IELTS Mock Test 9 - Reading Test 1 With Practice Test, Answers And Explanation
Passage
❓ Câu hỏi passage 1
Question 1 - 7
Do the following statements agree with the information given in the Reading Passage?
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if it is impossible to say what the writer thinks about this
1
One of Perkin's schoolteachers was the first to spot his talent for chemistry.
2
At the college, Hellwig worked as an assistant to Perkin.
3
Cinchona bark was more expensive than any other medicine available at the time.
4
In his experiments of 1854, Perkin managed to obtain quinine from aniline.
5
The dyes drawn from nature, used in earlier times, did not hold their colour for long.
6
The disagreement over Perkin leaving college ended his friendship with Hellwig for good.
7
Certain tiny organisms only became visible to scientists once dyes were applied to them.
Question 8 - 13
Answer the questions below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
8
In the past, which section of society was the only one able to wear purple cloth?
9
What did Perkin swiftly realise his new colour could become?
10
What term did the public eventually use for Perkin's first dye?
11
Who did Perkin turn to for advice on whether the dye was worth producing?
12
Fashion for the new colour took hold first in which country?
13
Which illness are scientists now working to prevent with these artificial dyes?
Passage
❓ Câu hỏi passage 2
Question 14 - 17
The Reading Passage has five paragraphs A-E.
From the list of headings below choose the most suitable heading for paragraphs B-E.
Write the appropriate numbers i-vii.
List of Headings
I
Deciding how to answer a message
II
What we currently understand about alien biology
III
Estimating the chances of life elsewhere
IV
The great gulf separating us from our nearest neighbours
V
Listening for broadcasts from the stars
VI
The starting premises behind the hunt
14
Paragraph C
15
Paragraph D
16
Paragraph E
17
Paragraph F
Question 18 - 20
Answer the questions below.
Choose NO MORE THAN THREE WORDS AND/OR A NUMBER from the passage for each answer.
18
What does the writer regard as the fundamental motive lying behind the search?
19
What does the writer say the majority of researchers disregard on the grounds that the case for it is unconvincing?
20
How many nearby candidate stars are covered by the more sensitive, targeted part of the search?
Question 21 - 26
Do the following statements agree with the information given in Reading Passage?
In following statements below, choose
YES if the statement agrees with the information
NO if the statement contradicts the information
NOT GIVEN if it is impossible to say what the writer thinks about this
21
The writer expects that any civilisation we detect is likely to be at a similar stage of development to our own.
22
The writer considers it improbable that living things could be based on any chemistry other than carbon and water.
23
The sheer number of stars and galaxies leads the writer to treat it as virtually certain that at least one other world somewhere is inhabited.
24
For the same amount of broadcasting energy, signals within one particular radio band reach further than the alternatives.
25
The survey that monitors the entire sky is carried out at a higher sensitivity than the one directed at selected stars.
26
The writer believes most national governments would prefer to keep the detection of an alien signal secret.
Passage
❓ Câu hỏi passage 3
Question 27 - 30
Answer the questions below.
Choose NO MORE THAN TWO WORDS from the passage for each answer.
27
According to the writer, what did migrating animals keep inside their bodies as a legacy of the sea?
28
Besides respiration, which function does the writer single out as most needing to be redesigned for life on land?
29
Unlike whales, turtles have not fully returned to the water; in what location do they still reproduce?
30
Ichthyosaurs shared their era with which other group of creatures?
Question 31 - 33
Do the following statements agree with the information given in the Reading Passage?
TRUE if the statement agrees with the information
FALSE if the statement contradicts the information
NOT GIVEN if it is impossible to say what the writer thinks about this
31
Seals have made a full return to a life in the water.
32
In some cases the writer says it is straightforward to tell where an extinct creature once lived.
33
The technique developed by Öberg and Solheim has since been used to study other kinds of reptile.
Question 34 - 39
Complete the flow-chart below.
Choose NO MORE THAN TWO WORDS AND/OR A NUMBER from the passage for each answer.
Method for working out where the ancestors of turtles and tortoises came from
STEP 1
From the bones of each of 63 species alive today, three separate readings were recorded.
STEP 2
Each species' three readings were then set out on a triangular grid, so that the species could be directly .
OUTCOME
The land tortoises were packed tightly together near the of the triangle, while the sea turtles collected in the lower region.
STEP 3
A number of living species then underwent the same procedure and were added to the chart.
