[♪ INTRO] Another year of the pandemic has come and gone.

A lot of science news this year  has been focused on the vaccines, new variants, and the rest of our  ongoing collective misfortune. But science has carried on -- and there  have been plenty of amazing discoveries this year that had nothing to do with COVID.

We think they deserve the spotlight  too, so here are three science stories you might have missed this year. Since the 1980s, scientists have been  pretty sure that dinosaurs died out during a mass extinction event caused  by a roughly 10 kilometer meteorite hitting the Earth’s surface  around 66 million years ago. We’ve even found the likely scene of the  crime: a crater about 180 to 200 kilometers in diameter on the Yucatan peninsula  of Mexico, called Chicxulub.

But where that meteorite came  from has continued to be a puzzle. It came from space, but we’re trying  to be more specific than that. We know from studying the crater that  the object was likely made up of carbon, along with other elements.

Previous research had pointed to the  breakup of the Baptistina asteroid family. So, it would be one of 2500  smaller asteroids that formed after a large asteroid broke  up 160 million years ago. But the Baptistina asteroids were  made of iron and magnesium silicates.

Also, the timing of the Baptistina  breakup didn’t quite fit. The other option was that it was  just a normal background asteroid that just happened to hit the Earth. But mathematically, an asteroid from the  main asteroid belt made out of carbon is only going to hit Earth  once every 3.5 billion years, which would be basically once  in the entire history of life.

Now, comets are more likely to be carboniferous. But a comet the right size hitting  Earth is even less frequent, like, once every 4 to 11 billion years. So, it’s been a pretty weird mystery.

But this year, astronomers at Harvard  did the math and realized that a carboniferous comet from the edge of the  solar system could have been knocked off course by Jupiter, changing its  orbit and sending it toward the sun. When it got close to the sun, gravitational  forces could have broken it apart, sending pieces flying into Earth’s orbital path. And the more objects crossing into Earth’s orbit, the more likely one is to actually hit the Earth.

Sure enough, their model predicts  carboniferous objects threatening Earth at just about the right time and frequency  to have made the Chicxulub crater. This is also consistent with  other carboniferous impact craters in South Africa and Kazakhstan. So these astronomers might  have actually figured out where the comet that killed  the dinosaurs came from.

And 2021 wasn’t just a big  year for ancient critters. It was also a big year for ancient art. Several discoveries this year  brought us some of the earliest known examples of hominin art.

In Israel, archaeologists  discovered a wild ox bone with six nearly parallel incisions  dating back 120,000 years. Now, I know that a bone with straight  lines carved into it may not seem like art, but anthropologically, it’s a reflection of  what’s called symbolic mediated behavior. These are practices where something  takes on a meaning more than just functional use that depends on  collectively shared beliefs.

Funeral practices, decorative  jewelry or clothing, cave painting, and engraving are all  considered symbolic behaviors. These practices are important in human  history, because they reflect the development of organized thinking and communication  between members of a group. Now, the researchers can’t say  for sure what the incisions mean, because they don’t match any  previously discovered patterns.

But whatever it represented, this bone  is the oldest known example of symbolic representation from the Middle East,  suggesting that collective symbolic meaning developed in the region  earlier than scientists had thought. And in the Tibetan Plateau,  archaeologists discovered the oldest known art made on an immobile rock surface. They found impressions of hand- and  footprints made in soft limestone that dated back to between  169,000 and 226,000 years.

They were able to rule out that the  handprints ended up there through everyday movement or from someone using  their hands to stabilize themselves. The impressions were made intentionally. And what’s more, they were made by children.

That’s right, these are basically the two  hundred thousand year old version of the concrete stepping stone with your handprint  in it that is still in your parents’ backyard. If that’s not adorable, I don’t know what is. Before this discovery, the oldest handprint  art was paintings that used hands as stencils dating back 40,000 years, and  these impressions are way older than that.

And maybe more importantly, it may reflect  that the earliest artists were actually kids, or at least that kids of  the time may have used art and creative expression when they played. Ancient hominins. They’re just like us!

And we didn’t just learn about  modern human development this year. We also learned more broadly  about mammal development. Traditionally, somewhere  between 20% and 92% of our DNA has been thought of as non-functional.

It’s often been maligned as “junk DNA”. And the junkiest of our DNA are transposons, also called transposable elements, or TEs. These segments of DNA make up  around 50% of the human genome.

They jump around to different places on  the genome, and as far as we’ve known for a long time, they don’t do much else except  sometimes cause problems when they land. A few transposable elements have taken  on new, more helpful functions over time, helping to determine how and when  other parts of the genome get used. But it’s not totally clear if  that regulation is helpful, or hurtful, or even makes any difference at all.

So this year, scientists took  a closer look at a specific TE. It’s known to regulate the  expression of a gene called Cdk2ap1. That gene is involved in  embryonic cell replication, and when and how embryos implants in the uterus.

They bred some mice that had the gene,  but were missing the TE regulating it. And half of the resulting  mouse pups died at birth. Normally, Cdk2ap1 becomes more active  around 24 hours before embryo implantation.

When that gene isn’t working,  embryos can’t implant correctly, causing serious complications  for the parent and offspring. Now, other mammals don’t have this same  TE, but the researchers did find analogous sections of DNA in seven other  mammal species, including humans. And all of them seem to switch  on before embryo implantation.

This is the first time that  scientists have reported a TE being critical to mammalian development. But if mice can’t survive without  it, that suggests they need the TE to help switch on Cdk2ap1, which in  turn is required for implantation. And that is pretty compelling!

Plus, it has implications for human fertility. A huge number of miscarriages in humans  don’t have a clear genetic reason. It’s possible that the answer  may actually lie in a transposon, just like with the knockout mice.

So it turns out that DNA that was once  trash, might actually be treasure. Thanks for watching this episode of  SciShow, which was brought to you today and for all of 2021 by the  generous support of our patrons. If you’d like to get involved next year,  you can get started at patreon.com/scishow.

We have some great perks for you there,  and also, we will be eternally grateful. [♪ OUTRO]