More than 120 million years ago, in a hot,
conifer-filled forest in whatâs now India, a small insect made a terrible mistake. While searching for a tasty meal of pine pollen
it wandered one step too far, only to find itself trapped in sticky, yellow resin. Tired from its flight, this small weevil was
quickly entombed in the fragrant yellow material, which eventually became the substance we know
today as Amber. Then, in 1993, scientists cracked open this
very same piece of amber. They took the body of the weevil, and they
sampled its DNA. Now, this is not a scene from the Jurassic
Park franchise. But this research IS from the 1990s, a decade
when scientists were rushing to find the most ancient DNA. And at the time, this weevil was the oldest
thing ever to have its DNA sampled. Or, at least, so we thought. The fact is, we can indeed get the DNA of
extinct organisms from some fossils. Itâs fragmented, and itâs imperfect, but
itâs possible. Itâs just not possible for every type of
fossil, and, most importantly, not from every time period. It took another few decades of research, and
a lot of take-backs, before scientists could figure out how we could truly unlock the genetic
secrets of the past. The first piece of ancient DNA ever replicated
was of an animal called the Quagga, a subspecies of zebra that went extinct in the 19th century. It was sampled in 1984, pretty much just to
see if ancient DNA could be sampled at all. But that research turned out to be extremely
useful, and not just because it inspired Michael Crichtonâs famous novel. The researchers used the size of differences
in the DNA sequence to determine when the Quagga, which is now known to be a subspecies
of Plains Zebra, diverged from another species, the Mountain Zebra. That split happened about 3 to 4 million years
ago, it turns out. So even though these species of Zebras look
very similar, we now know that they parted ways a long time ago, before the last Ice
Age began. Now, that DNA was tested from a sample of
dried muscle taken from a museum specimen. And for the next few years, the search for
ancient DNA drew from similar sources â soft tissue, preserved in things like permafrost
or ice, or mummified, or trapped in amber. And in the search for the oldest material,
amber seemed like the best place to look. After all, amber traps organisms in a perfect
medium for preservation. It dehydrates the DNA, which makes it more
stable, and tree resin has antimicrobial properties, which keeps the tissues from breaking down. So, in addition to our friend the Jurassic
Weevil, paleontologists sampled termites, bees, and other insects from their amber tombs. Not mosquitos though. Amber containing mosquitoes has not been sampled
for DNA yet. Still, these early efforts taught us a lot
about ancient DNA, and the organisms that managed to hold on to it for us. But there was a growing suspicion among scientists
that the oldest DNA to be extracted -- including the stuff from that weevil -â wasnât what
we thought it was. Experts already knew that such ancient DNA
wasnât perfect or pristine. Because, DNA is degrading all the time! Even in living things! Including you! The tiny components, or base pairs, that form
its code are always being changed by different processes. The most common of these is a process called
depurination. Itâs caused by water molecules in your cells
that attach to some of the base pairs, which makes them more likely to come off. Water is great for your cells, but over time,
it causes damage too, including to your DNA. But usually, damage like this isnât a big
deal. Your cells have countermeasures that straighten,
fix, or discard DNA thatâs been altered by things like depurination. However, all those repair services go out
of business ... once you die. But the degradation continues. Now, back in the 1990s, scientists knew all
this. It was part of why getting DNA from a Jurassic
Weevil seemed like a miracle to some, and an impossibility to others. What scientists werenât sure about was how
long it took DNA to degrade to the point where it was no longer readable. Was it 100 years? Or 100 million years? Today we know that DNA has a half-life, kind
of like radioactive elements do. That half-life marks the amount of time until
half of the DNA in a sample is degraded beyond use. But it can vary a lot, depending to some degree
on the organism, but to even greater degree on the quality of preservation. For example, recent research has shown that,
in cores of ocean sediments, the amount of DNA from single-celled algae known as diatoms
drops in half about every 15,000 years. So thatâs itâs half-life. But, by contrast, one study of the bones of
the extinct, large, flightless bird called the moa, showed that its DNA had a half life
of just 521 years. Now, as DNA decays, it doesnât just disappear
â it breaks apart into smaller, harder-to-read fragments. But these half-lives do mean that thereâs
an upper limit to how long DNA sticks around. This is where preservation comes in. Ideal environments for DNA preservation include
colder temperatures with very limited fluctuations. Closed environments are good, too. DNA on the inside of bones is better preserved
than DNA on the outside, because thereâs less interaction with the environment. But even in freezing cold temperatures with
best case preservation, thereâs a limit. A study done in 2012 of 158 well-dated fossils
concluded that, even in the best circumstances, DNA decays well beyond readability by 6.8
million years. Thatâs still slow enough that readable DNA
from the chloroplasts in diatoms can be found in marine sediments that are up to 1.4 million
years old. Here at Eons, we researched this a lot, and to our knowledge, thatâs the oldest confirmed DNA thatâs ever been sequenced. Yet, anyways. But with new genetic techniques, scientists
can read increasingly smaller chunks of DNA and put them together to make longer strands
â like the full genome of a 700 thousand year old horse, which was sequenced in 2013
from many, many small chunks of DNA. And it helps that shorter chunks of DNA, like
