CLUES TO THE PASTOne breakthrough that helped set the stagefor Darwin’s 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, and that all of them were still around. Neither ofthese ideas turned out to be true.

For example, amammoth may look like a modern elephant, but itisn’t one. Scientist GeorgesCuvier proved that in the1790s, when he comparedfossil mammoths with elephantsalive today. Mammoths were not onlydifferent 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 livinganimals. 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. Duringvery ancient times, Earth was home to many 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 Earth’s long history?

8

Mammoths helped prove

that different animals

existed in the very

ancient past.

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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 DARWINThe clues were there, but no one had yetfigured out what they meant.Then,in 1831, a young Englishman named Charles Darwin set out on a sailing ship called the Beagle — a voyage that would eventually lead him to solve the mystery of evolution.

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

Darwin’s job was to keep the captain company at dinner and to collect

plant and animal specimens at theplaces they stopped.

While on the isolatedGalápagos Islands (a chain ofvolcanic islands off the coast of 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 onlywith cobras, giraffes with

giraffes and ivory-billed woodpeckers withother ivory-billed wood-peckers. (Trybreeding a shark and a poodle andyou’re going to be out of luck.)

For most animals and plants,this is the most useful definition of species. But this definition doesn’t work for some life-forms —especially microscopic ones. Some of these life-forms don’t breedtogether at all. Instead, theyreproduce 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, theydeveloped 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 — howdid it happen?

Finches’ beaks are adapted to the different

foods available on various Galápagos Islands.

is

GA

LA

P

ÁG

OS

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 likelyto survive. He also knew that parentscould pass their traits on to theiroffspring. But how did newtraits come about?And how weretraits passed onto the nextgeneration? InDarwin’s time, 150 yearsago, people could onlyguess.Today, we know that it’sall 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” — forgrowing that living thing.Your genes instruct yourcells to divide in a way thatgives you certain traits. Forexample, you might have genesfor curly hair or straight, blue eyesor brown and so on — everything that makes you who you are.

Many people describe this genetic 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, DNA

is a set of instructionsfor a process ofdevelopment. Ifthose instructions

are followed, a cellcan divide over and

over and eventually growinto a plant or animal or otherliving thing.

This difference betweenblueprints and recipes is

important for understandingevolution. If living things

had blueprints, one part could be moved or redesigned orsubstituted without

much affecting thewhole organism. But

suppose you misread arecipe and accidentally

changed the cooking time or 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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We’re All MutantsYou are a result of mutations — and so isevery other living thing.

A mutation is an accidental, permanentchange in the genetic instructions formaking a living thing. Because mutationsare random changes, they can have eitherharmful 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 RiverBlindness, 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 be bad. For example, all animals share a “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 fewfortunate people immune to thevirus that causes AIDS.

When you’re talking aboutmutations, the smaller the better.But, as time passes, many smallchanges can add up to bigdifferences.

“Mutants” in the movies can have superhuman

body parts and abilities or super-bad personalities

and looks. These fantastic

exaggerations are inspired

by the real biological

idea of “mutation,”

or random genetic

change.

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Moths with this speckled pattern were most common

because they had better camouflage …

… until coal pollution altered their environment.

Then the previously rare darker moths had the

advantage, and soon became the most common type.

Yes. Occasionally, wecan watch evolutionin action.Take thepeppered moth,which is found in

many parts of England. Most of theseinsects were light colored with darkpepperlike speckles, while a rare few weredark all over. The common pepperedpattern was good camouflage against thelight-colored bark and lichens on the treeswhere 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 treeswith 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 easyprey 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 adaptation in the moths.That’s natural selection andevolution in action!

“Can we ever see evolution happening?”

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MARCH OF PROGRESS

Don’t be fooled by this drawing showing evolution as 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, we’re still working out exactlyhow they are related.

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

But knowing that one fossil is older than arelated fossil doesn’t tell us whether it is a directancestor of the other fossil. Think of your own

family: Your father and your uncle are both closelyrelated to you, and they’re both older than you. Butyou’re descended only from your father — not fromyour 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 can’t say forsure which one.

Consider the Neanderthals. These human relativeswere 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 nowstrongly suggests that Neanderthals were just an“uncle” to modern humans.

Humans

Bonobos

Chimpanzees

Gorillas

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Yes, eyes arecomplicated.Theyhave 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 notwork if any part were missing. For example,they say an eye without a lens couldn’tfocus.Therefore, eyes could not haveevolved in small steps from simpler designs — they had to be made all at once. Goodbye, Darwin!

This might sound pretty convincing,except for one thing: It’s just not true that eyes need all those parts to work. AsDarwin pointed out, nature today is full ofeye designs much simpler than ours.

For example, there are worms withclusters of light-sensitive cells on their skin.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 as complicated as my eyes?”

This sea creature, called

a flatworm, has many

simple cup-shaped eyes

on its body.

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Some animals have deeper cup-shapedeyes that give them better vision, althoughit’s still extremely blurry. But simplynarrowing the opening of cup-shaped eyesfixes that problem. A narrow “pinhole”opening 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 bit of transparent skin.Any slight bulge in thistransparent covering creates a crude lens thathelps focus light.This happens in manyanimals today, including the apple snail. It’seasy 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 pinhole

eyes of the

chambered

nautilus are just

open hollow spaces

with no covering lens.

Simple light sensitive cells … … lead to cup-shaped eyes … … that lead to pinhole eyes like

those of the chambered nautilus.

… and finally to high-quality

lenses like ours.

… which leads to crude lenses …A transparent cover develops to

protect the eye …

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