Hey smart people. Joe here. Charles Darwin
spent 5 years circling the world, to discover
what became The Origin Of Species. Alfred
Russel Wallace spent /8/ years traveling through
South Asia to witness the story of evolution.
But you can discover the same thing today,
without going anywhere. Because the story
of evolution is right there in your body.
You’re tailbone? Used to be a tail. If you
can move your ears, that’s thanks to muscles
you share with other mammals. That pink bit
in the corner of your eye? A leftover of a
third eyelid we can still find in lizards,
birds, fish, and others. Goosebumps are a
feature we share with lots of animals that
didn’t lose their body hair, and even hiccups
trace back to the brains of fish and tadpoles,
a brain signal they use to squeeze water through
their gills.
These body parts are just a few scattered
pages in a larger story about our evolution.
But there’s other fossils hidden inside
our bodies that tell an even clearer tale
about how we came to be the way we are.
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One of these stories is so simple, it can
be told with a single picture.
These are images of chromosomes, the big chunks
of DNA inside every one of our cells. When
we look at images of human chromosomes, next
to those of other apes, they line up nearly
identically. Except for human chromosome 2.
It seems to have no match next to the other
ape chromosomes. And chromosomes 12 and 13
in the apes, we don’t seem to have a match
for those.
Even weirder, humans have 46 chromosomes in
total, 23 pairs. These other apes? They have
48 chromosomes… 24 pairs. What could explain
this? If we’re supposed to be related, why
are we missing a chromosome?
Turns out we aren’t missing a chromosome
at all. Sometime during our species’ history,
those two chromosomes that are still separate
in other apes were welded to become our single
human chromosome 2. Us, chimps, gorillas,
orangutans, all inherited the DNA we have
today from some common ancestor with 48 chromosomes,
but two of ours got stuck together along the
way, and you can even see how they match up.
The sequence of DNA on our chromosome 2 carries
even more evidence. If two chromosomes were
glued together, you’d expect to find two
centers, or centromeres. And we do. We’d
also expect to find telomeres, the ends of
each original chromosome, stuck head to head
in the middle. And we do. Thanks to sequencing
the human genome, we can spot the exact DNA
base where this fusion happened: 113,602,928
bases from the end. A fossil of evolution,
hidden in our own bodies.
In Celtic mythology, there’s a place called
Tír nan Óg: a land of eternal youth. When
one scientist discovered a protein that’s
keeps stem cells forever young, he named it
after this place: NANOG.
The human NANOG gene is located on chromosome
12. At least, the version of the gene that
works. There are eleven other broken copies
of NANOG spread throughout our DNA. How did
these extra copies get spread through our
genome like lost socks?
When DNA is replicated, sometimes the same
bit gets mistakenly copied twice. And sure
enough, we see one broken copy right next
to the working NANOG gene, like a messed up
DNA photocopy. We call these broken genes
pseudogenes. And what about the other ten?
There’s another way these pseudogenes can
spread.
NANOG, like every gene, is copied, from DNA
into RNA, letter by letter, before that RNA
is used to build a protein. But every once
in a very long while, that RNA gets mistakenly
copied backwards, back into DNA, pasting a
new, broken version of the gene somewhere
in our chromosomes. After searching the billions
of letters in our genome, we’ve found ten
of these lonely NANOG leftovers in our DNA.
Their locations seem random, like they were
blindly tossed in. Now, say you were to read
another animal’s genome, and you found these
very same broken copies of NANOG, each pasted
in the very same locations, among billions
of letters of DNA. That would be a heckuva
coincidence. Unless… it wasn’t a coincidence.
This is what we see when we search for NANOG
in a chimpanzee’s genome. The original,
and nine of the lonely, broken copies, all
in the same locations as ours. This only makes
sense if, millions of years ago, you and me
and the chimpanzee share an ancestor whose
DNA was already littered with these broken
genes. We all inherited this pattern, because
we share an evolutionary link.
Of course, it’s one thing to share a link
with chimps. But chickens? That would require
some strong cluckin’ proof.
One thing you have in common with a chicken?
You both came from an egg.
Inside a chicken egg, the growing embryo produces
something called a yolk sac. It surrounds,
well… the yolk, and helps gradually absorb
nutrients to build a baby bird.
But human eggs are incredibly small, only
about a tenth of a millimeter across. Unlike
birds or turtles, our eggs don’t make a
yolk, or even a shell. We get our nutrients
and protection from our mom’s body. So you’d
think–no yolk, no yolk sac. But that’s
not the case. Humans make an empty yolk sac,
and it disappears halfway through pregnancy.
This is a clue, that our eggs and bird eggs
and even reptile eggs all share an evolutionary
link. But can we prove it?
There’s an essential ingredient in egg yolks
called vitellogenin. It’s a protein, which
means there’s a gene whose DNA codes how
to build it. Egg-laying vertebrates all have
at least one copy of this gene. But what about
us? If we really are descended from animals
that once laid eggs full of yolk, can we find
some leftover of that yolky gene hidden in
human DNA?
In 2008, researchers found just that: Fragments
of once-active egg yolk genes, hidden in our
own genome. and when they compared our versions
to the chicken versions, they even had the
same neighboring genes around them. This is
exactly what evolution would predict.
Just like fossil bones, we see remnants of
our ancestors, broken and buried, in these
hard to find places.
There are many myths of creation around the
world, and most agree on one thing: Humans
are special, and we need a special story for
how we got this way. When we take a close
look at our own biology, we can see that special
story is evolution, and it’s a story we
share with every other living thing on Earth–some
more than others.
To some people, the idea that humans–our
bodies, our brains, our thoughts–are also
the products of evolution removes some of
the magic of being human. But it doesn’t
have to. When Darwin wrote “there is grandeur
in this view of life” he meant that being
a creature molded by evolution is a totally
different kind of magic, the kind that still
amazes you, even when you know exactly how
the magic is done.
Stay curious.
