Leaf-based computer chips, and evidence that two early human ancestors coexisted
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This is a science podcast for November 29th, 2024. I'm Megan Cantwell. First up this week, host Sarah Crespi talks with newsletter editor Christy Wilcox about making circuit boards out of leaves. Next on the show, I speak with researcher Kevin Hatala about one and a half million year old fossilized footprints that suggest two species of early humans likely coexisted.

Now we're going to talk about making electronics greener using leaves. Christy Wilcox, our daily newsletter editor, is here to tell us how. Hi, Christy. Hi. How are you doing? I am doing fantastic. Yeah. We have some tape here from the researchers when you talk to them, and I do want to play some of that during this segment. But before we do...

What made you decide to write about Leaftronics, as it's been called? Yeah, well, it was one of those papers where as soon as I saw the title, I was like, what is this? This sounds fun. I, you know, obviously read it and thought it was really interesting. And then one of the researchers, Rakesh Nair, he reached out to me just randomly was like, hey, I've got this new paper. Are you interested in it? And I was like,

I was just looking at it. Yes. Can you please talk to me and explain it? And so I talked to him and I talked to his postdoc advisor, Hans Kleeman, about this fascinating idea of using leaves to make boards, the circuit boards in electronics technology.

more sustainable. So this is basically the bulk of a circuit board. You know, you have your little wires, you have your little tiny components made of expensive things that are hard to get out of the ground, but then you have this big plastic backing or glass backing. Why has this been such a tough problem to make this part of computers, electronics,

more sustainable, greener. These boards, as they are now, are just incredibly tough. You're talking about glass fibers and epoxy, plastics. These are going to last for thousands of years in the environment. And they have to be pretty tough because a lot of the manufacturing processes involve either like weird pHs, highly acidic, highly basic stuff, or they have really high temperatures.

There's been some work on people trying to, say, turn paper or silk into these especially flexible boards that are used in certain kinds of devices. But they're not all that great at handling these temperatures. You still have to coat them with something usually. And one of the things that Rakesh noticed is that paper isn't actually all that sustainable. You end up creating a lot of pollution and extracting a lot just to make paper.

But then the question was, OK, paper is not suitable. What's cheaper, more easily available, just as flexible, just as biodegradable as paper is? And the answer was nothing. I couldn't find anything that was better. There was always some reason that this new material was not as good as paper. Maybe it was more expensive. Maybe it couldn't be mass produced. One day going for lunch, there's a tree outside our institute.

It's a magnolia tree and I just, it just clicked. Ah, you know, leaves somehow keep their cells together and they are biodegradable. They are definitely the only thing that's even more ubiquitous than paper is. Let's remove the living cells, use the framework that keeps the cells together and instead of the cells, why not use polymers?

Can you talk about what they do to these leaves to make them suitable for use in electronics? Yeah, well, it's funny because they actually do something that I have done before for art, which is they skeletonize the leaves. You can put it in a particular kind of solution or whatever, and you get just this frame, this leaf skeleton.

which has this amazing, beautiful fractal pattern of these leaf veins, which is also very stable and very tough. They take that and they coat it in this bio-derived polymer. So again, something that came from nature and that is their circuit board. And

And then the way that Rakesh described it to me is like, he basically tried to do everything to make it not work as a circuit board. So the high heat you were talking about, like you can solder onto it. So he soldered onto it. He printed onto it. He etched it. He did everything to these. He even put it in one of these machines. So to make...

organic light-emitting diodes or OLEDs, right? They put them in this machine that creates a vacuum and then deposits the organic material in a very, very fine, very thin layer. Thankfully for having a guide like Hans, he was not complaining too much when I put a leaf in the vacuum chamber.

in our state-of-the-art physical paper deposition machine. It may have also been a result of me telling him after I had done this, but he definitely did not stop me from continuing the experiments.