OUTCOME
The points for these species settled near the of the triangle, between the two earlier groups.
STEP 4
The bones of A. vendhali and P. brannei were examined in the same way and their points added.
OUTCOME
The position of their points revealed that the two ancient species had once been .
Question 40
Choose the correct letter, A, B, C or D.
Write the correct letter in boxes on your answer sheet.
40
According to the writer, the single most important thing about tortoises is that
A
they can survive in some of the driest regions anywhere on the planet.
B
they have crossed between the sea and the land on more than one occasion.
C
their earliest ancestors were simple bacteria that lived in the sea.
D
they are very closely related to the sea turtles of today.
IELTS Mock Test Vol 9 - Reading Test 1 with Practice Test, Answers And Explanation
Luyện tập đề IELTS Online Mock Test Vol 9 - Reading Test 1 được lấy từ cuốn sách IELTS Mock Test Vol 9 với trải nghiệm thi IELTS trên máy và giải thích đáp án chi tiết bằng Linearthinking, kèm answer key và list từ vựng IELTS cần học trong bài đọc.
Born in the northern port of Hull on 27 September 1836, William Henry Perkin was a boy whose restless curiosity drew him early toward drawing, mechanics, photography and the natural sciences. What finally fixed his passion on chemistry, though, was an accident: while exploring the attic of a late uncle's house, he came across a shabby but still-usable laboratory, and it captivated him.
It was at Calverton School that chemistry took full hold of him. Frederick Nunn, one of his teachers, noticed how gifted and dedicated the boy was and arranged for him to hear a course of lectures delivered by the celebrated chemist Aldous Renshaw at the Institute of Natural Philosophy. Renshaw's talks stirred his ambition still more, and in 1851, when he was just sixteen, Perkin won a place at the Metropolitan College of Chemistry.
When Perkin arrived, the college was under the direction of a distinguished German chemist, Gustav Hellwig. It did not take long for Perkin's abilities to impress him, and within a single year the student had been promoted to become the youngest assistant Hellwig had. The great discovery that would make Perkin both famous and wealthy followed soon afterwards.
In those years the sole effective remedy for malaria was quinine, a substance taken from the bark of the cinchona tree, which grows in South America. Supplies of it, by the mid-1850s, could no longer keep pace with what physicians wanted. So when Hellwig remarked in passing how useful an artificial version of quinine would be, it surprised no one that his most promising student rose to the task.
Over the college holidays of 1854, working in a laboratory he had set up on the top floor of the family home, Perkin set out to build quinine from aniline, a cheap and plentiful waste product of coal tar. The quinine never came. What he got instead was a strange, dark, tarry residue. Rather than throw it away, his instincts and his training led him to look closer. By adding potassium dichromate and alcohol to the aniline at different points in the procedure, he eventually arrived at a solution of intense purple. The words of the chemist Anton Vahl, that fortune "smiles only on minds that are ready for it", were borne out, for Perkin grasped at once what his accidental product might be worth.
For most of history, the dyes used on cloth had come from natural materials: plants, and substances secreted by animals. A number of these were both hard to gather and enormously dear; the purple drawn from the glands of certain sea molluscs was one such, so expensive in its day that only royalty could dress in it. Natural colours had other drawbacks too, for they often looked dull and lost their brightness rapidly. This was the world into which Perkin's discovery arrived.
That his purple liquid could dye cloth, and would therefore be the first synthetic dye anywhere, Perkin understood almost immediately, and he patented it without delay. Most striking of all, perhaps, was how quickly he sensed that the colour could be turned into a profitable business.
At first Perkin called the colour Tyrian Lilac, though before long everyone knew it simply as violine, a name borrowed from the French word for violet. When he sought the opinion of Andrew Whitlock, who ran a dye works in Yorkshire, Whitlock told him the venture would certainly pay provided the dye held its colour without fading and could be made cheaply enough. And so, ignoring the furious protests of his teacher Hellwig, Perkin abandoned his studies and set about founding what became the modern chemical industry.
Backed by his father and his brother, Perkin built a factory a short way outside London. The plant drew on the coal tar that London's gas street-lamps threw off in almost limitless quantities, cheap and abundant, and by 1858 it was turning out the first synthetically dyed fabric in the world. A boost came from the Austrian court: when the Empress Adelheid of Austria concluded that the shade suited her, violine rapidly became the colour every fashionable woman in Vienna had to have. Britain soon followed, after the Duchess of Ellsmere was seen in public in a violine gown, and demand there grew just as feverish. Bold and fade-resistant, the dye left the public wanting more, and Perkin returned to his experiments.
