Evolution Past Present

8/10/2019 Evolution Past Present

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CLUES TO THE PASTOne breakthrough that helped set the stagefor Darwins theory of evolution was thediscovery that Earth was once home

to animals that no longer exist.For hundreds of years,

many people believed thatall creatures were createdat the same time, andthat all of them werestill around. Neither of these ideas turned outto be true.

For example, amammoth may look likea modern elephant, but itisnt one. Scientist GeorgesCuvier proved that in the1790s, when he comparedfossil mammoths with elephantsalive today. Mammoths were not only different from elephants they had goneextinct. They had died out and vanished

from the Earth.

The idea that some animals had becomeextinct was confirmed when people foundstrange fossils totally unlike any living

animals. One fossil hunter was a youngEnglish girl named Mary Anning. Around1810, she discovered the first completespecimen of an extinct ichthyosaur areptile with a sharklike body streamlinedfor life in the sea.

Anning went on to find other importantfossils. She was one of the great fossilhunters of all time.

The fossils discovered by Anning andothers were a real shock to people. During

very ancient times, Earth was home tomany kinds of animals that had since goneextinct. Fossils provided rock solid evidencethat life was different in the past.

But how far in the past? And what sortsof changes had occurred in living thingsduring Earths long history?

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Mammoths helped provethat different animals

existed in the veryancient past.

Reprinted from Evolution: How We and All Living ThingsCame to BePermission of Kids Can PressCopyright 2010 Daniel Loxtonwww.kidscanpress.com


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Ichthyosaurs were reptiles that evolved shark-shaped bodies. They ruled the sea for 150 million years,during the age of the dinosaurs.

An ichthyosaur fossil


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ENTER CHARLES DARWIN The clues were there, but no one had yetfigured out what they meant.Then,in 1831, a young Englishman

named Charles Darwin set outon a sailing ship called the Beagle a voyage that wouldeventually lead him to solvethe mystery of evolution.

A medical school dropout,Darwin was a rich kid with apassion for the natural world.It was his interest in sciencethat led him to volunteer aboardthe Beagle , which was on a map-making mission around the globe.

Darwins job was to keep the captaincompany at dinner and to collect

plant and animal specimens at the

places they stopped. While on the isolated

Galpagos Islands (a chain of volcanic islands off the coastof South America), Darwincollected tortoises and birds.He took specimens fromseveral of the islands. Later, in

England, a bird expert noticedsomething remarkable

although the birds looked different,they were actually all finches.

H.M.S. BEAGLE


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What exactly

a species? A species is a group of living thingsthat can reproduce with oneanother, but not with other life-forms.Cobras can breed only

with cobras, giraffes with giraffes and ivory-billed woodpeckers with other ivory-billed wood-peckers. (Try breeding a shark and a poodle and

youre going to be out of luck.)For most animals and plants,

this is the most useful definitionof species. But this definitiondoesnt work for some life-forms especially microscopic ones. Someof these life-forms dont breedtogether at all. Instead, they reproduce by splitting into two.

Darwin came up with a theory toexplain why the finches looked so differentfrom one island to another. He theorizedthat long ago, mainland finches had beenblown to the islands. Once there, they developed different adaptations to takeadvantage of the different foods available tothem. On one island, the finches had largebeaks for cracking tough seeds. On another,they had long thin beaks for catchinginsects and so on.

But if that was true if one speciescould turn into several new species how did it happen?

Finches beaks are adapted to the different foods available on various Galpagos Islands.

is

G

A L

A P

G

O S I S L A N D S

Pacific OceanSouth

America


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HOW CHANGE HAPPENSDarwin noticed that plants and animals hadtraits that made them more or less likely to survive. He also knew that parents

could pass their traits on to their offspring. But how did new traits come about?

And how weretraits passed onto the nextgeneration? InDarwins time, 150 yearsago, people could only guess.Today, we know that itsall in the genes.

Inside every cell of every livingthing (including you) is a long,complicated chainlike moleculecalled DNA. It contains thechemically coded instructions called genes for growing that living thing.

Your genes instruct your

cells to divide in a way thatgives you certain traits. For example, you might have genesfor curly hair or straight,blue eyesor brown and so on everythingthat makes you who you are.

Many people describe thisgenetic code as beinglike a blueprint.

After all, DNA doesinclude instructionsfor buildingsomething. Butbiologist Richard

Dawkins thinks DNA is more like a recipe. A blueprint describes a thing, but a recipe

describes a process for making a thing.

Like a cake recipe, DNAis a set of instructions

for a process of development. If those instructions

are followed, a cellcan divide over and

over and eventually grow into a plant or animal or other living thing.

This difference betweenblueprints and recipes is

important for understandingevolution. If living things

had blueprints, onepart could be movedor redesigned or substituted without

much affecting the

whole organism. Butsuppose you misread a

recipe and accidentally changed the cooking timeor substituted salt for sugar.

You would changethe entire cake.

So, DNA is a lotlike a recipe for growing aliving thing. And changehappens when there is anerror, called a mutation, in

those genetic instructions.


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Were All Mutants You are a result of mutations and so isevery other living thing.

A mutation is an accidental, permanentchange in the genetic instructions for

making a living thing. Because mutationsare random changes, they can have either harmful or helpful effects. For example,some women have a mutation that makesthem much more likely to get breast cancer.Some other people have mutations thatprotect them from diseases such as River Blindness, which, as its name suggests, cancause blindness.

