Climbing Our Family Tree

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Climbing Our Family Tree

  • Commentary
  • For the author, see David R. Holmes, Stalking the Academic Communist: Intellectual Freedom and the Firing of Alex Novikoff. Hanover & London: University Press of New England, 1989

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Citation preview

t LIM B N°G our family tree 01

by Alex Novikoff Illustrated by John English A YOUNG WORLD BOOK PUBLISHED BY INTERNATIONAL PUBLISHERS

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COPYRIGHT,

1945, BY INTERNATIONAL PUBLISHERS CO., INC.

CONTENTS LUNGS NEXT

4

THE OLDEST FISH

8

EVERYTHING CHANGES

TOO SLOW TO SEE

51

LAND HOt

10

SOME FAMILY TREES

ENTER THE DINOSAURS

HOW DO YOU KNOW?

EXIT THE DINOSAURS

THE VOYAGE OF THE

SMALL BUT MIGHTY 18

"BEAGLE"

21

THEY LOOK ALIKE

22

THEY GROW ALIKE

24

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68

HERE COMES MAN

70

ANCESTOR

26 .

60

PORTRAIT OF AN

71 73

THE OLD STONE AGE THE NEW STONE AGE

COUSINS UNDER THE SKIN

58

THE AGE OF MAMMALS

CLUES AND MORE CLUES

LEFTOVERS

57

THE TIMES

20

WONDER

52

SIXTY MILLION YEARS BEHIND

WHAT MADE DARWIN

27

ANIMAL) VEGETABLE,

~8

A DETECTIVE STORY

MINERAL

THE RACES OF MANKIND

32

THE OLDEST MEAT

34

FOOTPRINTS IN STONE

74

A NEW KIND OF CHANGE

30

STONE ANCESTORS

IS MAN HERE TO STAY?

ACTION! CAMERA!

WISHING DIDN'T DO IT

THE MISSING MAN

THE WHITE-EYED FLY

CLOCKS INSIDE ROCKS

LUCKY ANIMALS

LIFE BEGINS PRINTED IN THE U.S.A.

50

LUNGS AND LEGS

9

TOO SMALL TO SEE

!his Young World Book is produced In full .compliance with the Government's regulations for conserving p~er and other essential materials.

VERY DRY WEATHER

44

KING OF BEASTS DEAD END

BLIND ALLEYS

42

ANIMAL PROGRESS MAN'S FREEDOM

45

INDEX

IT HAD A BRAIN

27851

94

go

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THE OLDEST FISH Off the west coast of Africa a crew of native fishermen were hauling their nets into a trawler one day in December, 1938. ~udenly one of the men gave a shout. There in the net was a strange fish. Noone had ever seen anything like it before. It was a handsome creature-about five feet long, steel blue, with dark blue eyes . .Its fins were large and peculiarly shaped, something like flippers. The big fish leaped and thrashed among the smaller ones'. Presently the fishermen called the trawler captain to look at their o.d d catch. He bent over to examine it more closely. As he put out his hand to see whether it was alive, it snapped at him with its powerful jaws. The. fish stayed alive" for almost four hours. The m~re Captain Goosen thought about it, the more he suspected that here was something very unusual. Perhaps it might even have some scientific value. Just as soon as the trawler docked in East London. he sent word to Miss Latimer, curator of the local museum. But by that ·time the African heat was at work and the oily body of the fish was decaying, Miss Latimer, too, thought the fishermen's net had brought up ' something new and strange that should be kept for scientists to see and study. So she had a taxidermist skin, stuff and mount the big blue fish. She was right in guessing that here was a new specimen, and for what she did the fish has been named after her-La~i. But alas, no one realized just how important the strange fish wasneither the fishermen nor Captain Goosen nor Miss Latimer nor the taxidermist. If they had, they would somehow have found a way to