Fame and wealth were already his from that first success, yet Perkin kept up his research. Further dyes followed from his laboratory: aniline crimson in 1861, aniline black in 1865, and, as the decade closed, the shade known as Perkin's blue. His synthetic dyes, it should be stressed, mattered for far more than appearance. They proved indispensable to medicine as well, for by staining microbes and bacteria that had previously been impossible to see, they enabled scientists to pick out the organisms responsible for diseases such as cholera, leprosy and plague. Synthetic colours remain essential in laboratories today and, in a turn Perkin himself would surely have welcomed, they are now being enlisted in the effort to develop a vaccine against typhoid.
A. For centuries people have wondered whether life exists beyond our own world, and today, by listening for radio transmissions sent out by other intelligent beings, we may finally be close to settling the matter. Known widely by its acronym SETI - short for the search for extra-terrestrial intelligence - this undertaking is far from straightforward. Teams in various countries have hunted on and off for close to forty years, yet only recently has our technology advanced far enough to let us mount a serious, systematic sweep of every nearby star in the hope of finding some trace of life.
B
Curiosity lies at the heart of the search - the very same curiosity about the natural world that motivates every branch of pure science. Are we alone? Does life spring up of its own accord wherever conditions allow, or is there something exceptional about the Earth that has given rise to the abundance of living things around us? To resolve this most fundamental of questions, merely picking up a single radio signal would be enough. Viewed this way, SETI is simply one more part of the pure-science enterprise that keeps extending the boundaries of what we know. There are, however, further motives for wanting to know whether life exists elsewhere. Human civilisation, after all, has lasted perhaps no more than a couple of thousand years, and the dangers of nuclear war and pollution that have loomed over recent decades remind us how fragile our continued existence might be. Are we destined to survive a further three thousand years, or to destroy ourselves? Because a world such as ours can persist for roughly ten billion years, any civilisations that endure elsewhere in our galaxy could be anything from newly formed to billions of years old. It follows that whichever civilisation we do pick up will, on average, almost certainly be much older than we are. Simply knowing that such a civilisation exists would prove that long-term survival is achievable, which gives grounds for optimism; and it is even conceivable that a more ancient people might share what they have learned about surviving hazards like nuclear war, global pollution and dangers we have yet to imagine.
C
Two working rules guide most researchers as they weigh up the question of our solitude. The first is that unidentified flying objects are set aside, because the evidence for them strikes most scientists as too flimsy to warrant serious attention - though a genuinely persuasive case, should one ever surface, would of course deserve an open mind. The second is a deliberately cautious premise: the beings sought are assumed to be broadly similar to ourselves, for anything wildly unlike us might not register as life at all, quite aside from the problem of communicating with it. Such a creature might, admittedly, sport two green heads and seven fingers, and yet in the essentials it would still echo us - talking to its own kind, curious about the cosmos, dwelling on a planet that circles a Sun-like star, and, most constraining of all, built on a chemistry of carbon and water just as we are - the only chemical basis we currently know of that is capable of sustaining living systems.
D
Even granting these premises, how much we can say about other living things remains very slight. We have no firm idea, for instance, of how many stars possess planets at all, still less of the odds that life will emerge naturally once the right conditions are in place. And yet, faced with the several hundred billion stars of our own galaxy, the Milky Way, and the hundreds of billions of galaxies that fill the observable Universe, the notion that not one of these worlds bears life seems scarcely credible; indeed, the best estimate we can venture - drawing on the little we understand about what carbon-based life requires - puts the figure at something like one star in every 80,000 hosting a life-bearing planet. If so, our closest neighbours may lie about 120 light years off, which, on the scale of the cosmos, is practically next door.
