WEBVTT

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[NARRATOR:] Across the American 
 Southwest, golden deserts,

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dotted by cacti and 
 brush, stretch for miles.

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Yet here in New Mexico's 
 Valley of Fire,

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the landscape changes 
 dramatically.

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Patches of black rock 
 interrupt the sand,

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remnants of volcanic eruptions

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that occurred about 
 1000 years ago.

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The eruptions spewed 
 a river of lava more

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than 40 miles long, 
 across the desert.

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As the molten rock 
 cooled, it darkened,

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leaving any creature dependent

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on camouflage in 
 serious trouble.

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[DR. CARROLL:] In the 
 complex battle of life,

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one of the constant 
 struggles is between seeing

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and not being seen,

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the evolutionary game 
 of hide and seek.

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And we've come here to the 
 Valley of Fire in New Mexico,

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a battlefield, to find one 
 of the tiniest soldiers

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and what it can teach us 
 about how evolution works.

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[NARRATOR:] On the desert sands, 
 the rock pocket mouse blends

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in perfectly, its 
 light-colored fur concealing it

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from predators.

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But on dark lava, the same 
 fur makes the mouse stand out,

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attracting the many 
 creatures that see it as food.

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[DR. NACHMAN:] These mice are 
 the Snickers bar of the desert.

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They're eaten by foxes, and 
 coyotes, and rattlesnakes,

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and certainly by owls and 
 maybe even occasionally hawks.

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And most of those predators 
 are visual predators.

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[NARRATOR:] So what 
 happened to the pocket mice

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that found themselves 
 on this new terrain?

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When I accompany biologist 
 Michael Nachman onto the lava,

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it doesn't take long 
 to find out.

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[DR. NACHMAN:] Oh, 
 this one is closed.

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[DR. CARROLL:] Does it 
 have something in it?

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[NARRATOR:] Nachman has 
 been collecting mice,

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unharmed, in traps.

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[DR. CARROLL:] And 
 it's a dark one.

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[DR. NACHMAN:] It is.

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[DR. CARROLL:] Now are most of 
 the ones you find up here dark?

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[DR. NACHMAN:] Almost 
 all of them.

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[NARRATOR:] Not only have the 
 mice here evolved to be as dark

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as the rock, the color 
 change has occurred precisely

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where it will conceal 
 them from hunters.

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[DR. CARROLL:] It has a bit 
 of a white underbelly too.

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[DR. NACHMAN:] That's right.

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All of the dark ones here

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and on other lava flows 
 have a white underbelly

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and presumably there is no 
 selection for dark on the belly

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because predators are 
 coming from above.

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[NARRATOR:] Left to themselves, 
 the mice show no preference

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for light or dark rocks.

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It's the predators that 
 have made the difference.

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[DR. NACHMAN:] The change in 
 color over evolutionary time

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in the population is driven by 
 predators weeding out the mice

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that don't match 
 their background.

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[NARRATOR:] But how did the dark 
 mice arise in the first place?

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[DR. NACHMAN:] When 
 a black mouse appears

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in a light population of 
 mice, that is usually going

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to be due to a new mutation.

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And those are random 
 and rare events.

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[NARRATOR:] To fully understand 
 the pocket mouse transformation,

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Nachman moves from 
 the lava to the lab.

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He and his team extract DNA 
 from light and dark mice taken

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from one desert region.

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The aim? To find one or 
 more genetic mutations

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that caused dark coloration.

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A mutation is a change 
 in the chemical letters

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that make up our genes.

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It's a copying error that may 
 occur when our cells divide.

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[DR. CARROLL:] Mutation 
 seems to mean

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that something bad has happened.

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Well, mutations are 
 neither good or bad.

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Whether they are favored, or 
 whether they are rejected,

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or whether they're just neutral,

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depends upon the conditions 
 an organism finds itself.

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So, for the pocket mouse, a 
 mutation that caused the mouse

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to turn black, that is good if 
 you're living on black rock,

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and it's bad if you're living 
 out in the sandy desert.

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[DR. NACHMAN:] The 
 light mice are all

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on the bottom: here, 
 here, here...

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[NARRATOR:] Fur color is a 
 trait controlled by many genes.

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To figure out how dark mice 
 evolved, Nachman focuses

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on how these genes differ 
 in dark and light mice.