A mutation can have a big or smallimpact or no detectable impact at all. It alldepends on which DNA instructions arechanged, and how big those changes are.

Some genetic instructions are soimportant that any change will bebad. For example, all animals sharea master gene for growing eyes.Changes to that gene result indeformed eyes or no eyes at all.

Genetic instructions aredelicately balanced, so suddenlarge changes are likely to causeproblems. On the other hand, smallmutations have a reasonable chanceof being neutral (not harmful) andmay even be helpful. For example,many people carry the harmlessmutation that causes blue eyes.

Another mutation makes a few fortunate people immune to the

virus that causes AIDS. When youre talking about

mutations, the smaller the better.But, as time passes, many smallchanges can add up to bigdifferences.

Mutants in the movies can have superhumanbody parts and abilities or super-bad personalitiesand looks. These fantasticexaggerations are inspired by the real biological

idea of mutation,or random geneticchange.

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Moths with this speckled pattern were most commonbecause they had better camouflage

until coal pollution altered their environment.Then the previously rare darker moths had theadvantage, and soon became the most common type.

Yes. Occasionally, wecan

watch evolutionin action.Take thepeppered moth,

which is found inmany parts of England. Most of theseinsects were light colored with dark pepperlike speckles, while a rare few weredark all over. The common pepperedpattern was good camouflage against thelight-colored bark and lichens on the trees

where the moths liked to rest.The lesscommon darker moths were easier for birdsto spot and gobble up.

That was true until the IndustrialRevolution, which started in the late 1700s.

As people began to burn more coal topower new factories, coal smoke spread

over the countryside. It killed the light-

colored lichens and blackened the trees with soot.When the moths habitatdarkened, the light peppered patternstopped being good camouflage. Instead,the peppered moths now stood out morethan the dark moths, making them easy prey for hungry birds.

Suddenly, dark moths had a better chanceof surviving and breeding.The dark coloringstopped being a problem and became anadvantage.This advantage was passed alongto new generations.

Within a hundred years, almost all themoths were dark colored. A change in theenvironment led to a physical adaptationin the moths.Thats natural selection andevolution in action!

Can we ever seeevolution happening?


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M ARCH OF P ROGRESS

Dont be fooled by this drawing showing evolutionas a simple March of Progress. The real story

of human evolution is far more complicated, andscientists are still working on piecing it together. Although we know that the fossils of ancient humanrelatives are related, were still working out exactly how they are related.

We can tell that ancient hominids are relatedto us by studying their skeletons, teeth and otherevidence. We can also arrange the many species of extinct human relatives on a timeline: This speciesis older than that species and so on.

But knowing that one fossil isolder than a related fossil doesnt tell us whether it is adirect ancestor of the other fossil. Think of your own

family: Your father and your uncle are both closely related to you, and theyre both older than you. But

youredescended only from your father not from your uncle.

The same is true for fossils. We can often tellthat Species A was descended from either Species Bor a close relative of Species B. But we cant say forsure which one.

Consider the Neanderthals. These human relatives were powerful, muscular hunters who lived in Europeuntil they went extinct about 30 000 years ago.Scientists are still working out whether Neanderthalsare among our ancestors, or if they were a separatespecies related to our ancestors. DNA evidence now strongly suggests that Neanderthals were just anuncle to modern humans.

H u m a n s

B o n o b o s

C h i m p a n z e

G o r

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Yes, eyes arecomplicated.They have many parts that

work together: an auto-focus lens, an iris tocontrol the amount of light entering theeye, a bunch of little muscles to control thedirection of our gaze and so on.

Some people argue that eyes could not work if any part were missing. For example,they say an eye without a lens couldntfocus.Therefore, eyes could not haveevolved in small steps from simplerdesigns they had to be made all atonce. Goodbye, Darwin!

This might sound pretty convincing,except for one thing: Its just not truethat eyes need all those parts to work. AsDarwin pointed out,nature today is full of

eye designs much simpler than ours.

For example, there are worms with clusters of light-sensitive cells on theirskin.These cells let the worms tell nightfrom day.Then there are animals such asflatworms with eyes that are simple dentslined with light-sensitive cells.These cellscan detect the direction light comes fromand even detect motion.

How could evolution produce something ascomplicated as my eyes?

This sea creature, called a flatworm, has many

simple cup-shaped eyeson its body.


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Some animals have deeper cup-shapedeyes that give them better vision, although its still extremely blurry. But simply

narrowing the opening of cup-shaped eyesfixes that problem. A narrow pinholeopening helps focus, even without a lens.(Some photographers use pinhole camerasthat work in exactly this way.) In naturetoday, a squid relative called the chamberednautilus has this type of pinhole eye.

Then there are animals whose pinholeeyes are sealed over and protected by a bitof transparent skin.Any slight bulge in this

transparent covering creates a crude lens thathelps focus light.This happens in many animals today, including the apple snail. Itseasy to see how top-quality lenses like ourscould evolve any improvement in theclarity or focusing power of the lens is a stepin the right direction.

These many kinds of eyes illustratethe series of tiny steps that couldslowly transform ordinary skin cellsinto complex human-type eyes.Every step works. Every step is an

improvement. And every step is foundin animals alive today.

The pinholeeyes of thechambered nautilus are just open hollow spaceswith no covering lens.

Simple light sensitive cells lead to cup-shaped eyes that lead to pinhole eyes likethose of the chambered nautilus.

and finally to high-qualitylenses like ours.

which leads to crude lenses A transparent cover develops to protect the eye

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Evolution Past Present

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