E
To scatter information across the galaxy an alien society might use any number of methods, but most either demand enormous power or fade badly over the immense stretches between the stars. As it happens, radio waves in the band from 1200 to 3400 MHz carry furthest for a given transmitting power, which is why every search so far has focused on that part of the spectrum. A range of programmes worldwide has already made the attempt, among them a New Zealand effort using the dish at Rangimoana, on the country's South Island. From the couple of hundred stars examined to date, nothing has yet been picked up. Since 1995, however, the effort has been scaled up sharply: that year the Canadian federal legislature approved an eight-year grant of $12 million a year to fund a thorough hunt for life beyond the Earth. A large share of that money is going into the specialised equipment required to monitor many frequencies simultaneously. The work is split in two. One half is a targeted survey carried out on some of the largest dishes in existence - the Canadian-run instrument at Fenwick Bay in Newfoundland and the German telescope at Osterholt - which sweeps the nearest 1,500 promising stars at high sensitivity across the 1200-to-3400 MHz band. The other half, a broader and less sensitive watch over the whole sky, relies on the smaller antennas of the Orbital Tracking Network.
F
How we ought to respond, should a signal from another civilisation actually be detected, is a matter of real dispute. On one point there is unanimity: an immediate reply is out of the question. Setting aside the sheer impracticality of despatching an answer across such distances on short notice, the act would throw up a whole range of ethical problems that the world as a whole would need to work through before any message went out. Could humanity withstand the shock of confronting a civilisation far older and more advanced than itself? Fortunately, none of this is pressing. Because the stars under examination sit hundreds of light years away, their signals take centuries to reach us, and our reply would take centuries more to arrive; a pause of a few years, or even decades, while humanity argues over whether to answer and painstakingly composes a response, would therefore make little practical difference to how events eventually unfold.
Trace life back far enough and all of it once belonged to the sea. Time and again through evolution, bold pioneers from one animal group after another hauled themselves ashore — some reaching the driest of deserts — carrying a supply of their own private seawater with them, locked inside their blood and cellular fluids. The reptiles, birds, mammals and insects around us are only part of the story: scorpions, snails, spiders, worms, millipedes and centipedes, and crustaceans such as woodlice and land crabs have all made a go of it beyond the water too. Nor should the plants be forgotten, for until they had colonised the land, none of these animal journeys could have happened.
The shift from sea to land forced a thoroughgoing overhaul of the body, and two systems above all had to be rebuilt: respiration and water balance. And yet plenty of committed land animals later reversed course, gave up the terrestrial machinery they had acquired, and headed back into the water. The seals are only halfway through such a journey, hinting at what the in-between stages looked like on the road to extreme cases like whales and dugongs. Whales — dolphins among them, since dolphins are simply small whales — together with dugongs and the manatees, abandoned terrestrial life entirely and slipped back into a wholly marine existence. Not even breeding brings them ashore. Air, though, is still what they breathe: they never re-evolved anything like the gills their sea-dwelling ancestors once had. Turtles, too, returned to the sea in the remote past, and, like every backboned animal that went back to the water, they breathe air. In one respect, though, they have held back more than whales or dugongs have, since the beaches are still where they come to lay their eggs.
Turtles alive today, the evidence shows, all descend from an ancestor that lived on land in an era earlier than most of the dinosaurs. Close to the root of the present-day turtle and tortoise line sit two notable fossils, Archelochersis vendhali and Prototestudo brannei, laid down early in the dinosaurs' reign. But when only scraps of a fossil survive, how is its owner's habitat — land or sea — to be decided? Now and then the answer is plain enough. The ichthyosaurs illustrate this: reptiles that lived alongside the dinosaurs, fin-bearing and sleek of body, they left fossils so dolphin-like that they can only have lived, as dolphins do, in the water. With turtles the task is harder, and one route to an answer lies in the bones of the forelimb.
This is the route that Marta Öberg and Rurik Solheim, at Vendhal University, chose to follow. From the forelimb bones of 63 living species of turtles and tortoises they recorded three measurements, and for each species they plotted those three figures against one another on triangular graph paper, an arrangement that made the whole set of species readily comparable. On the resulting chart the land tortoises bunched into a dense cluster high near the top of the triangle, while the sea turtles gathered lower down, with no overlap between them. That gap closed only once species dividing their lives between land and water were added: these semi-aquatic forms came to rest at roughly the midpoint of the triangle, between the 'wet' assembly below and the 'dry' assembly above. The last step was to see where the fossils belonged. On this the bones of A. vendhali and P. brannei were unambiguous — their points sat squarely within the dry assembly, which meant that both long-extinct creatures had been land animals, from an age before the turtles ever went back to the sea.
