MARY LOUISE KELLY, HOST:
Time now for our science news roundup from Short Wave. That's NPR's science podcast. Reporting this week, we have Rachel Carlson. Hey there.
RACHEL CARLSON, BYLINE: Hey, Mary Louise.
KELLY: Hey. And Regina Barber - hello to you.
REGINA BARBER, BYLINE: Hey.
KELLY: So y'all have brought us, as always, three science stories that caught your attention this week. What do you got?
CARLSON: The oceanic origins of spiders.
BARBER: How you can see snapshots of a star's whole life in the Orion and Taurus constellations.
CARLSON: And why the likelihood of having a male child or a female child is a little more complicated than a coin toss.
KELLY: Wow. I want to get to all three of these, but my skin is already crawling and hasn't stopped since you mentioned spiders, so let's start there. What do they have to do with the - you said the ocean?
CARLSON: Great question. So based on the fossil record, researchers thought that the earliest spiders and scorpions existed around 450 million years ago on land. But a new study in the journal Current Biology suggests arachnids may have actually originated much earlier in the ocean.
KELLY: OK, so prehistoric ocean spiders - my skin is definitely crawling. I think we all are.
(LAUGHTER)
CARLSON: Yeah.
KELLY: What are scientists thinking? Why did they rethink this?
CARLSON: Yeah, it's pretty cool, actually. They looked at the brain and nervous system of a fossil from a now-extinct marine arthropod. It's called Mollisonia symmetrica. And one of the study authors, Nicholas Strausfeld, told me that it lived during the Cambrian period. That's around 500 million years ago or so, when most life was still in the ocean. And this fossil is about a centimeter-and-a-half long. They think that it probably had tiny little pincers by its mouth.
BARBER: Yeah, on the outside it doesn't look much like spiders - the ones we know today. But when they reexamined this fossil, they found that its brain and nervous system had a similar structure to arachnids, which made them think that this might push back the origin story for things like spiders and scorpions. Nicholas said that he thinks it's an early example of a modern spider brain with, like, organization that makes it easier to control movement, like walking around and weaving webs.
KELLY: And how is this new thinking landing with other scientists? - 'cause it sounds like a big shift.
CARLSON: It would be a big shift. So I got in touch with an evolutionary biologist who didn't work on the study. His name's Alejandro Izquierdo Lopez. And he told me in an email that Mollisonia's a really exceptional fossil and these findings are super cool. But looking at things like brains and the nervous system tissue of fossils like this is a really complex process, and it is possible that what seem like brains are other tissues. Plus, he says this finding would raise a whole host of other questions about arachnid evolution.
BARBER: But if these findings are supported with, like, more research, that would make the Mollisonia the earliest arachnid ancestor and confirm that spiders do really come from the sea.
KELLY: From sea spiders - maybe - to our next story...
BARBER: Yeah.
(LAUGHTER)
KELLY: ...Which has to do with the skies and the constellation Orion, which is one of two constellations I can actually find...
CARLSON: Nice.
KELLY: ...In the sky - Big Dipper or Orion - so I'm so glad we're on one of these. Why are we talking about it?
CARLSON: Scientists want to study it to understand life, or at least stellar life, because basically all stars are born in clusters of hundreds or thousands of stars. And an international team of astrophysicists studied open star clusters in Orion and its neighbor Taurus and determined that these clusters, or groups of stars, seem to grow old following some basic rules.
BARBER: This study used computer modeling to simulate how thousands of stars in these clusters might evolve and change over 800 million years. And the simulations confirmed that these star groups in these constellations are indeed snapshots of, like, different phases of a star cluster's life. Here's Hosein Haghi, one of the authors of this paper.
HOSEIN HAGHI: It's like seeing photos of the same person as a baby, teenager and elder.
KELLY: Baby, teenager and elder - so fascinating to think of stars in that way. Tell me more.
BARBER: So they looked at three clusters of different ages. So the baby cluster that they're talking about is the Orion Nebula. It looks fuzzy because the stars are still, like, forming in the gas and dust. It's part of the sword that hangs from Orion's Belt, if you remember, like, the three stars in the belt.
KELLY: Yeah.
CARLSON: And then the teenage star cluster is the Pleiades in Taurus. It's more spread out than the Orion Nebula stars.
BARBER: And the elder cluster is the Hyades, and that's the V-shaped group of stars that are part of the head of the constellation Taurus, the bull. And this one is so spread out that if you were to look at it with your naked eye, I think most people who aren't astronomers wouldn't even think it's a star cluster.
CARLSON: So, Mary Louise, looking at these three together, you can see how these types of clusters evolve, from bunched-up fuzzy balls to big, spread-out groups of stars.
KELLY: Although I'm wondering why they've only just been able to figure this out, like, why scientists couldn't do this before this - these latest simulations.
BARBER: So this hypothesis that these were snapshots of star cluster evolution - that already kind of existed. But to have it correspond with simulations just happened because the computing power just wasn't there yet. Until now, star cluster simulations were very simple. The stars had minimal interaction with the dust and gas they formed from. But in this study, they are able to, like, add that dynamic in and more accurately model, like, the relationship between stars over a long period of time. The findings were published in the journal Monthly Notices of the Royal Astronomical Society.
KELLY: All right. From maybe baby stars to human babies, our final topic. And you were telling me about the odds of having a male or female child, which I always thought would be, like, 50-50 - coin toss.
BARBER: Right. I mean, I also thought that, but in big families with more than, like, three children, that may not be the case, according to reporting from our colleague Scott Neuman.
KELLY: Say more. What's going on?
BARBER: Well, Harvard researcher Siwen Wang and her colleagues noticed something interesting among their own families and friends, which is that families with many children often have a string of male babies or, like, a string of female babies.
SIWEN WANG: To the extent that we're wondering - this happens so frequent - whether it's simply by chance, or is there any underlying biology explaining these sex clustering within families?
CARLSON: So to study this more, they turned to data that already existed. It's something called the Nurses' Health Study. It tracked over 100,000 pregnancies from 1956 to 2015. And examining the birth records of more than 58,000 women, Siwen's team found a pattern. In families with at least three children of the same sex, the next baby's sex tended to follow a weighted coin toss.
KELLY: I love studies like this, by the way, that start with the researcher just being like, huh, there's this thing in my family. Is this true everywhere?
CARLSON: Yeah.
KELLY: Let's investigate. But it sounds like you're telling me, Rachel, each family may have a distinct tendency towards males or toward females?
CARLSON: Exactly. So in families with three male children, the probability of having another male child was 61%. And in families with three female children, the probability of having another female child was 58%.
KELLY: Sixty-one and 58%. That's not 50-50 or...
CARLSON: No.
KELLY: ... Anywhere close to it. Do they know why?
BARBER: Well, one of the first things that stood out in the data were the mother's age at first birth. Women who started their families at the age of 28 had a slightly higher chance of having families with all females or all males.
CARLSON: We also know that maternal age during the reproductive years is associated with several physiological changes, including shorter menstrual cycles and a lower vaginal pH, so the age of the mom might influence the biological sex of the child. And in speaking with NPR, Siwen noted that parents' ages are often similar to one another. And since the study focused on maternal data and didn't include paternal age or genetic information, the potential influence of fathers on these outcomes remains an open question for the researchers.
KELLY: That's Rachel Carlson and Regina Barber from NPR's science podcast Short Wave, which you can follow wherever you follow podcasts for new discoveries, everyday mysteries and the science behind the headlines. Thanks, you two.
BARBER: Thank you.
CARLSON: Thank you.
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