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Welcome back smart people, Joe here.

Today I want to talk about why our species has evolved a life story that really isn't quite like any other species on this planet.

Understanding how the forces of nature, shape, an organisms life's story is the key to figuring out why our incredible living world is the way it is.

Today, we're gonna explore that by digging into one of our species biggest unsolved evolutionary mysteries, and that mystery is ....... Why your grandma exists.

It's not because of the cookies.

This tiny mite, less than a millimeter long may win the trophy for the strangest lifecycle of any animal.

The females are born pregnant.

She spends a few days feeding on the eggs of insects until several daughters and one son hatch inside her body and then mate with each other.

The daughters eat their way out of mom already pregnant.

Ready to do this all again a few days later.

And compare that reproductive strategy with the kiwi bird.

Females of this flightless feathered fluff ball grow a single egg that can be up to a quarter of her body weight.

That's like a human giving birth to a 4-year-old.

Why can a codfish release more than 10 million eggs each time its spawns?

While the Greenland shark doesn't reach sexual maturity until it's 150 years old?

And its pups grow for more than eight years before they're even born?

Why do some organisms grow, live and reproduce in the whirlwind of a season while others slowly stretch their lives for centuries?

These are the questions at the heart of the study of life histories.

This is the term we use in biology to describe how organisms allocate their energy and resources to grow, reproduce and survive.

It can tell us why individuals are the way they are, all the way up to why whole populations look the way they do.

Why organisms are large or small, why they mature quickly or slowly, why they have few or many offspring, or why they have a long or short life.

These kinds of traits are strategies that different organisms in different environments use to maximize their reproductive success over their lifetime.

Now, essentially the whole point of evolution in natural selection is to pass genes the next generation.

So whatever traits make an organism more successful at doing that, then those traits become more common.

In evolutionary biology, we call this concept of survival and reproductive success "fitness."

Consider that every organism has a fixed amount of energy and resources to support keeping itself alive and reproducing.

But because of this, maximizing fitness usually involves some trade-offs.

Having many offspring can shorten an organism's lifespan and leave fewer resources for each offspring.

If an organism matures quickly and reproduces early, it might remain smaller overall, which can affect its long-term survival.

Or consider that some species produce a large number of offspring while investing relatively little in each while others produce few offspring, but invest a ton of time and resources into each offspring's survival.

Now, when we look at our own species through this lens of life history theory, that's when we see just how weird humans are.

Okay, next to, I don't know, tortoises, humans have the potential to live longer than pretty much anything else on land with a backbone.

Our closest ape cousins, they cash out at just 30 or 40 years old, and we're pretty late bloomers.

We take a fifth to a quarter of our life before we stop growing.

That's a long time.

It's also weird that human kids need help eating and finding food for years after they stop nursing.

Most mammals can immediately fend for themselves once they're done with milk.

Humans: not even close.

I mean, how many 10 year olds can even make a grilled cheese sandwich?

Human offspring are just astonishingly needy, and if my kids are watching this, I love you.

I'm just spitting science facts, okay?

And because we wean our youngs so early, humans can really pump out babies back to back.

I mean, consider that female chimps typically go maybe five years between babies, while many of you might only be a year or two apart from your siblings.

Alright, I can hear you enough.

Jibber jabber science, man, you said you were gonna explain why grandmas don't make sense.

Well, now that we've introduced this idea of life histories and how they impact evolution, well, we can do that because the mystery of grandmothers is based on one more very weird thing about human lives: menopause.

That's right, we're going there.

Because it's pretty weird that human females living out their long human lives end their reproductive years so early.

I mean, especially considering the rest of their bodies can keep working at tip top shape as they get older.

Unlike human males who can reproduce throughout their lives, human females, reproductive years end in their forties or fifties.

What this all means is human females can live up to half their lives not being able to reproduce.

From an evolutionary standpoint, it doesn't seem to make sense, so let's see if we can figure it out.

Now, half the human population is unfortunately pretty ignorant with how female bodies, you know work.

I'm not gonna point any fingers, but if I did, the recipients of those points might look a lot like the average gender breakdown of this channel.

Anyway, let's get everyone up to speed.