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preserve every bit of it. Instead only the skin and a few parts of the skeleton were saved. This brand new fish was really the oldest fish ever caught! And all that 'scientists now have of it are the skin, the skull and a few bones. When scientists call this first Latimeria "old," they do not mean that it had lived for twenty years or fifty or a hundred before it was that fish almost exactly like it lived in the seas caught. They ~ean 300 million years ago, and no other fish we have today .res~bl so closely any of the creatures that swam about in those anCIent tlmes. We know about Latimeria's remote ancestors because they have and tail, the "been dug up as fossils. , They had the same kind of sca~e same jaws and gill covers and backbone. But more Important, t~ey had the same curious kind of fins, like limbs or paddles, from whIch they get their name-lobe-finned fish or simply lobe-fins. (Remembe.r those flipperlike fins; you will find out more about them later.) Latlmeria is the 'only lobe-fin ever seen alive. . There were plenty of lobe-fins· alive long ago when luxn~t forests and large-leaved evergreens grew in vast swamps covenng thousands of miles. At that time the only land-living creatures were amphibians (animals that had to spend ~art of their time in the water) and insects SOine with wings two feet wIde. Later,' great changes came over the earth. Mountains were heaved up and the Climate became drier and cooler. Now the strea~ ~n ponds began to dry up .and many fi~h disaper~. But not Latlmena s relatives. In spite of the changed chmate, they hved on and on. Then the earth's surface changed again. There were fewer and fewer lobe-fins in the streams. And finally, about 60 million years ago, the last of the'm were supposed to have died off. So to find Latimeria, 'a live lobe-fin, in 193 8 was like meeting a dinosaur in front of the Capitol!

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of the Dinosaurs, 80,000, years ago, North Americ was much bigger than it i now. Notice that the Wes Coast was still with Asia.

North America has a lot in 500,OOO,QOO That long ago, our Coast was connect Asia, and Wyoming 60 miles from the oce on the seashore.

Only 35,000 years ago (with About 300,000,000 years ago water covered a good deal of North America. It looked more like a group of islands than a real continent.

6

North America the same shape as it is today) the whole northern part of the continent was covered with

ice.

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we.eks late.T a cocoon; but by the time it breaks out of the cocoon a fe~ it has turned into a moth. The little gray tadpole of Apnl, ~lt a tall and no legs, by July has_ become the spotted leopard frog with strong

EVERYTHING CHANGES



Today dinosaurs no longer walk the earth. The woolly rhinoceros and the saber-toothed tiger have. disappeared. You cannot see a live mammoth in the zoo. New kinds of animals have taken their places. (The scientific way of saying this is: new species have developed.) 'The giant ferns and broad-leafed evergreens have died, and new species of plants grow out of the soil. Animals and plants are all different now. And that is what makes Latimeria so remarkable. Here is a species that has remained practically unchanged for millions of years. For everything in nature is continually changing. Some of these changes you can see for yourself. Ice melts to water, water vaporizes to steam. An apple blossom grows into an apple. Eggs hatch into baby chicks that grow into full-sized chickens. You can see the changes that take place in your own body as you get bigger and older. If you look around you can see many other changes. The earth itself changes. Perhaps you have watched a garden planted on a hillside. Every year the rain washes away some of the topsoil, unt:overing pebbles and leaving little gullies. Millions of years of rain and wind can wear down high mountains and turn them into low hills. The maps show how North America changed. Even our climate is not what it used to be. You may have heard your grandfather say that it doesn't snow the way it used to when he was a boy. Sometimes people imagine that the weather is different, but in certain places it is true that winters are slowly becoming warmer. We know that thirty thousand years ago Long Island, N ew York, was covered with an ice cap like that at the North Pole. Plants and animals change most of all. Some changes are so familiar to us that we hardly pay any attention to them. Seeds planted in the change into vegetables we eat in summer. A caterpillar spins

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hind legs and no tail. . But there are many changes in plants and anImals that we cannot see . It is the J·ob of scientists to find out how these changes work and to tell us about them.

