. Oddly enough, Swallow found no data on the Maryland study's primary variant. Tishkoff argues this disparity is due to geographic specificity of these mutations. Swallow, for her part, notes that Tishkoff's dramatic results may be a result of "the relative relatedness" of her sample. "If you have an ethnic group which is rather a small population in size but happens to migrate over geographic distances then they might be more related to each other than the surrounding people," she points out.
(D) Working with this highly correlated locus, Tishkoff's team sequenced a broad region of the chromosome around this nucleotide to determine whether it arose in concert with the European mutation. "It turns out they're on completely different chromosome backgrounds," she explains. "So it had a completely different origin." Next, the team tested to see if the mutation was positively selected, conferring a reproductive advantage and spreading quickly through the population. People who had this particular SNP on both copies of chromosome 2 had identical genetic scripts for the next two million base pairs--a phenomenon that occurs when there is a strong benefit to having a particular trait." Because this section has been preserved intact without being mutated or broken up by recombination, it indicates that it is very recent and very strong. In addition, Tishkoff's team determined the date range when the mutation likely occurred: 3,000 to 7,000 years ago, which matches up well with the archaeological record that places pastoralization coming to East Africa about 5,000 years ago. The European trait dates back about 9,000 years.
(E) Now, the results of a four-year, international research project find that communities in East Africa leading traditionally similar pastoral lives evolved their ability to drink milk rapidly and independently of the northern Europeans. According to University of Maryland biologist Sarah Tishkoff, the lead author of a study appearing in today's Nature Genetics, the mutation allowing them to "get milk" arose so quickly and was so advantageous that "it is basically the strongest signal of selection ever observed in any genome, in any study, in any population in the world."
(F) Nevertheless, both researchers are pleased that their studies found at least two genetic markers in common. Swallow concedes: "It looks jolly well as though drinking milk as an adult was good for some of us at some time in our history, that's for sure." (From Scientific American Online, December 11, 2006)
Exercise 9. Make up a list of the 10 key facts about human evolution discussed in this unit. Prove and explain your choice. Then summarize all the information discussed into one report.
Unit 12. Alcohol
We drink one another’s health and spoil our own.
Jerome K. Jerome
Exercise 1. What do you know about alcohol?
1. What is its main active compound?
2. In what regions of the world is alcohol produced?
3. How does alcohol affect the human body?
4. What are the risks of alcohol overdose?
5. What causes hangover after drinking alcohol?
6. What is alcoholism? How quickly does it develop?
Exercise 2. Read the information about alcohol provided by NewScientist.com to check some of your answers in Exercise 1.
What is it? Ethanol produced by the action of yeast on sugars.
What does it do? Ethanol is a biphasic drug: low doses have a different effect to high doses. Small amounts of alcohol (one or two drinks) act as a stimulant, reducing inhibition and producing feelings of mild euphoria. Higher doses depress the central nervous system, initially producing relaxation but then leading to drunkenness - characterised by poor coordination, memory loss, cognitive impairment and blurred vision. Very high doses cause vomiting, coma and death through respiratory failure. The fatal dose varies but is somewhere around 500 milligrams of ethanol per 100 millilitres of blood.
How does it work? At low doses (5 milligrams per 100 millilitres of blood), alcohol sensitises NMDA receptors in the brain, making them more responsive to the excitatory neurotransmitter glutamate, so boosting brain activity. These effects are most pronounced in areas associated with thinking, memory and pleasure. At higher doses it desensitises the same receptors and also activates the inhibitory GABA system.
How long is its history?
4000BC - Wine and beer making in Egypt and Sumeria
3500BC - Bronze-age vessels show evidence of wine consumption in eastern Mediterranean
800BC - Distillation of spirits in India
AD625 - Mohammed orders his followers to abstain from alcohol
1850s - New York bartenders invent the cocktail
1920-33 - Prohibition in the US. Alcohol was also illegal in Finland from 1919 to 1932 and in various Canadian provinces at various times between 1900 and 1948.
Exercise 3. Now read more detailed information about the effects of alcohol on the brain provided by Anthony Dekker D.O., Director of Ambulatory Care and Community Health at Phoenix Indian Medical Center.
What are the effects of alcohol on the brain?
The product of the oldest chemical reaction studied by man, alcohol, continues to challenge researchers. Since the original work on alcohol's neurological effects in the early 20th century, new theories have regularly emerged. What we have learned is that alcohol is a sedative-hypnotic in the acute intoxication phase for most patients. But it diminishes the quality of sleep. Individuals with sleep apnea often experience longer and more severe apneic episodes and hypoxia, or oxygen deprivation, after drinking alcohol.