- I'm Dr. Danielle Jones, a board certified O-B-G-Y-N. And some of you may know me online as MDJ or Mama Doctor Jones.

Sexual maturity occurs over a period of time known as puberty.

This generally starts somewhere around age eight 13 in females and a little bit later in males.

The menstrual cycle is a series of hormone changes that happen over the course of around a month in female bodies after they've gone through puberty.

The menstrual cycle is split up into phases.

The follicular phase being the time from when your period starts to the middle of your cycle when ovulation occurs.

The follicular phases job is to give estrogen the time to build up that lining and FSH, or follicle stimulating hormone, time to speak to the ovary, tell it: "Let's keep growing these little follicles until you have one that is nice and big and ready to be the lucky egg of the month."

Eggs basically go through a journey of being dormant all the way to ovulation.

Most of those eggs don't actually get chosen as the egg of the month and just end up dying out.

When an egg that has matured enough that it's ready to ovulate, the ovary responds, ovulates an egg, and then we've entered into the second half of the menstrual cycle, which we call the luteal phase.

In the luteal phase, that ovulation site has now turned into a corpus luteum.

The corpus luteum makes progesterone.

Its main role is to tell the uterus and the lining of the uterus: "right, stabilize, we're just here waiting on an embryo."

The cool thing about that corpus luteum is that it has about a 14 day lifespan unless there is implantation of an embryo.

But if an embryo doesn't implant and rescue the corpus luteum, it dies.

Progesterone levels drop and that sudden drop in progesterone leads to a withdrawal bleed, which is menstruation.

Menstruation is quite uncommon in the way that we understand it in the animal world.

Outside of a few mammals, including humans obviously, and some primates, elephant shrews and some bats.

Menopause marks the end of the reproductive years.

Technically it is defined as a full year without a menstrual cycle, and then we can diagnose, okay, this is menopause.

The ovaries no longer have enough follicles to mature into a mature ovulation site because of the way they are responding to the hormones in the body.

The average age range of menopause, which is generally considered 45 to 55, leads to symptoms such as hot flashes and difficulty sleeping, as well as changes to the vaginal skin or vaginal mucosa with a little bit more dryness, changes in mood and libido.

It's all related to those hormone changes that come along with the body trying to yell at an ovary that seems to not be listening, but really just can't because it's run out of follicles to allow it to grow into a dominant lucky lottery winning ovulation site of the month.

- Now before 1800, the average life expectancy of humans was about 40 years old, well before menopausal age for most females, but that average life expectancy was weighted down by really high infant mortality rates even before the modern industrial wellness influencer saturated era.

If you managed to live past age five, your life expectancy shot up to more than 70 years old.

Even our ancestor homoerectus, which lived 1.8 million years ago, could live past 50 - outliving their egg supply just like modern humans do.

That means human females could pretty regularly live 20, 30, even 40 years past their reproductive years.

Or jargonifically speaking, a long post reproductive lifespan.

Does that mean there's some evolutionary advantage to females living past reproductive age?

Many researchers believe that the existence of grandmothers is the answer to that question.

That what they do for their families is a big reason why humans evolved into the totally rad creatures that we are today.

But other researchers aren't so sure.

They argue that grandmas are just a weird side effect of human biology, like your appendix or wisdom teeth.

But what everyone can all agree on is that the existence of grandmas is super weird.

Humans along with elephants and a few species of whales seem to be the only species where grandmas help care for their grandkids by helping them get food and passing down important knowledge like the best watering holes, seal hunting spots, or how to make a cake at a three gram crackers in an expired packet of jello, there's an idea that argues that your nana is evolution's secret weapon.

It's called the grandmother hypothesis.

It basically argues that long post reproductive lifespan allows older females to stop worrying about their own babies, and instead help care for the next generation of humans and improve their odds of survival.

In other words, instead of investing in their own reproductive success and passing on half of their genes, grandmas invest in the reproductive success of their daughters, which, hey, still passes on some of the grandmothers' genes.

In the 1980s, an anthropologist named Kristen Hawks was observing the forging habits of the Hadza tribe of hunter gatherers and Tanzania.