And surprisingly, it worked. I deposited physical vapor, deposited gold and other metals, even semiconductors on top. And it just survived. The substrate survived. The layers look pretty good. We even optimized the surface roughness to really, really target difficult to make devices, even on glass like OLEDs. So, yeah, we try to push it to a point where it doesn't work so that we find a way to, OK, this is the limit. Now we optimize it.

but it already seems to be working pretty nicely. So it's now something that other scientists have to replicate and just let everyone know what doesn't work or where it could be improved. Because definitely there are ways to improve. We chose a polymer to bind onto this quasi-fractal structure.

But it was a random choice. It could be a much better polymer that we don't even know about yet. One of the things that comes up in the story is this idea that, you know, our phones, our TVs, they kind of are designed to work for a certain amount of time and then we are supposed to replace them.

with the newest model, the big upgrade. But the electronic components in there are designed to last, as you said, a thousand years. So if it was a leaf, how long would it last or how easy is it to recycle or upcycle into that next generation of electronics? So that's one of the really cool things that they actually show. It's pretty

particularly in the supplementary materials of this paper. You can degrade these leaves, but you can also recover everything that you've attached to it. So they were able to take the silver that they used to make the circuits, and they were able to break the leaves down and recover the silver and then make inks. The goal is a completely circular economy here, a completely recyclable product. And they envision having, you know, a grove of trees next to

The plant that makes the electronics next to the recycling plant that pulls the metals from it and puts these leaves into a biogas generator. The biogas fuels all these buildings. And so it would be like this one spot and you would have it all recycled and circular in one place. Okay. And this is what I always ask. This sounds great. Why is it not in my phone right now?

What are the next steps? What are the holdups that would make this difficult to, you know, get out there? While these Leaftronics do really well in almost every test that you would have for the industry, one thing that the industry has standards on is stability. This stuff that is robust and lasts for a thousand years. They have very strict standards on how durable the materials have to be.

The point is that we want these to biodegrade. So they're not necessarily going to reach that strictest standard. The way that Han said is that for these to meet standards, the standards would probably have to compromise and be like 5% or 10% lower. But surviving heat, surviving use, wear and tear, it's fine. It's just, it's not going to last 500 years. And that's the interesting thing too, right? You're making these cell phones, these smartwatches or whatever that...

fail in five years, 10 years? Like, why do you need something that lasts a thousand years? There will have to be some give and take from the industry if we want to have devices that are truly recyclable and sustainable in that way. The other thing we really need to prepare ourselves for if this comes to market is all the puns. I mean, you guys, I do have Hans sharing some of his favorites here. I'm just going to let that play.

We have so much ideas of creating new acronyms for projects. So we have Leaftronics, we have Unbelievable, we have Believe, we have Exfolia, which is Latin for From the Leaf. So we have a lot of funny word games just based on the word leaf. One of the things that really struck me about this was how the researchers talk about being inspired by nature and also inspiring.

guided by nature. So this idea of fractals, I don't know if we really went into that when we first mentioned what a skeletonized leaf looked like. I thought this was so cool. This kind of fractal structures and using one material over and over again at different dimensions is something we have

and said, this is a very nice and beautiful way, but in our technology, we never adapt this way. We are always either making just nano scale or micro scale or macro scale, but always using different methods and different materials for every different scale. But nature is just doing always the same material again and again and building from a piece of sugar

building an entire tree. The same material always. Just use the material in a slightly different way. And this is so inspiring and I think the leaf is just a very beautiful representation of how this can be done. They also talk about how nature has made all these things that are very durable and recyclable and that if nature can't support it, maybe you shouldn't do it. I

I sort of asked them about what are the limits here? They tried to make OLEDs. They succeeded. They tried to, you know, etch it. They tried to solder on it. All of these things worked. I asked, what's not going to work? Where is this going to break? I might just add this hypothesis saying everything we can't add onto the leaf substrate is simply not worth it.

Because the leaf itself or wood, a tree, is like a radical solution from nature. It's something super stable but still biodegradable and can withstand the most harshest condition you could imagine in terms of pH, temperature, et cetera. So we could, for example, never grow a silicon wafer on top of our leaf substrate because you would need 1,300 degrees C in order to grow silicon on top of that. That would never work.