TOO SMALL TO SE E You have felt your overcoat get thinner and thinner as the threads wore away. Your skin, your tongue, your stomach and other .organs, like the overcoat, also wear away. Yet they stay the same thIckness. How can that be? Your body is made up of millions and millions of tiny cel~ which wear out just like the threads. But as the old cells wear out, new ones Every second?f your lifetime ten mil~on red cells of your are pro~uced. blood break down and ten million new cells take thelT places. You cannot see an old cell wearing out or a new one growing except through a microscope. For even the largest. cell of the human, body, the egg cell, is less t.han six-thousandths of an Inch long. It would take about two million of them to fill a sewing thimble! Some changes are so small that they cannot be seen even through the strongest microscope. Scientists know by their results that they must take place, just as you know someone must have thrown the ball you see sailing over the fence even though you cannot see the thrower . Scientists know that the chemical substances of , our cells are continuously ,c hanging in a process called metabolism, which means "working over." The ' living matter of our bodies is worked over, destroyed and built up again all the time.

TOO SLOW TO_ SEE

ter, together with a great many other stars. A close-up view of sun and other stars would show a fiery ball of gas 600 times as hot the temperature at which water changes to steam. Although scientists Some changes occur so slowly that we cannot see them at all. 't altogether agreed _on the details in the first half of the mqvie, By observing the Tesults of these changes. too, scientists have figured t of them think it should be something like this: ?ut wh~t lnust have happened. These are the changes that take place A star from another cluster comes racing past the sun. It doesn't In specles of plants and animals. T~day there are over a lnillion different living species of plants it the sun , but it comes along close enough to exert an enormous pull its surface. This pull tears away huge mass of fiery gas, like a tidal and anImals. If you COIn pare pictures of them with pictures taken ve. As the stray star moves a-way, it leaves this mass of burning gas seventy-five years ago, you cannot see any real difference. A horse today uspended in space and spinning round the sun. Then the mass conenses into nine parts-nine planets, all gaseous fireballs , still revolving . ~¥·.n"Y'Id the sun. The earth is one of them. Soon the earth begins to cool and chemicals form out of the gases. ron and other metals form first and produce the central core of the rth. Then comes carbon which combines with the metals to form first chemical compounds. When the temperature falls to 200 times the heat of steam, a layer solid rock forms over the core of the earth which is still fluid. Above crust is the atmosphere, made up of various gases. The temperature looks like the horses in the Civil War. In fact, our horses might very •.r,~ " .., down still more, and torrents of boiling water begin to fall on well have been the models for Greek statues made three thousand years earth's surface, bringing down with them many chemicals that had ago; and our kind of wheat was found in the tomb of Tut-Ankh-Ahmen buried over three thousand years ago. Wheat has been wheat and horses have been horses for several thousand years. Now suppose we go further back than that and compare our plants and animals with the plants and animals that lived 60 million years ago. A very few speci~, like Latimeria, resemble their ancestors closely. Others have changed so much in that time that you can hardly see any resemblance at all. Look at the pictures on page 16, and you will discover how surprising the differences can be. of a fox terrier, with four toes on its Every species that we know today has come from some other nt feet: a much later three-toed horse the size of a collie: today's liorse, species. Among its ancestors there was one change after another, but larger, with only one toe. Above. these changes sometimes took millions of years. If we had a motion the Horse's toe bones changed picture projector that could speed up so fast that we could watch the years. ten million years go by in one minute, we would be able to see the InroUtlh changes of species from one to another, starting with the first tiny living things and ending with modern man. At this speed, " The March of the Species" would take about an hour and forty minutes. If the movie began further back in history, when ' the earth itself was being born, you would have to sit through a three-hour-and-twentyminute show. You would see the sun speeding through space in a

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already formed in the atmosphere. These chemicals, dissolved in t primItiVe oceans, produce more and more ,c omplicated chemica Eventually very special chemicals appear. These are the proteins, t most important chemicals in living matter. Scientists still do not quite understand how it happened but som how proteins were able to combine with each other and with ot~ chemicals until, after a long while, they developed into living matte At last the first living cell evolves; the movie is already half over N ow the real "March of Species" begins. At first there are ve simple cells. Then simple plants and anilnals develop from these cell From these in turn come all the plan ts and ani~ls we know toda Man appears only as the movie is coming to an end. The first speci of man .comes in less than 40 seconds before the close; the mode species of man only seven-thousandths of one second before the end This movie shows us that all plants and animals today, howeve different they seem, are relatives who have descended from the common ancestor and belong on the same family iree. This slow descent with change of one species from another . called evolution.