In other individuals, though, alcohol may act as a stimulant. Indeed, its association with violent and self-abusive behavior is well documented*. At intoxicating levels, alcohol is a vasodilator (it causes blood vessels to relax and widen), but at even higher levels, it becomes a vasoconstrictor, shrinking the vessels and increasing blood pressure, exacerbating such conditions as migraine headaches and frostbite. Researchers have also thoroughly documented the effects of alcohol on the developing fetus. Approximately one third of all babies born to alcoholic mothers will develop Fetal Alcohol Syndrome or Effects (FAS or FAE), causing central nervous system dysfunctions including Attention Deficit Disorder (ADD) and impaired IQ. There are also growth and facial abnormalities associated with these infants.
In the early 1900s, H. Meyer and Charles Ernest Overton originally theorized that the effect of alcohol was achieved by altering the lipid environment of cell membranes. This theory, however, requires much higher concentrations of alcohol than are clinically observed. A recent theory, supported by several researchers, pins alcohol's effect on voltage and ligand-gated ion channels that control neuronal activity. Two distinct ligand-gated channels have been identified, inhibitory ones (GABA receptors and strychnine-sensitive glycine receptors) and excitatory ones (N-methyl-D-aspartate (NMDA) and non-NMDA glutamate-activated channels and the 5HT3 subtype of serotonin receptors).
The inhibitory aspect occurs due to a hyperpolarization of neurons, secondary to an influx of chloride ions. The neuron becomes less likely to achieve the threshold membrane potential. The excitatory receptor is dependent on the NMDA and non-NMDA glutamate receptors that control the influx of sodium and calcium, which bind to endogenous neurotransmitters (glutamate or aspartate) and depolarize the neuronal membrane. The NMDA receptor seems to have a high permiability to calcium, which acts as a catalyst to several intracellular events.
Chronic exposure to alcohol seems to alter the NMDA receptors and this may play a role in the clinical symptoms of alcohol withdrawal. In vitro studies have demonstrated an increase in the binding sites for MK801 (dizocilpine) in neurons chronically exposed to alcohol. This rise may account for the acclimation process, in which greater concentrations of alcohol are needed to cause experimental and clinical symptoms of intoxication. NMDA can cause seizure activity. Mice that have been exposed to chronically elevated levels of alcohol reveal increased numbers of NMDA receptors and NMDA related seizure activity. The NMDA antagonist MK801 has been shown to decrease the severity of seizures in these mice during withdrawal. Through a complex process of cell membrane ion pumps and neurotransmitter stimulation, the multi-faceted effects of alcohol and alcohol withdrawal are becoming better understood.
*Under the influence of alcohol, the brain experiences impairments in the following regions:
•Frontal Lobe - Loss of reason, caution, inhibitions, sociability, talkativeness and intelligence.
•Parietal Lobe - Loss of fine motor skills, slower reaction time, shaking.
•Temporal Lobe - Slurred speech, impaired hearing.
•Occipital Lobe - Blurred vision, poor distance judgement.
•Cerebellum - Lack of muscle coordination and balance.
•Brain Stem - Loss of vital functions.
Exercise 4. Do you agree with the statements below? Give reasons for your choice.
Exercise 5. Some scientists argue that alcohol can produce significant positive effect on one’s health. The article below provides evidence for this point of view. Read the article and find answers to the questions:
Drink to Your Health?
By Arthur L. Klatsky
Three decades of research shows that drinking small to moderate amounts of alcohol has cardiovascular benefits. A thorny issue for physicians is whether to recommend drinking to some patients.
America has always had trouble deciding whether alcohol is a bad thing or a good thing. Millions who remember Prohibition, when all alcoholic beverages were illegal, now witness a constant stream of advertisements from producers of alcoholic beverages encouraging people to drink. Despite alcohol’s popularity today, however, many still consider abstinence a virtue. Certainly, heavy drinking and alcoholism deserve deep concern for the terrible toll they take on alcohol abusers and society in general. But worry about the dangers of abuse often leads to emotional denials that alcohol could have any medical benefits. Such denials ignore a growing body of evidence indicating that moderate alcohol intake wards off certain cardiovascular conditions, most notably heart attacks and ischemic strokes (those caused by blocked blood vessels). A few studies even show protection against dementia, which can be related to cardiovascular problems.
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