But maybe it's exactly for this reason. It's not worth considering that because nature already said, this is the limit of what our planet can withstand and everything that goes above that is simply too stable. You should not produce that.

Christy, anything I missed from the story? Anything else you want to share from your reporting on it? So yeah, one thing that just stood out to me is just they look so cool. They sent me pictures and I was just in awe. And it was just, they are so neat looking. I just, personally, I find that, you know, from an artistic perspective, very, very cool. There's photos on the website. You can see in the newsletter and I'm sure in the podcast page. Well, Christy, this has been really fun. And yeah, everyone should go look at those pictures and think about having a leaf in your phone. I love it.

Christy Wilcox is the editor for our daily newsletter, Science Advisor. Stay tuned for my conversation with researcher Kevin Hatala about newly discovered footprints in Kenya.

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Based on fossil evidence, we know that two species of early humans, Paranthropus boisei and Homo erectus, lived in similar regions of southern and eastern Africa over a million years ago. But it's been unclear whether these species just live near each other or if they actually coexisted. This week in science, Kevin Hatala and colleagues published a paper with evidence that just might clarify that story. Thank you so much for joining me, Kevin.

Thank you for having me. Of course. So I guess for listeners, it might be helpful first to paint a picture of exactly what Homo erectus and Paranthropus boisei looked like. Yes. So Homo erectus, they look, especially from the neck down, a whole lot like modern humans. People think that they are maybe as strong of a candidate as there is within this time period for being a direct ancestor of ours. Then Paranthropus boisei, on the other hand, they look very,

very different. They are characterized by skulls that seem to be built for chewing very mechanically challenging foods. They have very large jaws and very large teeth. And so it seems like they were adapted to a very different kind of diet from Homo erectus. And from the neck down, we don't know too much about what Paranthropus poisei looked like. A lot of the fossils that are attributed to that species are

skulls or teeth. Those are the most distinctive pieces of their anatomy. There have been skeletal fossils of these species that have been found in similar regions and time periods, but why haven't researchers been able to kind of make that leap that they were actually sharing habitats with each other? There are

skeletal fossils co-occur within sedimentary sequences that represent maybe 100,000 years or so of time. And it's very difficult to clarify and understand

where within that 100,000 year or so span, each of these species was present. You could have that sort of alternating occupation where they're not actually there at the same time, but you would never know that from the sort of resolution that you can get in the skeletal fossil record. So we've just been talking about skeletal remains so far, but footprints are

Another really great way to understand exactly what habitats these different species were in. Could you talk a little bit about the site where you discovered these footprints? The site is located just off the eastern shore of Lake Turkana in northern Kenya. This is an area known as Kubifora, an area where paleoanthropological research has been going on for quite some time, for decades. This particular location where this site was found

It wasn't like a long-running excavation that had been going on for decades. A team was excavating skeletal fossils, and then one member of that team, Richard Loki, identified that there was a footprint layer and found the first hominin footprint at this particular site. The time period that we're looking at here is roughly like 1.6 to 1.4 million years ago. Seems like it has to be pretty pervasive.

perfect conditions for that to happen and for it to preserve the level of detail that you need to really analyze them, right? So you need a mud surface that is just soft enough to accept the impressions of the feet that are walking on it. And then you need some kind of overlying sediment to be sort of

but gently washed over top of it in order to distribute an overlying layer of sediment that's going to sort of cover up and end up preserving the shapes of those footprints that are made in the mud. You would think that they would be very rare, but when you think about how frequently that might happen over the course of 100,000 years or so, maybe it's not quite that

that rare, at least if you're in this kind of environment on the margin of a lake that has been there for quite some time. How many footprints are there? Are we seeing someone kind of walk

around the shoreline for a while? Or how much did you have to work with from this site? There's one very long trackway where one individual took 13 steps in a row. And then there are three other footprints that are oriented in a different direction, walking almost perpendicular to that very long trackway.