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SOME FAMILY TREES A family tree, as you might imagine from the name, is a sort diagram or map of a family. Here is your own family tree that sho how you are related to your brothers and cousins, your father, unc and aUl)t, grandfather and great-grandfather. If you start at the tips of any two branches and move a penc downward and inward along each branch, you will find that the t pencil lines will eventually meet at a point on the trunk. At this poi is the com1?1on ancestor of the two people at the tips. Thus, if yo start with you and your cousin and keep moving past uncle and fathe the lines will eventually meet at grandfather. He is the common a cestor of both you and your cousin. You are both descended from hi A scientist makes much the same sort of family tree to show ho different species of plants and animals are related to each other. picture the million plant and animal species in the world would ta a space far bigger than this page, so here is just a small part of t family tree that includes man and some of his nearest relative the vertebrates. (A vertebrate is an animal that has a backbone.) Just as you found your own and your cousin's common ancest on your family tree, you can find the common ancestor of any t species on his tree. If you start with man and gorilla and work dow ward, you will come to a meeting point at ape ancestor, the ancestor both gorilla and man. These apes lived on earth some twenty or thirt million years ago. We know about them because their skeletons hav been found in many parts of Europe and Asia. You can see fro this family tree that men are not descended from gorillas any more tha gorillas are descended from men; both gorillas and men are decende from this common ape ancestor. Notice that the branches of man and apes are nearer together tha those of man and monkeys. The nearer the two species are to each othe on their two branches, the more closely they are related. Man and ap are closer relatives than man and monkeys. A bird and a reptile a closer to each other than a fish and a monkey. Trace sQme relationshi out for yourself.

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DINOS ~)l.TIN

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HOW DO YOU KNOW? If these changes happen so slowly we can't see them, how do we know they have happened at all? A bird and a fish certainly don't look much alike. How can we be sure that they are related to each other? Up until 400 years ago most people were convinced the world was Hat. It was Fernando Magellan and his crew who proved that the world was round by sailing around it. In the same way, most people believed that everything in the world had been created in the beginning just exactly as it is now. It was Charles Darwin, more than any other person, who convinced people almost a hundred , years ago that the different species were all related to each other and showed how ' one species was descended from another.

THE VOYAGE OF THE BEAGLE ·

fa~t,

that his father once wrote him angrily, "You care for nothing but shooting, dogs, and rat-catching, and you will be a disgrace to yourself and all your family." , Charles' father then decided that he should be a doctor and sent him to medical schol~ But it soon became obvious that young Darwin was not at all interested in medicine. So his father tried to make a clergyman out of him and sent him to the' University at Cambridge. Still Darwin couldn't really make himself care for anything but hunting and natural history. As soon as he was ,graduated, one of Darwin's professors, a scientist who understood him better than his father, urged him to apply for the job of naturalist aboard H.M.S. Beagle. The ship was to make a voyage around the world, surveying trade routes and looking for ways to improve trade for British merchants in the far-off corners of the earth. The captain was willing to give up part of his own cabin to any young man who would go with him, without pay, as naturalist. Today no one remembers how much the Beagle helped British merchants. The information the trip yielded about trade was far less important than the knowledge Darwin obtained-knowledge that was to change people's way of thinking. It was during his trip on the Beagle that Darwin first began to develop his theory of evolution. Darwin's father obje.cted to the whole idea of his going. Finally Charles' uncle persuaded the father to agree. And even then young Darwin almost didn't get the job-ecaus~ the captain of the , Beagle thought you could judge a man's character by his nose. Darwin's, he said, showed he did not have enough energy and detennination for

Charles Darwin was born in Shrewsbury, England, on the very day on which another great man, Abraham Linc'o ln, was born in America-February 12, 1809. In those days schools did not teach science as they d~ today. Twelve-year-old Darwin, who wanted to spend his time out of doors collecting plants and watching animals, had to stay inside and learn how to write poetry. He was very bad at it-so bad, in

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the voyage. But at last he, too, said that Darwin could come a Later, when the two of them became good friends, the captain admit that his idea about noses was pretty farfetched. So Charles Darwin set sail from Plymouth on December 27, 1831. For almost five years he lived on the ship. Everywhere it sailed he collected facts and more facts-about rocks, plant; and animals. The more facts he gathered from different parts of the world, th~ more he became convinced that the things he observed in nature could not be explained by the old idea that each species had been separately created.

time, Darwin thought, the common ancestors of both the island and the mainland species must have traveled from the mainland to the islands. Later, all the species in both places, through slow changes, became different from each other.

