[MUSIC]
[BIRD CHIRP]
A smart person once said “nothing in biology makes sense except in the light of evolution.”
Evolution is what lets us understand the history of life.
We’re used to seeing things change over time.
But biological evolution is about something more.
Centuries of observing nature has shown us
that all life is related. And just like how the physics here on Earth
is the same physics that rules the Andromeda galaxy, the same basic biological rules apply
to all creatures great and small.
If life has a point, it’s making more life.
The instructions parents give to the next generation come with a few random changes.
Every new generation ends up slightly different than the generation before it.
But nature doesn’t always make survival easy, so any creature that lives long enough
to make more life passes its traits on more often.
This is what Darwin and Wallace figured out, the beautiful theory
we still study today. That all life is descended from a common ancestor, that new generations
show variation, and the more often a trait survives, the mores common it will become.
It might not tell us how life began, but it tells us how all living things came to be
the way they are, bacteria, bugs, or Beagles.
[BIRD CHIRP]
Some people think nature is like a tornado that picks up a pile of raw materials and
then spits out a palace.
But nature doesn’t work that way.
Copying DNA, UV rays, chemicals, the shuffling of genes, they all introduce tiny changes,
and those changes are random. But natural selection is anything but random.
An organism builds its house using DNA blueprints. But when it copies those blueprints, they’re
a little different thanks to that random genetic variation. When construction begins…
Some of the houses that get built fall down. Some of the houses aren’t better or worse…
just different. But some of the houses turn out even better!
The blueprints for the houses that stay standing get put back into the genetic shuffling machine
so nature can do it all again. And again and again and again.
Nature doesn’t set out to build one perfect house. It’s constructed billions of houses,
with trillions of blueprints, and a lot of nature’s houses fell down along the way.
Life isn’t just chance. It’s the non-random selection of random variation, and it built
this beautiful city called Life.
[BIRD CHIRP]
Male crickets love to sing.
But on the Hawaiian island of Kauai, that singing comes with a price.
Several years ago, a parasitic fly arrived on the island. Those flies listen for crickets’
chirping wings, lay eggs inside them, and the crickets are devoured from within.
Thanks to these flies, Kauai’s crickets almost disappeared, except for a few.
These survivors were missing the ridges that crickets normally rub together to chirp, and
today Kauai is filled with quiet crickets.
Silence saved their lives. This wasn’t just a change in behavior. The
silent wing shape was caused by a mutation on the cricket’s X chromosome.
Kauai’s crickets became the strong silent type in fewer than 20 generations. Not only
did we see natural selection in action, it happened in the blink of an eye.
A few years later, those parasitic flies showed up on neighboring Oahu, and in response, most
of the crickets there had gone silent too. Surprisingly, they had developed a completely
different mutation in the same gene. Nature arrived at the same destination by
two completely different paths.
While evolution is often a long gradual process, rest assured we’ve witnessed plenty of fine
tune-ing right before our eyes.
[BIRD CHIRP]
Your eyes are beautiful. They’re amazing bits of biological engineering. Just like
a camera, remove one of their intricate parts, and the eye can’t do its job.
For this reason, many people think eyes couldn’t possibly arise from a blind process like natural
selection. But that idea doesn’t hold vitreous fluid.
Light-sensitive cells first showed up in simple single-celled creatures, helping them swim
towards the sun. Cup-shaped light spots then began to let creatures
sense light’s direction. Deeper pits eventually formed a pinhole iris,
increasing the eye’s resolution. Some animals developed a protective, crystalline
covering over the iris, which later allowed them to focus and control the light entering
their eye. This is essentially the eye that we have today.
Computer simulations have replayed this selective process in just 350,000 generations, showing
simple light patches can evolve into camera-like eyes in tiny, adaptive steps, 1,829 to be
precise. Nature took a little longer than that, but
genes, biochemistry, fossils, and anatomy all tell the same story. Eyes are pretty easy
to evolve. So easy that nature has done it independently 50 to 100 times.
This kind of complexity, rather than overthrowing Darwin’s theory, is proof of its power.
So whether it’s an eagle observing ants from atop a skyscraper, a mantis shrimp scanning
circularly polarized light, or you watching YouTube videos… there’s room for an eye…
in evolution.
[BIRD CHIRP]
During WWII, Londoners took nightly refuge in the Underground as German buzz bombs exploded
above. But they were greeted with a different buzz: Mosquitoes.
In the 19th century, when London’s Tube was being dug, a population of mosquitoes
colonized the tunnels. They originally preferred biting birds above ground, but since pigeons don’t ride
the subway, they found a new target in rats, mice and people.
Now, just more than a century later, when the tube-dwellers try to mate with their above-ground
cousins, it almost never results in viable offspring. The subway skeeters are becoming
a new species right before our eyes, with different behaviors, and different genes.
