This Arabian desert was once a lush paradise. Why did ancient humans avoid it?

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0:55Welcome back to The Nature Podcast. This time, a desert trek to discover what ancient humans needed to flourish. And how blocking out the sun is starting to look less like science fiction. I'm Nic Patry-Chow. And I'm Sharmini Bundel.

1:21First up on the show, what do you think ancient humans would have needed to be able to cross vast deserts. Water, right? Well, a new study says, yes, that's right, but there was another secret ingredient that helped humans thrive. Dan Fox has the story. The Rubal Kali is a huge desert covering nearly a third of the Arabian Peninsula. Its name translates as the Empty Quarter, and this vast sand sea holds a secret. This sort of romantic landscape of utter desolation, except for all of these little scraps of evidence all around the desert of Paleolithic or prehistoric occupation. This is Jeff Rose, an archaeologist who spent years trekking through this desert in Oman.

2:13And he knows that people once lived here because this desert was once a flourishing lake-filled savannah, part of what is known to researchers as Green Arabia. We thought we had Arabia figured out. I kind of kid around sometimes. It's impossible to stop the car in Oman and not find an archaeological site. They're everywhere from a million years of Brit history. But the empty quarter still had some surprises left. And so this expedition crossed the RuPaul Collie. Nobody was expecting them to come back completely empty-handed. And that's exactly what happened. Despite the evidence of Paleolithic people that had been found in the past, a recent expedition into the interior of the desert didn't find signs of ancient settlement.

3:02Something that may be surprising as the region has been filled with legends. Don't get me started. There's a lost city in there called the Atlantis of the Sands that Lawrence of Arabia got everybody excited about. I mean, it's completely mistranslation of Arabic. But anyway, I mean, the space shuttle Challenger flew over it in the 80s looking for this lost city. And there's all kinds of folklore and mythology circling around this desert. While myths of ancient cities abound, today the region is incredibly dry, with little more than three centimetres of rain falling here each year. One of the things that I find so interesting about the RuPaul Kali, it's one of the first places we knew about the lakes of Arabia.

3:43And this is back in like the 1920s, the 1930s, when you'd have these explorers crossing it, and they would come back and they'd say, oh, we found these lake basins with evidence for freshwater shells and stone tools all around them. And I remember even when I was a kid seeing this, and that was what ignited my imagination and my interest in Arabia, was the middle of the biggest desert in the world. You have these, supposedly you have these stone tool sites. Those dried up lake beds are part of the evidence that the empty quarter wasn't always a desert. So green Arabia is the term we use for any interval of time when rainfall increased over the Arabian Peninsula.

4:25And this is happening periodically throughout the ice ages. So typically, like we're in Europe or Americas, we think of the ice ages as these continental ice sheets coming down from the poles and then covering the landmass. But in the Arabian Peninsula, the effects of the ice ages are different. Instead of going through these sort of hot, cold ice periods, you go through these wet, dry periods. So every time an ice sheet comes down across Europe, it's pushing the intertropical convergence zone, the jet stream. It's pushing that jet stream south. Suddenly, Arabia is getting all that rainfall. And so we're seeing lakes forming over the Rubal Collie.

5:06We see lakes over the whole peninsula. The rivers fill up. And I mean, it's really dramatic. And this dramatic change didn't go unnoticed by early humans. It becomes this attractive landscape that's bringing populations in, whether they're from Africa or they're from the Levant or possibly even from Asia, coming from the Iranian plateau. So Arabia becomes this kind of land grab from all directions every time it turns green. Jeff had seen plenty of evidence for these ancient migrations during his work in Oman, but the interior of the Rubaq Ali Desert was another story. It's such a difficult place to access because it's 1 ,500 kilometers of desert that you can't just go in and expect, well, we'll just camp for a few nights and come back out.

5:52So it really was this kind of archaeological frontier with all of these questions around it about early humans. Did they go into the RuPaul Collie? Did they live around the lakes? So when the opportunity to join an expedition across the desert arrived, Jeff jumped at the chance to be involved. bringing co-author geologist Rudolf Dalmore, known as Rudy, on board. Sort of at the last minute I had to back out, Rudy ended up going anyway. He was so up for it. And Rudy is 70 and retired and, you know, but he was up for the adventure. They launched in, I think it was November of 2023. And then at the end of December, after the 35 days in the desert, and they come back and I get this phone call and Rudy is just utterly dejected.

6:37And he said, I'm so sorry. So what's wrong? He said, we failed. We didn't find anything around the lakes. And so did you find the lakes? He said, yeah, ostrich, eggshells and lake deposits and, you know, freshwater shells, all the things, but no stone tools. And so I thought, OK, we might have something here. So the expectation going into the expedition was there. We are going to be up to our ears in stone tools. We're not going to know what to do with all the stone tools we're going to be surveying around. There's even an email exchange from before they left where they said, look, there's going to be so much.