You might therefore conclude that today's land tortoises have stayed ashore ever since those distant terrestrial beginnings — as most mammals did after a few of their number returned to the sea. Not so. Sketch out the family tree of modern turtles and tortoises, and nearly every branch proves to be aquatic; the land tortoises form just a single twig buried deep among otherwise water-dwelling lineages. This implies that they did not occupy dry ground unbroken from the days of A. vendhali and P. brannei onwards. Their forebears, instead, were among the creatures that slipped back into the water, only for the line to climb ashore again in comparatively recent times.
What tortoises embody, then, is a striking double homecoming. Like every mammal, reptile and bird, they count marine fish among their distant ancestors, and behind those fish stretched a line of worm-like beings reaching back, still within the sea, to the earliest bacteria. A later set of ancestors took up life on land for many generations; later ancestors again abandoned it for the water and became sea turtles. And then, at the last, the line hauled itself ashore a second time to become the tortoises — a few of which now live in the most arid deserts on Earth.
Born in the northern port of Hull on 27 September 1836, William Henry Perkin was a boy whose restless curiosity drew him early toward drawing, mechanics, photography and the natural sciences. What finally fixed his passion on chemistry, though, was an accident: while exploring the attic of a late uncle's house, he came across a shabby but still-usable laboratory, and it captivated him.
It was at Calverton School that chemistry took full hold of him. Frederick Nunn, one of his teachers, noticed how gifted and dedicated the boy was and arranged for him to hear a course of lectures delivered by the celebrated chemist Aldous Renshaw at the Institute of Natural Philosophy. Renshaw's talks stirred his ambition still more, and in 1851, when he was just sixteen, Perkin won a place at the Metropolitan College of Chemistry.
When Perkin arrived, the college was under the direction of a distinguished German chemist, Gustav Hellwig. It did not take long for Perkin's abilities to impress him, and within a single year the student had been promoted to become the youngest assistant Hellwig had. The great discovery that would make Perkin both famous and wealthy followed soon afterwards.
In those years the sole effective remedy for malaria was quinine, a substance taken from the bark of the cinchona tree, which grows in South America. Supplies of it, by the mid-1850s, could no longer keep pace with what physicians wanted. So when Hellwig remarked in passing how useful an artificial version of quinine would be, it surprised no one that his most promising student rose to the task.
Over the college holidays of 1854, working in a laboratory he had set up on the top floor of the family home, Perkin set out to build quinine from aniline, a cheap and plentiful waste product of coal tar. The quinine never came. What he got instead was a strange, dark, tarry residue. Rather than throw it away, his instincts and his training led him to look closer. By adding potassium dichromate and alcohol to the aniline at different points in the procedure, he eventually arrived at a solution of intense purple. The words of the chemist Anton Vahl, that fortune "smiles only on minds that are ready for it", were borne out, for Perkin grasped at once what his accidental product might be worth.
For most of history, the dyes used on cloth had come from natural materials: plants, and substances secreted by animals. A number of these were both hard to gather and enormously dear; the purple drawn from the glands of certain sea molluscs was one such, so expensive in its day that only royalty could dress in it. Natural colours had other drawbacks too, for they often looked dull and lost their brightness rapidly. This was the world into which Perkin's discovery arrived.
That his purple liquid could dye cloth, and would therefore be the first synthetic dye anywhere, Perkin understood almost immediately, and he patented it without delay. Most striking of all, perhaps, was how quickly he sensed that the colour could be turned into a profitable business.
At first Perkin called the colour Tyrian Lilac, though before long everyone knew it simply as violine, a name borrowed from the French word for violet. When he sought the opinion of Andrew Whitlock, who ran a dye works in Yorkshire, Whitlock told him the venture would certainly pay provided the dye held its colour without fading and could be made cheaply enough. And so, ignoring the furious protests of his teacher Hellwig, Perkin abandoned his studies and set about founding what became the modern chemical industry.
Backed by his father and his brother, Perkin built a factory a short way outside London. The plant drew on the coal tar that London's gas street-lamps threw off in almost limitless quantities, cheap and abundant, and by 1858 it was turning out the first synthetically dyed fabric in the world. A boost came from the Austrian court: when the Empress Adelheid of Austria concluded that the shade suited her, violine rapidly became the colour every fashionable woman in Vienna had to have. Britain soon followed, after the Duchess of Ellsmere was seen in public in a violine gown, and demand there grew just as feverish. Bold and fade-resistant, the dye left the public wanting more, and Perkin returned to his experiments.
