Did Spiders' Ancestors Come From The Ocean?

Summary

Short Wave Podcast Summary: "Did Spiders' Ancestors Come From The Ocean?"

Released: July 25, 2025 | Host: NPR’s Mary Louise Kelly | Hosts: Emily Kwong and Regina Barber

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1. Ocean Origins of Spider Ancestors

Timestamp: 02:24 – 04:24

The episode kicks off with a fascinating exploration into the evolutionary history of spiders. Traditionally, scientists believed that the earliest spiders and scorpions emerged on land approximately 450 million years ago. However, new research published in Current Biology challenges this view, proposing that arachnids may have originated in the ocean much earlier.

Mary Louise Kelly explains, "They found that the 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" (03:25).

Regina Barber elaborates on the study, highlighting that researchers examined a fossil named Molisonia symmetrica, an extinct marine arthropod from the Cambrian period, around 500 million years ago. "Nicholas Straussfeld... thinks it's an early example of a modern spider brain with organization that makes it easier to control movement, like walking around and weaving webs" (03:25).

The revelation has sparked debate within the scientific community. Evolutionary biologist Alejandro Izquierdo Lopez expressed cautious optimism, noting, "Molisonia is a really exceptional fossil. These findings are super cool," but also emphasized the need for further research to confirm whether what appears to be brain structures are indeed indicative of arachnid ancestry (04:24).

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2. Stellar Lifecycles Viewed Through Orion and Taurus

Timestamp: 04:34 – 06:51

Transitioning from the depths of the ocean to the vastness of space, the podcast delves into astrophysical research concerning star clusters in the Orion and Taurus constellations. An international team of astrophysicists utilized advanced computer simulations to model the evolution of thousands of stars over 800 million years, offering unprecedented insights into the life cycles of star clusters.

Mary Louise Kelly shares, "They looked at three clusters of different ages... to see how these types of clusters evolve from bunched up fuzzy balls to big spread out groups of stars" (05:56). The study, featured in the Monthly Notices of the Royal Astronomical Society, confirmed that the observed star clusters represent different evolutionary stages, akin to "seeing photos of the same person as a baby, teenager, and elder" as quoted by Hossein Haki, one of the paper's authors (05:29).

Regina Barber connects this research to everyday observations, noting, "It’s like seeing photos of the same person as a baby, teenager and elder" (05:29), adding a relatable analogy to the complex processes governing stellar evolution.

The breakthrough was made possible by increased computing power, which allowed for more dynamic and accurate simulations. "Star cluster simulations were very simple... Until now," Mary Louise Kelly remarks, highlighting the technological advancements that enabled this deeper understanding of star formation and cluster dynamics (06:12).

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3. Complexities in Determining Baby’s Sex Beyond a Coin Toss

Timestamp: 06:51 – 09:14

The final segment addresses a common misconception about the probability of having male or female children. Contrary to the simplistic 50/50 belief, recent research reveals that the likelihood of a child’s sex becomes more nuanced in larger families.

Harnessing data from the Nurses Health Study, which tracked over 100,000 pregnancies from 1919 to 2015, Harvard researcher Siwen Wong and her team discovered patterns where families with multiple children of the same sex exhibited a higher probability of subsequent children being of that same sex. Specifically, "in families with at least three children of the same sex, the next baby's sex tended to follow a weighted coin toss" (07:59).

Regina Barber summarizes the findings: "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%" (08:26). Mary Louise Kelly adds that maternal factors, such as the mother's age at first birth, appear to influence these probabilities. "Women who started their families at the age of 28 had a slightly higher chance of having families with all females or all males" (08:15).

The study also touches upon physiological changes associated with maternal age, including shorter menstrual cycles and lower vaginal pH, which may contribute to the observed sex clustering. However, as noted by Regina Barber, the research primarily focused on maternal data, leaving the role of paternal factors an area for future investigation (08:43).

Mary Louise Kelly reflects on the personal nature of such studies: "I love studies like this... Let's investigate," highlighting the relatable genesis of the research questions posed by real-life observations within families (07:03).

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Conclusion

The episode "Did Spiders' Ancestors Come From The Ocean?" skillfully weaves together diverse scientific topics, from evolutionary biology and astrophysics to human genetics, providing listeners with engaging insights into each field. Through expert interviews and detailed explanations, Mary Louise Kelly, alongside co-hosts Regina Barber and Rachel Carlson, delivers a rich narrative that illuminates the interconnectedness of scientific discovery.