7:16Just take a GPS point and then when we go back, we can actually piece plot in all the artifacts. So nobody was expecting them to come back completely empty handed. There was one obvious explanation for the lack of stone tools. Ancient humans never settled here. And so the revelation is it's not just water. It's water and stone. Here in the centre of the Rubel Kali, there was no stone. Previous models of early human migration assumed that access to fresh water was the key ingredient. And so Green Arabia was an ideal habitat. But this expedition showed that lakes, greenery and verdant savannah were not enough.

7:55After we did our study, we found 57 % of that landscape that we thought was habitable was actually uninhabitable, or at least undesirable. And so it's really kind of turned our thinking about migration on its head, that they're not just sort of opportunistically filling out this entire landscape. They're specifically going to distinct locations where there's rich raw material sources. But proving that the expedition team simply hadn't missed the evidence was a trickier proposition. Trying to present the negative test of we didn't find anything was one of the trickiest parts of the paper. Because how do you go and say, hey, we looked and we failed?

8:37One of the basic principles in archaeology is absence of evidence is not evidence of absence. They were helped, though, by the unique nature of desert archaeology. You can see everything because there is 99 % archaeological visibility when you're in the desert. So, you know, OK, you have a sand dune which formed in recent years. But between the dunes is essentially a landscape that has been deflated over a million or two or three million years. So anything in the last few million years is going to be sitting there on top of that landscape. So we're able to show that, hey, we could find the ostrich eggshells.

9:14We could find, you know, these tiny little freshwater shells, all that stuff which was deposited from 100 ,000 years ago or from a known time period. There's no archaeology to go with it. And they could combine this negative result with years of archaeological finds at other sites. It was also having the positive evidence case on the flip side. So having that database of 13 years of surveying the desert and showing we didn't find them here, but we found a thousand plus sites on these landscapes where water and stone were found together. And while this expedition may have ended empty handed, previous work has shown that when both the key ingredients, rock like flint and water are present, then tools are found, backing up the idea that these two need to be together for ancient humans to settle.

10:02But this doesn't mean that some ancient humans didn't venture into the Rubal Kali. So it's not to say that there couldn't have been a middle Paleolithic explorer who, you know, brought a stone tool with them and tried to cross the desert. That's entirely possible. It's the fact that they wouldn't want to live there. And one of the guys on our paper who does experimental archaeology was really crucial in building out this idea. So what he would say is it's not that they're being pushed away from the desert. It's not a preventing force. They're being drawn to the flint outcrops. They're being drawn to the raw material.

10:40So it's an attractive force. At the end of the day, we're lazy. We don't want to go that far for the one thing that is required to survive. There are a number of studies which have already pointed out that people tend to be within five and 10 kilometers of a raw material source. So I think it's already understood among archaeologists. But what was missing is the massive large scale impact. And I think the data set from Arabia is what's unique. And it's what's given us this insight was that, OK, we've seen it act on local scales all across Europe and all across Africa. Now we're seeing on an entire continent when you can see everything on the surface how important Flint really is.

11:23Jeff hopes this finding will help other researchers working in deserts to narrow their searches, helping them find more sites. And there is already evidence that similar raw material requirements exist in other environments, like high-altitude settlements. This work could also start to unlock a bigger mystery. One of the things that happens after the Middle Paleolithic is everything shrinks down. So there's this miniaturization process. And we know it happens in Arabia. We know there's a subsequent presence of people who make really small stone tools. So we were sort of wondering, is miniaturization possibly a response to this need to be near stone tools?

12:06In other words, if we get past our addiction to stone, can we go further away from stone? And as the world starts drying out after 70 ,000 years ago, things become much more challenging. And so if you have to go further to look for food or for water or for resources, if you don't have to go back to the raw material source so often and you can bring your toolkit with you further distance, then that might be one of the keys to the successful colonisation of the globe. Because all of this mystery, what we're really trying to answer is how did modern humans leave Africa, spread across Arabia and then colonise the globe overnight?

12:48That was Geoff Rose from the University of Algarve in Portugal. For more on that and Dan's video on the story, check out the show notes for some links. Coming up, assessing whether distant satellites could help cool the Earth. Right now, though, it's the research highlights with Zoe Valbray.