Fame and wealth were already his from that first success, yet Perkin kept up his research. Further dyes followed from his laboratory: aniline crimson in 1861, aniline black in 1865, and, as the decade closed, the shade known as Perkin's blue. His synthetic dyes, it should be stressed, mattered for far more than appearance. They proved indispensable to medicine as well, for by staining microbes and bacteria that had previously been impossible to see, they enabled scientists to pick out the organisms responsible for diseases such as cholera, leprosy and plague. Synthetic colours remain essential in laboratories today and, in a turn Perkin himself would surely have welcomed, they are now being enlisted in the effort to develop a vaccine against typhoid.
B. Curiosity lies at the heart of the search - the very same curiosity about the natural world that motivates every branch of pure science. Are we alone? Does life spring up of its own accord wherever conditions allow, or is there something exceptional about the Earth that has given rise to the abundance of living things around us? To resolve this most fundamental of questions, merely picking up a single radio signal would be enough. Viewed this way, SETI is simply one more part of the pure-science enterprise that keeps extending the boundaries of what we know. There are, however, further motives for wanting to know whether life exists elsewhere. Human civilisation, after all, has lasted perhaps no more than a couple of thousand years, and the dangers of nuclear war and pollution that have loomed over recent decades remind us how fragile our continued existence might be. Are we destined to survive a further three thousand years, or to destroy ourselves? Because a world such as ours can persist for roughly ten billion years, any civilisations that endure elsewhere in our galaxy could be anything from newly formed to billions of years old. It follows that whichever civilisation we do pick up will, on average, almost certainly be much older than we are. Simply knowing that such a civilisation exists would prove that long-term survival is achievable, which gives grounds for optimism; and it is even conceivable that a more ancient people might share what they have learned about surviving hazards like nuclear war, global pollution and dangers we have yet to imagine.
Curiosity lies at the heart of the search - the very same curiosity about the natural world that motivates every branch of pure science. Are we alone? Does life spring up of its own accord wherever conditions allow, or is there something exceptional about the Earth that has given rise to the abundance of living things around us? To resolve this most fundamental of questions, merely picking up a single radio signal would be enough. Viewed this way, SETI is simply one more part of the pure-science enterprise that keeps extending the boundaries of what we know. There are, however, further motives for wanting to know whether life exists elsewhere. Human civilisation, after all, has lasted perhaps no more than a couple of thousand years, and the dangers of nuclear war and pollution that have loomed over recent decades remind us how fragile our continued existence might be. Are we destined to survive a further three thousand years, or to destroy ourselves? Because a world such as ours can persist for roughly ten billion years, any civilisations that endure elsewhere in our galaxy could be anything from newly formed to billions of years old. It follows that whichever civilisation we do pick up will, on average, almost certainly be much older than we are. Simply knowing that such a civilisation exists would prove that long-term survival is achievable, which gives grounds for optimism; and it is even conceivable that a more ancient people might share what they have learned about surviving hazards like nuclear war, global pollution and dangers we have yet to imagine.
C. Two working rules guide most researchers as they weigh up the question of our solitude. The first is that unidentified flying objects are set aside, because the evidence for them strikes most scientists as too flimsy to warrant serious attention - though a genuinely persuasive case, should one ever surface, would of course deserve an open mind. The second is a deliberately cautious premise: the beings sought are assumed to be broadly similar to ourselves, for anything wildly unlike us might not register as life at all, quite aside from the problem of communicating with it. Such a creature might, admittedly, sport two green heads and seven fingers, and yet in the essentials it would still echo us - talking to its own kind, curious about the cosmos, dwelling on a planet that circles a Sun-like star, and, most constraining of all, built on a chemistry of carbon and water just as we are - the only chemical basis we currently know of that is capable of sustaining living systems.
Two working rules guide most researchers as they weigh up the question of our solitude. The first is that unidentified flying objects are set aside, because the evidence for them strikes most scientists as too flimsy to warrant serious attention - though a genuinely persuasive case, should one ever surface, would of course deserve an open mind. The second is a deliberately cautious premise: the beings sought are assumed to be broadly similar to ourselves, for anything wildly unlike us might not register as life at all, quite aside from the problem of communicating with it. Such a creature might, admittedly, sport two green heads and seven fingers, and yet in the essentials it would still echo us - talking to its own kind, curious about the cosmos, dwelling on a planet that circles a Sun-like star, and, most constraining of all, built on a chemistry of carbon and water just as we are - the only chemical basis we currently know of that is capable of sustaining living systems.