CLUES AND MORE CLUES

While the Beagle was sailing in and out among the Galapagos Islands off the west .coast of South America, Darwin noticed a very remarkable and important thing: Although ' the plants and animals on the islands resembled those on the mainland, most of them belonged to different species. He saw also that the birds on one island were almost the same as those on another island-but not quite. There were all sorts of little differences in their coloring, their songs, their nests, their eggs. He collected no less than 23 .different species of land birds on the islands, not one of which was found on the mainland. He also saw two kinds of giant tortoise found only in the Galapagos, some so large that it took eight men to lift them. And he found that the natives could tell which island a tortoise came from just by looking at it. All these similarities and differences must mean something. "It strikes me with wonder," Darwin wrote in his diary. The more he wondered and observed, the more he began to realize there was only one possible answer to the puzzle. If all these species of plants and animals had developed from common ancestors, then it was easy to understand their similarities and differences. At some

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After the Beagle returned to England, Darwin began his first notebook on the origin of species. During the next twenty years he filled notebook after notebook with still more facts that he and others discovered about the world of living things. These facts all led to one conclusion: that all living things are descended from common ancestors. Darwin proved the truth of evolution, the descent with change of one species from another. Where others before him had failed, Darwin succeeded in convincing the world that he was right about evolution. He succeeded for two reasons: He collected an enormous number of facts and put them together so that they told the whole story. And he not only declared that evolution occurred, but he also explained how it worked and what caused it. This he called the theory of natural selection. You will find out more about it later in the book. Nearly a hundred years have passed since Darwin's great book, The Origin of Species by Means of Natural Selection} was published. People have found out new facts about evolution, and especially about inheritance. These facts have made more precise our ideas of how natural selection works. This does not mea,n the theory was wrong. On the contrary, a true theory is alive; like everything else in the world, it changes and grows. Only a dead, useless theory stays the same down to the last detail. Here are some of the things that proved to Darwin and other scientists that living things are related to each other.

THEY LOOK ALIKE Living things that are related to each other look like each other. And the closer the relationship, the more they look like each other. Has anyone ever told you that you have a family resemblance to your brother or your sister or your cousin? Have you ever noticed that people in the same family often look like each other? Look at the skeletons in the picture. The one on the left is a frog; the one on the right is a man. They look alike, don't they? Notice the skull, the arms, the legs, the backbone-especially the backbone. This means that both man and frog belong to the great family of vertebrates.

Man and the frog are not very close relatives. But see how much ore alike man and the chimpanzee are. Not only do they both have : skull, a backbone, arms and legs, but both also have hair, ,nipples and belly button. They have the same kind of face and nose. ~oth have fingers that can pick t~ings up. Both can ~alk on two feet. either erect or stooping.

Here is a picture of a man's arm, a dog's front' paw, a .bat's wing and a seal's flipper. They certainly do not look much alIke on the outside. But look at the second picture showing the bones underneath. Notice that each one has the same number of bones in pretty much th~ same position. ~ All these are certainly family resemblances. They can mean only one thing: All these living things are related to each other; they must have come from a COlnmon ancestor.

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THEY GROW ALIKE ' All animals related to each other grow and change in much the same way during their very early life. EmbTYo is the name ~e use for very young living creatures-chickens before they are hatched from the egg, for instance, or puppies and babies before they are born while they are still inside their mother's body. Here are the very early embryos of a man, a mouse, a monkey and a rabbit. Can you tell which is which? Here are the later embryos of a shark, a salamander, a chicken, a rat and a man. Look back at the family tree on page 16. Now you can see that the elnbryos of the close relatives look like each other. They have a family resemblance. But embryos of different vertebrates resemble each other in more than looks. They change and grow along a similar pattern. For instance, a human enlbryo at an early stage looks first like an embryo fish, then like an embryo amphibian, then like an embryo reptile, and finally it 'begins to acquire the characteristics that make it a mammal. Someone once called this process "man climbing his family tree."