Species are always more likely to emerge when a population gets isolated.
Maybe one goes underground. Maybe a river runs through them. Or maybe one drifts out
to sea.
Once separated, a different drift occurs: genetic drift. As time goes on, mutations
accumulate and traits are selected by each environment, until the two groups are sufficiently different
that they can’t mate.
Or, as scientists call it, reproductive isolation.
London’s mosquitoes didn’t invent this. It happened with the dozen or more tortoise
species that Darwin found in the Galápagos. It created us and our extinct Neanderthal cousins.
Small, random changes, challenged by nature, have spawned each of the tens of millions
of species alive today, and the hundreds of millions that are stuck… underground.
[BIRD CHIRP]
A giraffe looks perfectly suited for its environment. But that long neck hides a stunning error
of evolutionary engineering.
Although it’s just a few inches from the brain, the nerves controlling a giraffe’s
voice box take a 15-foot detour before reaching their destination.
We’re wired up the same way. The nerve letting me to talk to you right now is wrapped around
my aorta. In our fish ancestors, that nerve connected
brain to gills. But as gills gave way to lungs and heads moved forward, things got a little tangled.
Our bodies are full of examples proving evolution can be kind of dumb.
Why else would the waste management system be built by the amusement park?
Why else would we eat, drink, and breathe all through the same hole?
Why do we get cavities, or cancer? Why else would I be wearing these?!
Because natural selection isn’t about “survival of the fittest”, it’s more like survival of the
“good enough’.
Nature can’t draw up a body from scratch, it can only tweak existing plans. And if a
plan works well enough to allow an organism to reproduce, it gets passed on, tangled wiring
and all.
Life may not be logical, but you know I’m speaking from the heart when I say, “Congratulations
on being good enough.”
[BIRD CHIRP]
Look down. No matter what your biological sex, I’m willing to bet most of you have
two nipples. If you can’t see them, they’re probably under your shirt.
Some people have even more than two! But that’s for another video.
It’s easy to see why females have two nipples. We’re mammals. It’s right there in the
name. Dogs, cats, pigs and whales all have nipples too. Like us, they nourish their
offspring with milk.
Male nipples? Not much going on there.
If they’re not an evolutionary advantage, why hasn’t natural selection erased ‘em?
Because my nipples are cheap, and they’re not hurting anything.
In the eyes of evolution, male nipples don’t have much cost: Just a little more tissue.
But deleting them from the recipe altogether, reprogramming the genes and body development
in just in one sex? That would cost more.
Traits don’t have to provide a specific advantage to get passed on.
Why is your blood red and an octopus’s is blue? It’s not because one color’s better,
it’s just a side effect of nature selecting different chemistry for different creatures.
And sometimes, traits can end up being used for a function different from what nature
originally selected.
Not every trait is an adaptation, and they don’t all have a point. Sometimes they have
two.
[bird chirp]
There’s only a few unbreakable laws in the universe.
One says that disorder always increases. We see this play out everywhere.
Unfortunately, some people think this law’s a problem for evolution.
Some people have said that life is basically applied chemistry
and chemistry is essentially applied physics
So then in that case is biology applied physics?
Although, you might say that physics is applied math so I don't know how far we want to go here.
Well without biology there wouldn’t be physicists, but you have a point. Like anything else,
life obeys the universe’s laws.
A physicist might say you and a bacterium are both ordered bunches of particles.
You are just a fancier and more complex arrangement. Surely saying you evolved from something like
that breaks the 2nd law? How could evolution add order and complexity to life?
There’s something you haven’t been told about the 2nd law: it only applies to closed
systems, and Earth is not a closed system.
If you took a pan of cookie dough, half frozen, half room temperature and put it in a cold
oven, eventually, thanks to entropy, you’d get a bunch of equally uncooked cookies, even
though the total amount of energy has remained the same.
But if we turn on the oven, if we add energy to the system, our disordered dough can transform
into something deliciously ordered.
The sun is like our oven, it’s constantly showering Earth with more energy than we can
use. That energy changes DNA, plants use it to
make more plants, animals make more animals, and we turn it into cookies… all without
breaking the law.
[BIRD CHIRP]
Take a perfectly good book and start changing letters, and it won’t take long before it’s
unreadable. Information has been lost.
Then how did evolution, through a similar process, morph the simplest life forms into
the most complex? Evolution can create information.
Many people think this is impossible, but there’s several ways we know it happens.
When DNA is copied or shuffled to make sperm or eggs, genes or sets of genes can be duplicated.
With an extra copy no longer constrained by natural selection, the duplicate is free to
take on a new role.