D. Even granting these premises, how much we can say about other living things remains very slight. We have no firm idea, for instance, of how many stars possess planets at all, still less of the odds that life will emerge naturally once the right conditions are in place. And yet, faced with the several hundred billion stars of our own galaxy, the Milky Way, and the hundreds of billions of galaxies that fill the observable Universe, the notion that not one of these worlds bears life seems scarcely credible; indeed, the best estimate we can venture - drawing on the little we understand about what carbon-based life requires - puts the figure at something like one star in every 80,000 hosting a life-bearing planet. If so, our closest neighbours may lie about 120 light years off, which, on the scale of the cosmos, is practically next door.
Even granting these premises, how much we can say about other living things remains very slight. We have no firm idea, for instance, of how many stars possess planets at all, still less of the odds that life will emerge naturally once the right conditions are in place. And yet, faced with the several hundred billion stars of our own galaxy, the Milky Way, and the hundreds of billions of galaxies that fill the observable Universe, the notion that not one of these worlds bears life seems scarcely credible; indeed, the best estimate we can venture - drawing on the little we understand about what carbon-based life requires - puts the figure at something like one star in every 80,000 hosting a life-bearing planet. If so, our closest neighbours may lie about 120 light years off, which, on the scale of the cosmos, is practically next door.
E. To scatter information across the galaxy an alien society might use any number of methods, but most either demand enormous power or fade badly over the immense stretches between the stars. As it happens, radio waves in the band from 1200 to 3400 MHz carry furthest for a given transmitting power, which is why every search so far has focused on that part of the spectrum. A range of programmes worldwide has already made the attempt, among them a New Zealand effort using the dish at Rangimoana, on the country's South Island. From the couple of hundred stars examined to date, nothing has yet been picked up. Since 1995, however, the effort has been scaled up sharply: that year the Canadian federal legislature approved an eight-year grant of $12 million a year to fund a thorough hunt for life beyond the Earth. A large share of that money is going into the specialised equipment required to monitor many frequencies simultaneously. The work is split in two. One half is a targeted survey carried out on some of the largest dishes in existence - the Canadian-run instrument at Fenwick Bay in Newfoundland and the German telescope at Osterholt - which sweeps the nearest 1,500 promising stars at high sensitivity across the 1200-to-3400 MHz band. The other half, a broader and less sensitive watch over the whole sky, relies on the smaller antennas of the Orbital Tracking Network.
To scatter information across the galaxy an alien society might use any number of methods, but most either demand enormous power or fade badly over the immense stretches between the stars. As it happens, radio waves in the band from 1200 to 3400 MHz carry furthest for a given transmitting power, which is why every search so far has focused on that part of the spectrum. A range of programmes worldwide has already made the attempt, among them a New Zealand effort using the dish at Rangimoana, on the country's South Island. From the couple of hundred stars examined to date, nothing has yet been picked up. Since 1995, however, the effort has been scaled up sharply: that year the Canadian federal legislature approved an eight-year grant of $12 million a year to fund a thorough hunt for life beyond the Earth. A large share of that money is going into the specialised equipment required to monitor many frequencies simultaneously. The work is split in two. One half is a targeted survey carried out on some of the largest dishes in existence - the Canadian-run instrument at Fenwick Bay in Newfoundland and the German telescope at Osterholt - which sweeps the nearest 1,500 promising stars at high sensitivity across the 1200-to-3400 MHz band. The other half, a broader and less sensitive watch over the whole sky, relies on the smaller antennas of the Orbital Tracking Network.
F. How we ought to respond, should a signal from another civilisation actually be detected, is a matter of real dispute. On one point there is unanimity: an immediate reply is out of the question. Setting aside the sheer impracticality of despatching an answer across such distances on short notice, the act would throw up a whole range of ethical problems that the world as a whole would need to work through before any message went out. Could humanity withstand the shock of confronting a civilisation far older and more advanced than itself? Fortunately, none of this is pressing. Because the stars under examination sit hundreds of light years away, their signals take centuries to reach us, and our reply would take centuries more to arrive; a pause of a few years, or even decades, while humanity argues over whether to answer and painstakingly composes a response, would therefore make little practical difference to how events eventually unfold.