Early in its development, the embryo of man has gill pouchesjust like those in the embry~ fish. In the fish, t?ese. pouches fin~ly form a series of slits through whICh the water comIng Into the fish s mouth leaves its body. In a human embryo, these pouches appear, then disappear two or three weeks later. Your heart has four chambers. But a human embryo, before it develops a heart with four chambers, for a little while has one with only two-like a fish-and then with three-like a reptile. And did you know that you once had a tail? Every human embryo during its fifth week of life, when it is only one-third of an inch long, has a tail about one-sixth the length of its tiny body. This usually disappears by the eighth week of life-but we all have a few of the bones, muscles and nerves with which to wag the tails we haven't got. All of us once had a coat of "fur," too-thick silky hair that, in the seventh month of life before birth, covered every bit 'o f our bodies. This fur coat disappears just before or soon after birth. So man's embryo develops for a little while like a ,vater-swallowing fish , like a reptile with three divisions in its heart, like a mammal with hair all over its body, like an anilnal with a tail. All these resemblances in the way man and the other vertebrates develop and grow can mean only one thing: All these living creatures are related to each other'; they must have come from a common ancestor.

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A HUMAN

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EMBRYO

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LEFTOVERS Can you wiggle your ears? If you were a rabbit or a. zebra, y could stretch your ears and catch the first rustle of danger. Long a man 's ancestors could do the same thing, but man cannot. Today still have a set of muscles for ear wiggling. Some people can actua use these muscles, but all of us havoe them whether we use them or n There is a special name for things like the ear-wiggling muscl which are no longer useful to an anim:aI but which are left over fro an earlier stage of development. They are called vestigial structure, They are like the umbrellas you see people carrying on a sunny aft noon, left over from a rainy morning when they were us~fl. l\fan h no less than 180 of these vestigial structures in his body! That is w the British scientist Julian Huxley called the human body "a museu of evolution." Long before the days of clothes, the body of man's ancestor, Ii the body of the ape and other mammals, was covered, with hair. Toda the hair has nlostly disappeared from the human body, but we sti have the muscles for moving it. Have you ever 'watched a cat besid the fire? Its hair lies down smoothly. But when it goes out into the col its hair stands on end. This traps air between the hair and the body an keeps the cat warm. Notice how the little hairs on your arm lie fl when you are warm. Then step into the cold for a minute and see ho some hairs stand up straight and how at the base of each hair the ski is raised in a little bump. The bumps-gooseflesh-certainly don't hel to keep you warm. They no longer have any function. 'They are ju one more proof that man is related to the animals with fur. Do you know what the little red fold of tissue at the inside corn of each of your eyes is? It is a vestige of a third eyelid that is no longe any use to man. Frogs and similar animals still have a well-develope third eyelid-a thin, transparent one that they close when they swi under water.

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A newborn baby can hang by its hands for several minutes, just as a baby monkey clings to its mother's fur. This is a ·leftover from a far-off past when the baby hung on while the mother swung through the trees. All these leftover vestigial structures can mean only one thing. You and the cat and the frog and the other vertebrates must have had a cOlnmon ancestor.

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COUSINS UNDER THE SKIN The shapes of bones and the development of embryos are resemblances that can be seen either with the naked eye or through a microscope. But there are invisible resemblances between living creatures, and the closer the two are on the family tree, the stronger these resemblances are. Every animal has certain specific chemicals in its blood and tissues. Blood chemists have developed a very accurate method of determining just how similar these chemicals are in any two animals. They have shown that certain groups of animals, li~e men and apes, or turtles and alligators, are close chemical relatives. Many disease germs are fOtlnd only in animals whose blood chemistry and cells are siln~ar. Specific germs need these specific chemical substances in order to live. Thus only men and apes can get measles. And the common louse will l~ve only on men and apes! If you know about the Red Cross blood bank, you have heard about blood groups. All human beings fit into one of four groupsA, B, AB and O-according to the chemical differences in their blood. These same chemicals, ' which cause the differences in human blood, are also found in the blood of apes and monkeys. All these chemical resemblances can mean only one thing: All these living creatures are related to each other; they must have had a common ancestor.