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One by one the genes 
 prove identical.

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But at last, something 
 does turn up.

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The difference between 
 dark and light mice boils

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down to a difference of 
 four chemical letters

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in a gene called Mc1r.

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Because the gene controls 
 the amount of dark pigment

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in a mouse's hair follicles,

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a mouse with these 
 mutations grows dark fur,

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which gives it an advantage 
 on a dark background.

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But still, that's one mouse.

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How would its dark fur 
 spread to a whole population?

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[DR. NACHMAN:] This lava flow 
 is about a thousand years old.

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And so you might wonder,
 has there been enough time?

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It's only been a thousand years.

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It's a very short period of time 
 for a new mutation to come along

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and spread, so that 
 all of the mice

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on this lava flow are black.

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Because really, they all are.

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[NARRATOR:] Indeed, 
 such a rapid spread

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of a mutation may 
 seem unlikely--

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until you do the math.

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[DR. CARROLL:] And 
 the reason is,

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that while only one new mouse 
 born in 100,000 may be black,

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hundreds of thousands of mice 
 are born in any given year.

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And then those mice that are 
 black have enough advantage

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that their babies do better 
 and they have more offspring.

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And their offspring 
 have more offspring.

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And just about a 5% advantage 
 compounded year in and year

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out can very quickly turn 
 the whole population black

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as we see today.

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[NARRATOR:] If dark color gives 
 mice a 1% competitive advantage,

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and you start with 1% of 
 the population being dark,

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in about 1000 years, 95% 
 of the mice will be dark.

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If instead the dark color 
 gives them a 10% advantage,

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then it only takes 100 years.

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Thanks to Nachman's mice, 
 science has an example

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of evolution, crystal 
 clear in every detail.

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[DR. NACHMAN:] What's 
 exciting about this is

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that we have a system that's 
 very simple ecologically.

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You have dark rocks and 
 you have light rocks.

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And you have dark 
 mice and light mice.

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It couldn't be simpler.

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[DR. CARROLL:] We know 
 who the predators are,

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what the selective force is.

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We know precisely 
 the genetic basis

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of what makes the 
 mice have an advantage

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or a disadvantage depending 
 upon where they live.

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All the pieces are 
 finally together.

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It's a perfect illustration

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of Darwin's process 
 of natural selection.

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[NARRATOR:] In fact, 
 it's more than that.

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For Nachman's mice also 
 counter a common misconception:

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that evolution is 
 a random process.

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[DR. CARROLL:] Well, there 
 is one random component,

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and that's the process 
 of mutation.

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Mutations occur at 
 random throughout our DNA.

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Every new organism is born 
 with a new set of mutations.

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But while mutation is random, 
 natural selection is not.

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Natural selection sorts 
 out the winners and losers

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and that's really 
 what the whole process

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of evolution is driven by.

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[NARRATOR:] But if natural 
 selection is not random,

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would it produce the same result 
 under the same conditions?

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It does. And here's proof.

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Rock pocket mice collected by 
 Nachman from other lava flows

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in other parts of the Southwest.

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[DR. NACHMAN:] These are 
 two different black mice

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and they each evolved 
 on different lava flows.

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And the lava flows are hundreds 
 of miles apart but the changes,

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the genetic changes that 
 made these mice black,

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were different in each case.

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And what's amazing to me is 
 how similar the black mice are.

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We didn't know when we started 
 this whether we would find

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that they were the same 
 genes or different genes.

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And we were really 
 surprised to find

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that they were completely 
 different genes.

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And yet, if you look at the 
 mice they look almost identical.

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[NARRATOR:] Clearly, there 
 are different genetic ways

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to make a mouse dark.

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But once the beneficial 
 mutations appear,

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natural selection, the 
 non-random part of evolution,

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can, under very similar 
 conditions,

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favor very similar adaptations.

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[DR. NACHMAN:] In effect, 
 each of these lava flows is

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like rewinding the tape of 
 life and allowing evolution

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to occur again and again.

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And in each case, we find 
 the dark mice have evolved.

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[NARRATOR:] The rock pocket 
 mice show us that evolution can

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and does repeat itself...

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and why evolutionary 
 change is never-ending.

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As environments transform, 
 so must the species

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that inhabit them, adapting 
 and re-adapting in the great

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and complex battle of life.