Thank you to 23andMe for supporting PBS Digital Studios

This is Harrison.

Harrison's favorite hobbies include pooping, peeing, not sleeping, and costing me a fair

amount of money.

Not exactly the number one qualities you go looking for in a friend.

Yet, I knew he was the world's most perfect human the instant I laid eyes on him, I knew

I'd give up everything I have for him, even my life, without hesitation.

All of that, even though just a few months ago, we'd never even met.

The reason why?

Evolution, of course.

[MUSIC]

Staying alive long enough to have healthy offspring, and getting our traits from one

generation to the next is how we win the big-picture game of evolution.

It's kind of the whole point.

Some traits, whether it's being tall, or having stripes, or six fingers on each hand,

help us win more than others.

If they increase an organism's chances of surviving and reproducing, then they'll

become more common in the future.

This is one of Chuck D's basic principles in his theory of natural selection.

The measure of how successful a trait is at making it to the next generation, is called

fitness.

A trait that's more likely to be passed on has higher fitness compared to one that's

less likely to be passed on.

What we know today, and what Darwin didn't know back when he was figuring all this out,

is that all these traits from how our bodies are built to how we think are the product

of thousands of genes, all interacting with each other and the environment.

Knowing that makes you realize we are survival machines.

We exist to get our genes into the next generation.

Congrats!

Now you know the meaning of life.

Harrison represents my genes in the next generation.

Well, half of them, anyway.

He's my son.

Did I not mention that before?

What did you think, that I just stole someone's baby?

Now, just for fun, let's say a pack of velociraptors try to snatch him away.

But I step in and sacrifice my life to save his!

Which I would totally do.

Because he's awesome.

This self-sacrifice is an extreme example of altruism, which basically means your pain

for someone else's gain.

Thing is, this kind of risk or ultimate sacrifice–doesn't immediately make sense when you think of evolution.

If I become velociraptor food, my future chances of reproducing are now zero.

Because I'm dead.

So why would I give up 100% of my genes, the ones in my body, to save just half of them,

the ones he carries?

It might seem like a really bad exchange on my part, but it actually makes sense.

And some pretty basic math can help explain why.

Whether an act of altruism is worth it depends on the relationship between the two parties

involved.

The biologist WD Hamilton actually came up with an equation for this: It factors in the

cost to you, C; the benefit to the other individual, B; and how closely related the two of you

are, r.

Let's unpack this equation a bit.

If the cost to you is less than the benefit gained by the other individual, multiplied

by that individual's relatedness to you, then acting altruistically is worth it in

the eyes of evolution.

When two individuals aren't related at all, r will be zero.

Let's say a bus–full of velociraptors–is careening towards a stranger, completely unrelated

to me.

If I push them out of the way and die myself, I completely lose while the stranger completely

wins.

They might print something heroic about me in the newspaper, but technically this is

not the kind of behavior that evolution rewards.

But if the relatedness is greater than 0, things work differently.

Let's say the raptor bus is careening towards my first cousin, and I die saving them.

Because my cousin and I have one-eighth of our genes in common--our r is 0.125.

In this case, I don't completely lose.

I only *seven-eighths* lose.

If I push my sister out of the way of the raptor express?

Well, my sister and I have one-half the same genes, so I only *half* lose.

Understanding this, biologist JBS Haldane reportedly that he was “prepared to lay

down his life for eight cousins or two brothers.”

But let's say I'm given an even more difficult choice.

Two busses of velociraptors, one heading towards my brother and the other my child.

My brother and Harrison both share 50% of my genes.

I'd half-lose either way!

How do I decide what to do?

Well, I really really hope I never actually have to make this choice, because I love my

brother too, but I suspect most parents would save their kid first, and evolution has a

good reason for that.

Because Harrison is a tiny baby with his whole life ahead of him, and my brother and I are

adults, Harrison has the greatest “reproductive potential” among us.

That's a fancy way of saying that Harrison is likely to have more children from this

point on than either my brother or I will.

This is where we see the influence of genes through generations.

Each one of Harrison's future offspring will share about 25% of my genes.

If he has just two kids, I break even on my sacrifice.

And if he has more than that, or if any of his kids have kids of their own, I'd actually

be ahead.

All of these potential grandchildren and great-grandchildren mean sacrificing myself for my child is a

better decision for the future of my genes than by letting my child die.

Which I would never let happen, okay buddy?

Daddy promises.

Now we've been focusing on the most extreme examples of altruism, giving up your life,

but the fact is altruistic acts are fairly common among social animals.

Meerkat “guards” keep watch over the colony and raise an alarm if they spot a predator,

which puts them at risk.

Vampire bats regurgitate blood meals to neighbors that don't get enough food, and scrub jay

chicks hang around the nest after they grow up and help their parents raise their siblings.

If the individuals in these groups were completely unrelated, it'd be difficult to explain

why they do this.

But!

If the group members are related to each other, even only distantly, genes that underlie altruistic

behaviors like caring for children, protecting members of their group, sharing food, might

spread through a population not by upping the odds of an individual's own survival,

but by helping an individual's genes survive in their relatives--a mechanism known as kin

selection.

Kin selection can help us to understand how complex human behaviors might have evolved.

Of course, none of us sit there and run equations in our head to decide if it's worth helping

the people we care about.

And we can't trace an emotion as complex as love or kindness directly to a gene or

two.

But the way humans behave has been influenced by evolution the same as any other animal.

Helping each other helps us survive, and not just in this generation.

It ensures some part of us will make it into the next, and the next, and the next.

So, why do we care for our family?

Why would I, or any other parent, sacrifice everything for our children?

It's evolution.

I mean, at least in part.

And you can't spell evolution without love, right?

Stay curious.

A big thanks to 23andMe for supporting PBS Digital Studios and our show.

23andMe comes from the fact that human DNA is organized into 23 pairs of chromosomes.

23andMe is a personal genetic analysis company that was created to help people understand

their DNA.

If you and your family each get your 23andMe results you can unwrap how much Neanderthal

DNA you each have, and you know, you gotta hurry, because you never know when a bus full

of velociraptors might come around the corner.

23andMe has a special holiday offer now through December 26, in case you're looking for a

sciencey gift.

You can go to 23andMe.com to check out their holiday offer and get kits for your family.

Show your support for this show by checking out 23andMe.com/OKAY

A big thank you to our special guest Harrison.

You guys like his shirt?

Let me know in the comments.

Maybe we've got another merch idea.

How'd you enjoy your YouTube debut?

Was it fun?

Did you have fun?

Leave him a nice comment, and I'll read them to him right before he goes to sleep.

Thanks for joining us, we'll see you next time.

Stay curious!