How we ought to respond, should a signal from another civilisation actually be detected, is a matter of real dispute. On one point there is unanimity: an immediate reply is out of the question. Setting aside the sheer impracticality of despatching an answer across such distances on short notice, the act would throw up a whole range of ethical problems that the world as a whole would need to work through before any message went out. Could humanity withstand the shock of confronting a civilisation far older and more advanced than itself? Fortunately, none of this is pressing. Because the stars under examination sit hundreds of light years away, their signals take centuries to reach us, and our reply would take centuries more to arrive; a pause of a few years, or even decades, while humanity argues over whether to answer and painstakingly composes a response, would therefore make little practical difference to how events eventually unfold.
Trace life back far enough and all of it once belonged to the sea. Time and again through evolution, bold pioneers from one animal group after another hauled themselves ashore — some reaching the driest of deserts — carrying a supply of their own private seawater with them, locked inside their blood and cellular fluids. The reptiles, birds, mammals and insects around us are only part of the story: scorpions, snails, spiders, worms, millipedes and centipedes, and crustaceans such as woodlice and land crabs have all made a go of it beyond the water too. Nor should the plants be forgotten, for until they had colonised the land, none of these animal journeys could have happened.
The shift from sea to land forced a thoroughgoing overhaul of the body, and two systems above all had to be rebuilt: respiration and water balance. And yet plenty of committed land animals later reversed course, gave up the terrestrial machinery they had acquired, and headed back into the water. The seals are only halfway through such a journey, hinting at what the in-between stages looked like on the road to extreme cases like whales and dugongs. Whales — dolphins among them, since dolphins are simply small whales — together with dugongs and the manatees, abandoned terrestrial life entirely and slipped back into a wholly marine existence. Not even breeding brings them ashore. Air, though, is still what they breathe: they never re-evolved anything like the gills their sea-dwelling ancestors once had. Turtles, too, returned to the sea in the remote past, and, like every backboned animal that went back to the water, they breathe air. In one respect, though, they have held back more than whales or dugongs have, since the beaches are still where they come to lay their eggs.
Turtles alive today, the evidence shows, all descend from an ancestor that lived on land in an era earlier than most of the dinosaurs. Close to the root of the present-day turtle and tortoise line sit two notable fossils, Archelochersis vendhali and Prototestudo brannei, laid down early in the dinosaurs' reign. But when only scraps of a fossil survive, how is its owner's habitat — land or sea — to be decided? Now and then the answer is plain enough. The ichthyosaurs illustrate this: reptiles that lived alongside the dinosaurs, fin-bearing and sleek of body, they left fossils so dolphin-like that they can only have lived, as dolphins do, in the water. With turtles the task is harder, and one route to an answer lies in the bones of the forelimb.
This is the route that Marta Öberg and Rurik Solheim, at Vendhal University, chose to follow. From the forelimb bones of 63 living species of turtles and tortoises they recorded three measurements, and for each species they plotted those three figures against one another on triangular graph paper, an arrangement that made the whole set of species readily comparable. On the resulting chart the land tortoises bunched into a dense cluster high near the top of the triangle, while the sea turtles gathered lower down, with no overlap between them. That gap closed only once species dividing their lives between land and water were added: these semi-aquatic forms came to rest at roughly the midpoint of the triangle, between the 'wet' assembly below and the 'dry' assembly above. The last step was to see where the fossils belonged. On this the bones of A. vendhali and P. brannei were unambiguous — their points sat squarely within the dry assembly, which meant that both long-extinct creatures had been land animals, from an age before the turtles ever went back to the sea.
You might therefore conclude that today's land tortoises have stayed ashore ever since those distant terrestrial beginnings — as most mammals did after a few of their number returned to the sea. Not so. Sketch out the family tree of modern turtles and tortoises, and nearly every branch proves to be aquatic; the land tortoises form just a single twig buried deep among otherwise water-dwelling lineages. This implies that they did not occupy dry ground unbroken from the days of A. vendhali and P. brannei onwards. Their forebears, instead, were among the creatures that slipped back into the water, only for the line to climb ashore again in comparatively recent times.
What tortoises embody, then, is a striking double homecoming. Like every mammal, reptile and bird, they count marine fish among their distant ancestors, and behind those fish stretched a line of worm-like beings reaching back, still within the sea, to the earliest bacteria. A later set of ancestors took up life on land for many generations; later ancestors again abandoned it for the water and became sea turtles. And then, at the last, the line hauled itself ashore a second time to become the tortoises — a few of which now live in the most arid deserts on Earth.
