[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