13:09Scientists have found that people in rural communities sleep less after getting access to electricity. In a 12-year study, scientists tracked the changing sleep patterns of indigenous people in Argentina who gained access to electricity and to smartphones. They found that the residents slept, on average, 35 minutes less per night after getting access to the grid. That may not sound too surprising, because it's widely thought that artificial light and digital technologies disrupt sleep, but there's actually been little long-term evidence for this. The team in Buenos Aires analysed more than 12 ,000 bouts of sleep involving 156 members of an indigenous community in Argentina from 2012 to 2024, They used data from activity tracking devices and compared rural communities that just got access to electricity after 2016 with communities that already had it.

14:05So while smartphone and electricity seemed to delay bedtime for everyone, the group that just got electricity also lost 35 minutes of sleep. The researchers found that neither electrification nor smartphone use alone fully explain the delays in bedtimes, and they suggest that broader social and cultural changes linked to modernisation might also be reshaping sleep. So before you go to bed tonight, get out your phone and read that research in Current Biology.

14:37AI doctors may be pricier for patients, according to a new study. A team of researchers gave 24 artificial intelligence systems notes from 200 medical visits and they asked them to propose plans for diagnosis and treatment. They then compared these plans proposed by the AI models with ones crafted by humans. The AI suggested more diagnostic tests, made more specialist referrals and prescribed more medications than humans did, which led to ballooning costs. AI plans were on average US$314 per visit, compared with just US$97 for the care provided by human doctors. A physician that reviewed the AI plans judged that most of the extra testing was reasonable, but not likely to change a diagnosis or to improve care.

15:27When researchers encouraged the AIs to be more frugal, the costs of the suggested treatments decreased, but it was still more expensive than human plans. You can read that research in the New England Journal of Medicine.

15:46it's getting quite hot here on earth despite global agreements to constrain rising temperatures it's looking increasingly like humanity will soar past the agreed limit of 1.5 degrees celsius above pre-industrial times the world is starting to edge towards ever more dangerous levels of warming. So many people are starting to talk seriously about geoengineering, where often large-scale projects aim to cool the Earth in some way. These ideas can be controversial, but one prospect has attracted space engineers from across the globe, some of whom say it could be viable in just a few decades. That idea is blocking sunlight with satellites.

16:34Reporter James Dineen has been writing all about this for Nature and he joins me now. James, hi, how's it going? Good, how's it going? It's going well for me. So James, when I first heard about this, the first thing that popped into my head was the classic Simpsons episode where Mr Burns takes a large disc and blocks out the sun. Is that the sort of thing we're talking about here? What exactly are we talking about when we're talking about blocking out some of the sun's light? Yeah, it does sound like a Simpsons episode. It sounds really out there and it is. What they are talking about here is placing a large amount of material, whether in the form of satellites of some sort of design or even like a cloud of dust of some kind into stable orbit between Earth and sun.

17:17And if this disk of material was large enough to block about 1 % of sunlight reaching the Earth, shading the Earth, climate models suggest that would cool the climate by about one degrees Celsius. And there are lots of different concepts about how that disk of material might be placed into orbit between sun and Earth. Again, all very sort of hypothetical engineering concepts that are in some ways strain credulity, but also earnest space engineers say that something like this could be possible if you had enough rocket launch ability. In this report, the sort of two main architectures that they look at are this idea around using these giant, extremely thin satellites called heliogyros that are kind of like a solar sail where you have a central hub.

18:14And then around that hub, there are blades of extremely thin reflective material that extend from that central hub. And then the structure of the satellite is maintained by the centrifugal force of the rotating satellite in space. And all of this is about minimizing the amount of material that would be required to block a sufficient amount of sunlight. So in one of these versions with these heliogyro spinning reflective satellites, they imagine 5 ,000 of them each 40 kilometers in diameter, about just under two and a half million kilometers away from Earth. There's also an idea they propose around using some sort of dust or engineered particle in a sort of a cloud of dust in between Earth and sun.

19:02Though that would come with difficult problems of its own, for instance, in order to prevent the dust from floating away into space, you might need some sort of fleet of what they call Shepard satellites that are keeping the dust cloud in place using some sort of repulsive electric field. And what might it actually take to do some of these ideas? What would you need to actually send out into space? Well, it would be very simple. All you would need is an industrial base on the moon. I'm just kidding. It would not be simple at all. The amount of material we're talking about here is a huge amount of mass.

19:38And mass is famously difficult to launch into orbit. In the spinning satellite version of the idea, they estimate you would need something like 70 million tons of mass, which is like 1500 times more than the total amount of stuff humans have ever sent to space. In order to make that look more feasible in their concept, they envision that a small number of satellites, the sort of central hub of the heliogyros would be launched from Earth. but the majority of material would actually be launched from the moon where lower gravity means there's greater launch efficiency so when you say what would you need one thing you would need is a large mining industry on the moon and the ability to launch that material from the moon to meet up with material launched from Earth somewhere between the sun and us.