For more episodes and science stories, visit Short Wave Plus.

Transcript

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[Mary Louise Kelly]

You're listening to Short Wave from npr. Hey short wavers.

[Regina Barber]

Regina Barber here and Rachel Carlson. We with our bi weekly science news roundup featuring the host of All Things Considered. And today we have Mary Louise Kelly. Hi Mary Louise.

[Mary Louise Kelly]

Hi.

[Rachel Carlson]

I'm so glad to be back. And I hear you've got some new research on why the ancestors of spiders may have originated in the oceans instead of land.

[Mary Louise Kelly]

Yep. Plus how you can see snapshots of a star's whole life in the Orion.

[Regina Barber]

And Taurus constellations and why the likelihood of having a male or female baby is much more complicated than a coin toss.

[Rachel Carlson]

I have so many questions. Can't wait.

[Mary Louise Kelly]

I mean, I hope we answer them on this episode of Short Wave, the science podcast from npr.

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[Mary Louise Kelly]

So, Mary Louise, where would you like to start?

[Rachel Carlson]

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.

[Regina Barber]

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.

[Rachel Carlson]

Okay, so prehistoric ocean ocean spiders. My skin is definitely crawling. I think we all are. What are scientists thinking? Why did they rethink this?

[Regina Barber]

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 Molisonia symmetrica. And one of the study authors, Nicholas Straussfeld, 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 pincers by its mouth.

[Mary Louise Kelly]

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.

[Rachel Carlson]

And how is this new thinking landing with other scientists? Because it sounds like a big shift.

[Regina Barber]

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 Molisonia is 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.

[Mary Louise Kelly]

But if these findings are supported with, like, more research, that would make the Molsonia the earliest arachnid ancestor and confirm that spiders do really come from the.

[Rachel Carlson]

Sea, from sea spiders, maybe to our next story, which has to do with the skies and the constellation Orion, which is one of two constellations I can actually find in the sky. Big Dipper, Orion. So I'm so glad we're on one of these. Why are we talking about it?

[Regina Barber]

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.

[Mary Louise Kelly]

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 Hossein Haki, one of the authors of this paper.

[Regina Barber]

It's like seeing photos of the same person as a baby teenager and elder.

[Rachel Carlson]

Baby teenager and elder. So fascinating to think of stars in that way. Tell me more.

[Mary Louise Kelly]

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.

[Rachel Carlson]

Yep.

[Regina Barber]

And then the teenage star cluster is the Pleiades in Taurus. It's more spread out than the Orion Nebula stars. 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.

[Rachel Carlson]

Although I'm won why they've only just been able to figure this out, like, why scientists couldn't do this before these latest simulations.

[Mary Louise Kelly]

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.

[Rachel Carlson]

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.

[Mary Louise Kelly]

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 Newman.

[Amazon One Medical]

Say more.

[Rachel Carlson]

What's going on?

[Mary Louise Kelly]

Well, Harvard researcher Siwen Wong 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.

[Regina Barber]

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? 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 191956 to 2015. And examining the birth records of more than 58,000 women. Siwan'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.

[Rachel Carlson]

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? Let's investigate. But it sounds like you're telling me, Rachel, each family may have a distinct tendency towards males or toward females.

[Regina Barber]

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.

[Rachel Carlson]

Was 58%, 61, and 58%. That's not 50, 50 or anywhere close to it. Do they know why?

[Mary Louise Kelly]

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 mal.

[Regina Barber]

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, Siwan 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.

[Rachel Carlson]

Okie dokie, queue up the next study on the daddies.

[Regina Barber]

Yeah, absolutely. Mary Louise, it is always such a pleasure having you on.

[Mary Louise Kelly]

Agreed. You should always come back.

[Rachel Carlson]

I will indeed. Invitation accepted. Thank you.

[Regina Barber]

Thank you.

[Mary Louise Kelly]

You can hear more of Mary Louise Kelly on consider this NPR's afternoon podcast about what the news means for you.

[Regina Barber]

This episode was produced by me, Rachel Carlson, and Jordan Marie Smith. It was edited by Rebecca Ramirez, Christopher Inteliatta and William Troup.

[Mary Louise Kelly]

Tyler Jones checked the facts. Tiffany Vera Castro and Robert Rodriguez were the audio engineers. I'm Regina Barber.

[Regina Barber]

And I'm Rachel Carlson. Thanks so much for listening to Short Wave, the science podcast from npr.

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