Nearly all flowering plants, and many grasses, have had their entire genome duplicated at
least once.
Organisms can also swap or steal genetic material. This is most common in bacteria and other microbes,
But a good portion of your genome is the remnants of ancient retroviruses, while nearly half
can be traced to even smaller genetic invaders. These DNA parasites are actually natural selection
at its smallest scale, individual units of information trying to pass on to the next
generation via you.
Evolution can create information. Nature’s story may not have an end in mind, but it
has no problem adding new pages to the book of life.
[BIRD CHIRP]
If humans evolved from monkeys, then why are there still monkeys?
There’s many things wrong with this question, like the fact we’re more closely related
to apes than monkeys.
Okay then so why are there still apes?
Asking why there are still apes is like saying, if some Americans and Australians came from
British people, then why are there still British people?
When we see two adjacent branches out here on the tree of life, what we mean is that
they have a shared ancestor, some organism, a long time ago, whose descendants took different
evolutionary journeys.
That’s the way it is for turkeys and T. rexes, whales and hippos, even hyraxes and
elephants.
There are still apes because apes are good at being apes.
The point is this: We’re not more evolved than a chimpanzee. We just evolved differently.
It’s often said that evolution is survival of the fittest. But it’s more like survival
of the “fit enough”. As long as a species’ traits are good enough
to survive and reproduce in their environment, that species will live on.
The tree of life is not a ladder.
So to all my cousins out there, gorilla, orangutan or chimpanzee, you’re the best ape you can
be. That’s good enough for evolution, and me.
[BIRD CHIRP]
A thousand years ago, if you had been born with genes that made you diabetic, you’d
probably die. But not today.
Head to small for those extra wisdom teeth? Well, no more deadly infections, just go to the
dentist.
Today we don’t face many of the challenges our ancestors did. Now that we can modify
our environment and we live long past the age we reproduce, some people think we’ve
moved beyond natural selection.
Recent history is full of examples that we’re still changing.
Many of us carry a mutation that keeps a milk-digesting enzyme turned on into adulthood, allowing
those people access to another source of nutrition.
Even more recently, a genetic mutation has made Tibetan highlanders uniquely adapted
to living at high altitudes.
That’s practically yesterday when it comes to evolution.
One copy of the mutation that causes sickle cell disease can protect people from malaria,
which is why we see it selected in populations that live here.
In areas where HIV and AIDS continue to be hard to treat, a gene mutation that provides resistance
to the virus will likely become more common.
Like every living thing, we’re not an end product of natural selection, we’re a work
in progress. So if you ever wonder if we’ve reached our
evolutionary peak, remember that going farther is just what we do.
[BIRD CHIRP]
In 1938, scientists found a very weird fish swimming off the coast of S. Africa. They
were surprised to see it, because the last time anyone had was in a 66 million year old
fossil.
The coelacanth has been called a “living fossil”, but that’s a really dumb name,
because it’s very much alive. It’s just barely changed since the time of the dinosaurs.
Now look at us, we inhabit every corner of the planet. We shape our environment, we
control energy, we have big brains that we use to understand our world and the universe
in which it resides. We‘re pretty awesome.
But in evolution’s eyes humans and coelacanths are equally successful. We’ve both made it.
Each of us, in our particular environment, has conquered enough challenges to survive.
A side effect of being the only species that can figure out where it is and where it came
from is believing that we’ve somehow climbed to the top of nature’s ladder.
But viewing ourselves as the pinnacle of evolution, the brightest star in a the galaxy of life,
is no more correct than viewing Earth as the center of the universe.
Every species evolves, at different rates, in response to different environments.
Some make it, some don’t, but as long as we exist, as long as that fish exists, evolution
will continue its journey to… well, nowhere in particular.
That evolution doesn’t have a purpose, realizing there’s no destination in mind, or no trophy
for “best species” or “most improved” waiting at some finish line, can be an uncomfortable
thought. This is how it is with evolution.
So… what’s the point? We’ve studied the night sky for centuries,
the same night sky that ancient fish lives under, and it’s shown us that we’re just
one small part of something bigger, that we’re made of the same stuff as what’s up there.
It’s a profoundly humbling experience.
But we’ve also learned that our planet has as much life ahead of it as it’s lived so
far, and that the universe has even more still. Right now, Earth is a tiny bit of life in
a tiny corner of a vast ecosystem, and we’re just a few steps into this journey. That
gives us incredible potential. We understand much of how the universe works,
even if we can’t perfectly predict exactly what it will look like tomorrow.
We might not know where we’re going, or what we might find there, but we’re definitely
going somewhere. Ultimately, we’re just doing what we can with what we’ve got, to
reach tomorrow, like everything else in the universe.
And we should feel good about that. That’s the best we can do.
Stay Curious