20:37So when you talk about Mr. Burns, I think this idea is somewhat more ambitious than anything Mr. Burns ever thought of. But the report you mentioned suggests that this sort of thing would be viable in a few decades. What's making them believe that even though this sounds like a huge challenge, it might be doable in that time frame? So the biggest thing is the falling cost of launching material into space. You maybe have heard discussions about things like data centers in space. There's discussion around SpaceX launching its constellations of satellites. All of that sort of changes the economics of what might be possible to launch into orbit.

21:19And so in some ways, this concept, the solar geoengineering idea is sort of saying, OK, in this new world where it's cheaper to launch a bunch of stuff into space in the future, does that change what might be viable around these seemingly science fiction ideas to reflect solar radiation? One thing to mention on that, one idea they also considered is the idea that some people have suggested in the past of reflecting sunlight from orbit around Earth, like most other satellites orbit. But they quickly ruled out that idea because they found that a satellite large enough to block enough sunlight to impact temperatures would rapidly be destroyed by collisions with all the other space junk and debris that are orbiting, which could create a cascade of debris in space and render near Earth orbit unusable.

22:10So what's the thinking behind this? Why are people looking at this approach now? You started out by mentioning rising global average temperatures, damages from climate change. That's very real. And as the United Nations has said recently, it's become abundantly clear that the world has failed to cut greenhouse gas emissions fast enough to keep temperatures in line with climate targets. And because of that, people have started to look at other approaches that might be needed to cool the planet. And solar geoengineering is among those. It remains very controversial, but there are researchers looking at it.

22:47And now this adds space-based approaches to the conversation. But it's important to say that the researchers on this report and in general aren't advocating for the launch of some sort of big effort to do this. They're not saying it should be done, but are exploring the idea should it ever become necessary or should the world's governments decide that they might need to do something like this. And to be clear as well, no one's actually suggesting that they're going to do this. This is all hypothetical at the moment. So what has been the insights from the broader scientific community about this?

23:23Exactly. As you said, this is an engineering study at a very conceptual level. No one's proposing this be done or certainly not saying that it would be viable to do now or suggesting someone goes about building it. It's still in the stage of, is this something that in the future might ever be done? And if the world were to decide to do that, what would be involved in that sort of effort? One research I spoke to sort of laid it out as there's sort of like tier one is like, is this thing physically possible? Tier two is from an engineering standpoint, is it viable? Is it economically feasible and on?

23:59And then tier three is the social political governance dimension of whether that would be something that would ever be reasonable to do. And this report maybe brings it over tier one. Here's something that seems like there aren't any deep physical principles that would make it impossible. And maybe even there are some things that make it seem viable from an engineering standpoint, but it doesn't broach any of those other really important questions that would emerge before something like this actually is proposed or starts to get built or anything like that. But do we know what other consequences there could be from this?

24:38Because obviously blocking out 1 % of the sun's rays, it doesn't sound like much, but it's a bit of an unpredictable system that we would be affecting there. Do we know anything about that? There's a huge amount of unknowns, like other ideas around planetary solar geoengineering, like ideas in the atmosphere. There are all sorts of unknowns about how this would affect the world's water cycle, how that would affect agriculture. In the space-based version in particular, we're talking about these thousands of rocket launches. Those would come with emissions, pollution, which has already become a problem with just the growing number of rocket launches even now.

25:16the mining requirements to support those rocket launches could also cause problems. So there are a huge amount of unknowns and hypothetical risks for what is itself a very theoretical, at this point, idea. So where do we go from here, do you think? So the report that the story is about kind of summarizes the findings of a bunch of different papers that are currently going through peer review. And the researchers who are working on I think we're excited about how this report has brought together a sort of sense of a global community thinking about space based approaches to solar geoengineering that didn't really exist before.

25:54And so more research from this group is what I anticipate. James, thank you so much. Thanks for having me. Niche reporter James Dineen there. For more on that story, you can read his news article by following the link in the show notes. And that's all for this time. Let us know if you've enjoyed the show. You can leave us a review on your podcast app. You can send us an email to podcast at nature.com or you can message us on social media. We are at Nature Podcast. I'm Sharmini Bandel. And I'm Nick Petridge Howe. Thanks for listening.

26:40State Fair food is getting smaller and pricier. We'll have room for fried cookies. Okay, you never overspend on food. What's your deal? What's his deal? He went to Metro and got$450 in value on the new Samsung Galaxy A37 5G for free. And no activation fees. Free phone? Oh, sign me up. Get that more for your money feeling. Only on Metro by T-Mobile. Bring your number and activate$40 period plan. 450 based on device discount wave activation fee. Not available if with Metro T-Mobile in the past 180 days.