[MUSIC] Is there something you'd change about yourself?

Not just your hairstyle, but down at the genetic level?

For the first time in human history, the ability to build and select genetic traits is within reach.

Problem is, no one's played this game before.

[music] Mutation and selection.

These two principles have molded basically all life on Earth, from pond scum to platypuses.

Evolution's rule book says changes to life's instructions happen more or less at random, and not because an organism needs something, while selecting which traits are good enough to be passed on, is decided by... ya know, the universe.

But humans are rewriting these rules in incredible ways, putting us in charge of our evolution.

For starters, we've massively changed our environment so it's much harder to die.

There's almost no chance a predator or another human will kill you today, science can repair our bodies or make us invincible to enemies we can't see.

And more people die today from having too much to eat than too little.

For thousands of years we've been deciding which plant and animal traits are useful, which are passed on, and which are not.

Thanks to more and more selection under our control, human life expectancy has doubled in just a few generations.

But this simply lets us steer evolution.

Once we discovered the molecule that guides these traits, we realized we could hit evolution's gas pedal.

DNA's information is encoded in long chains of nucleotides, and natural mutations, good or bad, happen very rarely and in random spots.

In the mid-20th century scientists realized they could speed up mutations using radiation or chemicals, but the effect was still up to chance.

And contrary to popular belief never, ever, ever leads to superpowers.

Later, in the 1970s, scientists exchanged entire chunks of DNA between species, creating microbes that manufacture insulin, plants resistant to viruses, even mice with human genes.

They could control what was being edited, just not where in the genome this new DNA was inserted.

What scientists needed was a tool so precise it could make a single change in 3 billion DNA bases, and cheap enough that anyone could use it.

In 2012, they got it: CRISPR.

It sounds like a breakfast cereal, but CRISPR has the power to literally reshape humanity, combining unnatural selection with non-random mutations.

Evolution could be in our hands.

CRISPR was invented by one of life's simplest organisms.

Just like you and me, bacteria are constantly under attack by viruses.

We can afford to sacrifice a few cells to fight an infection, but a single-celled microbe doesn't have this luxury.

If a microbe survives an infection, it saves some viral DNA in part of its genome filled with clustered regularly-interspaced short palindromic repeats.

That's where CRISPR gets its name, it's a bacteria's immune system, a memory of past infections, to protect it and its offspring in the future.

Those viral mugshots are copied into pieces of RNA, and loaded into a special protein called Cas9.

If the virus infects again, and CRISPR sees a match, the Cas9 protein cuts up the viral DNA like a ninja.

Scientists realized that CRISPR could do this in any type of cell, and by reprogramming the target, CRISPR could cut any genetic sequence down to a single DNA needle in a 3 billion letter haystack.

Suddenly we have the power to edit genomes like a word processor.

When cells repair cut DNA, they can glue the ends back together, often trimming a letter or two out and disrupting the genetic code.

Or the cell can use another template to write in new DNA, letting us splice in new genes with surgical precision.

Genes can be switched on or off.

Infections like HIV cut out.

Our own immune system can even be reprogrammed to hunt down cancer cells.

Thousands of human diseases are caused by mutations in single genes, and each could be reversed with CRISPR.

Even complex traits like height or heart disease, caused by many genes interacting with the environment in ways we don't fully understand, might be within reach.

But to repair every cell in a body, human genetic modification has to be done at the earliest stages of embryonic development, and those changes will be passed on to future generations.

It makes you wonder: if CRISPR is one day used in embryos to Make People Better, would we also use it to Make Better People?

Using CRISPR, if parents wanted, say, a blue-eyed baby, could they order the necessary change in the OCA2 gene?

If they wanted a more muscular baby, could they edit its myostatin gene?

Curing diseases with CRISPR and making designer babies aren't technically that different, but they're miles apart ethically.

Beyond curing diseases, CRISPR makes us to ask tough questions.

Who decides what is a better baby?

What if only the rich can afford to edit genes?

Should parents even be allowed to determine their baby's genetic future?

Maybe they already do, thanks to birth control and techniques like in vitro fertilization?

Welcome to a new evolutionary balance.

On one side: most of nature, with natural selection and random mutation molding a whole planet's worth of diverse species.

On the other side is us, a single species with tools that could match or maybe exceed the speed and power of evolution as we know it.

What's clear is the ability to control nonrandom mutation and unnatural selection in humans, to control a piece of our evolution, is no longer a question of if we can.

It's a question of if we will.

Stay curious.