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. The ancestors of the ' ~ert ' ebrats are 'the iuvetbl:a~s ~ . the ·: ~n'l·I · n~ . J\ without backon~s. But.fr~ . m which of the ma~y 'groii}:;s: ~f .i:nvert~bl "' i(lJIo~ did the vertebrate.s devlop~frm -· . womis ~ lobst~, .. · jellY·fish? Or., . " jMU"'Vt their particular ~netoF the ear~hwom '~ or : th~ ' ~taifsh ,.: or ,' : · wliatP .T 0 find the answer, b'iologists observed th~ .' way ~ in, .~l1th embryqs of all thes.e groups oeveloped'. Th~ ' e~bryo e~hirod ' ~m:s ~ is, the starfish and ·their cousins-cleve'l oped . dtir~!lg. the J~rs : t ' 'day ,qf, in very much the same. 'Yay as. some''- o{ t,he' @mbr:yq . vertbTa~ .. . structure, (iook ' ~t the pi~ture) is ' found in' both : e~bryds ' a'rid _ n' 'n' ·u'·11:10",'' ' '' else in the animal kipg~on:I. This ,~as ' oril y one " clue~b f . · to · pin h,is 'whole ·important one. Perhaps a detective,wo'u ld not li~e and reputation on suc.h a sm~l fact. Yet..for m~riy. y'ea~s . 't h,is',w,as the' 'clue there was, and biologists were co~viqed it' m~ant the ,ec' JtlUIO(e] ~ I1l I , and the vertebrates' were relat , ed. ~ ". " . , .. _ And not very long ago ~t was ptov~d th,it the.y .were right:. in two British scientists showed that the ' vertehrates', .and,_ the 'echr.... of animais to contain certain , chemiCal · , suDt:an . ce! ~ were the, only group~ Then in 1942 all doubt was removed by ~ery acurte ' ~lod , .tes~ · 'W . rn.lc: ~ ~A showed the same thing-the echinoderms were the only inverte,b ra group closely related to the vertba~. . ' "

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, Chemistry, vestigial ·organs, development of em.bryos, hone , . ture and other similarities· all' point td 'the fact ' that the "animals ' plants we know today developed from ~omn ~ncestor by the ",'Y"n.~p(!,C " of evolution. But where are' these ancestors? .H ave we ever se~n them

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STONE ANCESTORS Have you ever picked up a stone from some rocky hillside or river bed and noticed on one side the outlines 'of a leaf or the shape of a like a piece of sculpture? If you have, the chances are that you ha found a fossil. Fossils are the remains of plants and animals which have been preserved for millions of years in the rocks under the earth's surface. Long chapters of the earth's history are written in fossils. They tell us what the earth was like millions of years ago, what kind of animals and plants grew on its surface and how they changed. They tell us a s half a billion years long. Most important of all, they actually show us the common ances from which different modern creatures are descended. They are real ancestors preserved in stone. Men have been digging up fossils for centuries, but it is only in the last two hundred years that they have been generally recognized as the remains of living things. In the Middle Ages people had all sorts of funny ideas about what fossils were. Almost everybody believed that

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God created the world in six days, so some people thought fossils might be God's failures, discarded after the hasty creation of the world. Others thought they might be "sports of nature" mysteriously created for some unknown reason. Still others thought that the little rock figures were molds and casts fronl which real animals and plants had been created. People who disapproved of science declared that fossils had been placed in the earth to confuse the geologist and humble him by making him 'feel how useless it was to try to fathom the mysterious ways Providence. We know now that, to become a fossil, the body of an animal or plant must be buried very quickly, so that it will not be eaten by other animals, decayed by the action of bacteria or dissolved by the weather. Sometimes animals have been buried speedily in quicksand or even lava and volcanic ash. But most fossils are found in spots that ~er once the shores or bottom of a shallow sea where animals were easily covered by layer upon larer of mud that eventually turned to stone. Sometimes great upheavals of the earth changed the fossil rocks, crushing or folding them. Then the fossils were destroyed. Fossils are rarely found in old rocks which have been worn down for millions of years by wind and weather.

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