the DNA found in your mitochondria or a diatomâs chloroplast, are more stable and can last
longer. So if DNA becomes unreadable in less than
6.8 million years, how the heck do we have DNA from a weevil thatâs 120 million years
old? Well it turns out, that âancient weevilâ DNA
wasnât actually from an ancient weevil. And the problem was in the methodology. In order to read a DNA molecule, you need
a LOT of it to make sense of what youâre reading. This means you need to make many copies of
it, in a process called amplification. The easiest and most efficient way to amplify
DNA is a process called PCR, or Polymerase Chain Reaction. PCR can quickly make even small amounts of
DNA into large, consistent samples that are easy to test. And itâs really sensitive: All you need
is an itty, bitty bit of DNA to start with. But because itâs so sensitive, it can also
accidentally replicate things you didnât want. Like, if a single human skin cell should fall
into the sample, it could be replicated so quickly and thoroughly that its genetic code
would overwhelm the sample. And thatâs exactly what happened with the
sample from the weevil. In the late 90s and 2000s, when lab conditions
became better controlled, samples that were tested in the early â90s were re-tested. And a lot them couldnât be reproduced successfully. The DNA that we thought was from that Jurassic
weevil actually turned out to be mostly from a modern fungus that had gotten into
the sample. And the rest of the DNA was from a modern
weevil, probably because the scientists were comparing the old DNA to DNA from living species,
and accidentally cross-contaminated. Likewise, the termites and the bees preserved
in Amber were all re-tested⌠and their DNA was found to be from humans, trees, fungi
and other modern contaminants. And when youâre dealing with tiny snippets
of DNA, itâs actually not that hard to mistake one organism for another. After all, we all share a lot of our DNA with
other organisms, even ones that bear no resemblance to us. So if these scientists happened to pick the
wrong section of DNA to replicate, they could end up reproducing a section thatâs in a
weevil, but is also in a tree, or a human. So⌠does that mean there isnât DNA from
fossils after all? Nope! We can get great DNA samples from some fossils,
as long as theyâre more recent, and most importantly - if youâre really careful about
preventing contamination. Nowadays, you have to wear a bodysuit and
two pairs of latex gloves to keep your DNA from falling into the mix. Labs have to be sealed off from outside air,
and surfaces must be bathed frequently in UV light to kill any lingering genetic material. And if youâre comparing ancient DNA to modern
DNA, you have to use two separate labs so they doesnât get mixed up. But all these precautions are worth it, because
when itâs amplified properly, ancient DNA can reveal to us some wonderful things! For example, DNA from fossil humans has shown
us a lot about where different human populations came from. Itâs demonstrated that humans, Neanderthals,
and Denisovans were all probably interbreeding during the last 100 to 200 thousand years. And in 2014, ancient DNA also showed us that
the extinct flightless Elephant bird from Madagascar was most closely related to the
Kiwi of New Zealand, and not Ostriches, like we once thought. So even though it doesnât reach back to
the days of the non-avian dinosaurs, some DNA that weâve sequenced is still pretty
darned old â like that 700,000 year old horse from the Yukon Territory. In 2013, it helped to illuminate the story
of horse evolution, and showed that bone DNA is better preserved in permafrost than we
previously thought, possibly storing readable pieces for up to a million years. And recent research has changed what we know
about DNA decay rates, too. In 2016, scientists studying diatom DNA found
that even though it decays rapidly for the first hundred thousand years, the older stuff
decays more slowly, and no longer follows the regular half-life pattern. Likewise, an analysis in 2017 found that older
bones of large mammals held more DNA than expected, given the half-life of DNA. And other research has even shown that certain
types of bone, like the dense bones of your inner ear, hold more DNA and more likely to
preserve DNA for longer. So, all of this suggests that bigger chunks
of DNA could last longer than we thought. Weâve learned a lot about the limitations
we currently face when it comes to studying the DNA of long-gone organisms. And all of this adds up to the knowledge that
extracting DNA from a 75 million year old velociraptor is impossible. At least, for now. Remember: 25 years ago, it seemed impossible
weâd ever have ancient DNA at all. And right now, truly ancient DNA â dinosaur
DNA â is out there, but itâs dissolved into pieces. Itâs impossible to read using our current
technology. Itâd be like trying to piece together an
entire book thatâs been chopped up into individual words⌠or letters. But that doesnât mean itâs always going
to be impossible. It just means that, at the moment, we canât
make out what that book says. Maybe one day, perhaps even in our lifetimes,
weâll find a way to crack that code. But for now, the mysteries of that weevilâs
genetic code remain a jigsaw puzzle of base pairs that we have yet to put together. Thanks for joining me! And special thanks to our four eontologists,
David Reed Rasmussen, Jon Ivy, Eric Lawrence, and Steve. Thank you so much for your support! If youâd like to join them, head over to
patreon.com/eons and pledge for some neat n nerdy rewards. Now, what do you want to know about the story
of life on Earth? Let us know in the comments. And donât forget to go to youtube.com/eons
and subscribe!
It's a shame the half-life of DNA is so small. Get maybe 10-20 million years before you hit a wall where none is left :(
So they happened to do this in 1993? The same year Jurassic Park came out.
Can someone post a useful tldr/w?
Ummm, it was a mosquito and they got the DNA of a lot more than a bug! Lol....youâre SO stupid.
The silly comments above notwithstanding, this was an interesting watch.
tl;dw
1 million year old, if you're lucky