Post reproductive women spent more time foraging and collected more hard to get food than young females of childbearing age.

That's right, I granny always knows where the snacks are, right?

More food improved kids' chances of surviving to maturity and having kids of their own.

That's a win for evolutionary fitness.

Maybe you're thinking, well, couldn't females do all of this while remaining fertile?

Nice.

You're thinking I like that.

But there's ways that living your reproductive years could make thi.. First, you're not juggling the extra caretaking work while caring for your own babies.

And since your baby years are behind you, there's no need to compete with younger females for partners.

Not to mention that pregnancy gets increasingly risky as females get older.

So having a long post reproductive lifespan could translate to evolutionary benefits - from healthier babies to longer lives and better health for your tribe members.

And scientists have actually seen some of these benefits in the real world, in other species where females have long post reproductive lifespans.

In toothed whales like orcas, grandmothers care for th... by sharing food and knowledge.

One study found that calves were more than four times more likely to die in the two years after their grandmothers passed away.

So yeah, females living past their reproductive years can have big benefits for their species, but there's just one catch for the grandmother hypothesis to work, females living past their reproductive years would have to be rare in nature, something that we only see in humans and a few other species.

Because if it happened to lots of species, it couldn't be a special evolutionary advantage just for us.

You can probably guess where this is going.

In recent years, scientists tracked female fertility in captive and protected mammal populations, and they did that by using MRIs to count egg supply and by analyzing hormones and urine samples.

The last one's definitely a job for an intern.

Now, you can count the number of species that menstruate on like one hand, and a species can't go through menopause if they don't menstruate.

But, that research on captive mammals proves that multiple species, from cows to Reeses monkeys, do go through oopause, or the end of ovulation and can live well past their fertile years.

And the reason might just be because of how eggs are made.

- For the most part, from the time that you are born around 20 weeks gestation, those ovaries have six to 7 million follicles inside them and by birth that has dropped down to about a million.

If they're not the follicle that is trying to mature that month, they just stay basically frozen in time in a state of meiosis.

Once they decide that they're going to be in that gr.. of follicles that's trying to compete for follicle of the month, they then move back into the process of cell division and get out of that frozen state of meiosis and start developing.

By the time you are going through puberty, there are maybe 400 to 500,000 left and those follicles result in 400 to 500 eggs that are actually ovulated.

Over time, staying in that frozen state of miosis gradually means there's more and more possibility of poor egg quality, meaning when they do start cell division again, they have a higher chance of making mistakes in that process.

And that's why you see more genetic abnormalities and miscarriages and infertility as you move through the reproductive years and get closer to the age of menopause.

At menopause, less than a thousand follicles remain, and that is where the level becomes low enough that it is no longer supportive of continuing through the reproductive years and you move into menopause.

- So let's put all of this together.

Since all female mammals are born with all the eggs they'll ever have, they're all at least hypothetically capable of outliving their fertility if they can survive long enough.

The problem is nature is pretty metal.

Recent research suggests that the reason we haven't observed more mammals with long post reproductive lifespans isn't because it's rare or something that only happens to humans.

It's simply because for most species it's hard to make its old age without getting eaten, or meeting some other horrible demise.

This might also explain why the few species where we do see long post reproductive lifespans in the wild are species like whales and elephants 'cause they're just good at living a long time.

That means grandmas might just be an evolutionary side effect of figuring out how to live longer in general.

So it seems like this whole idea of grandmothering being a unique evolutionary superpower for modern homo sapiens, might not hold up, at least according to what we know today.

But it's a really fascinating case of how studying the life histories of different organisms can help us figure out how natural selection has shaped their bodies and their behaviors to be successful in their environments with that limited amount of time and energy that we all have at our disposal.

Because just like all of our other traits, the way we grow and the ways that we behave have been molded by evolution and natural selection.

Sometimes in strange and mysterious ways.

Sure, we're a weird species with a unique way of growing and living, and maybe grandmas are still a bit of an evolutionary mystery.

But whatever the reason they exist, I think we can all be glad that they do.

Stay curious.

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Essentially the whole point of evolution.

Evolution from healthier babies to longer lives and lives.

That's not a word, just made that up.