And those footprints are different sizes and different shapes. And so it looks like those represent three different individuals. And so in total, it looks like there are four different hominin individuals that were walking across this particular surface at that moment in time. The footprints that were laid down on this surface, they would have been formed within hours to days of one another.

Based on our experiments that we've done on the modern Lake Turkana shoreline, it seems like footprints will preserve there for about three days or so at most. In either scenario, these hominins would have been coexisting and sort of sharing the same landscape if they were there within hours or within days of each other. Interesting. Okay, so I guess...

not as a researcher in this field, if I were to look at these two footprints, would I be able to tell that maybe that they're very different? Or is this really like you have to look at a very detailed way to understand that potentially two different species could have left these footprints? It's possible that you could tell them apart. There are differences that I think sort of stick out to me in my mind. When we were excavating these, one of the first footprints that we were looking at was one where the big toe connective

kind of sits off to a slightly different angle than what you typically see in modern human footprints. And my colleagues and I, we sort of said to each other, oh, that looks a little bit strange. That looks a little bit different. What's going on with the big toe there? I don't think...

Any of us knew while we were in the field that these were footprints that might represent two different species. Once we excavate the footprints, we, in this case, used a method called photogrammetry, where we take a whole bunch of photographs and then use computer software that can stitch those together to create 3D models of the footprints. And most of our analyses are actually analyzing the 3D morphologies.

You don't want to jump to any conclusion too early. So it really wasn't until we got back to the lab, analyzed the 3D shapes, you know, could see these things separating. I went through that analysis and then went through with several different colleagues on this paper saying, like, can you think of any reason why this could be wrong? And we spent months going through and trying to figure out what could the other possible interpretations be

I'm curious if these early human footprints fall within the range of what we see today with modern human footprints, or were they totally outside of this range? You're going to have some variation within any sample. And so modern human footprints, they can look quite different from one another. And among the fossils, they can look quite different from one another as well.

The interesting thing in this particular case, when we look at the footprints from this site and from some of the other sites we analyzed in the paper, there are some fossil footprints that fall pretty squarely within the sort of human range of variation. And then there are other sets of footprints that fall far outside the human range of variation. And they kind of separate somewhat nicely into these two distinct clusters, which

And based on that analysis, that's when we started to say like, oh, I think these are these footprints are representing two different ways of walking. And can you kind of tell how the motion of their foot and their gait differs from Homo erectus based off those footprints? It's hard to know exactly how it differed, but we can tell that the motions were different. The arch shape of footprints tells us something about how the foot was moving through mud. If

If you walked in sand, you made a footprint, you would see that it has an arch shape to it. People a lot of times assume like that's just reflecting the arch shape of your foot itself. Two of my co-authors here, Steve Gatesy and Peter Falkingham, we've run experiments where we've tried to figure out how a footprint forms.

We used bi-planar x-ray, so two x-ray beams that overlap and they're recording x-ray video. And then in the area where those x-ray beams overlap, you can actually reconstruct what's happening there in 3D. What we're finding is that that arch shape has more to do with the way that your foot rolls and rotates through the mud as you walk. And so we walk and we strike the ground with our heel first and then our foot smoothly rotates so that it's flat.

Then we push off the ground with our forefoot. And as you rotate your foot in that fashion, that is what's actually pushing the sediment around and creating that arch shape of your footprint.

Now that arch shape, that's what we actually see looking different when we compare what we assume are Paranthropus footprints to what we've hypothesized are Homo footprints. They're sort of differently arched and that tells us that the foot was moving differently. The sort of finer grain details of the exact differences that were occurring, that's a question that we're still trying to figure out. Can you tell anything about the morphology of the foot from these footprints as well?

I mentioned that the big toe was one of the characteristics that kind of distinguishes these. And so looking at the footprints that we hypothesize were made by Paranthropus, it seems like the big toe was a little bit more mobile than what we see in modern humans. And so the big toe would sort of splay outwards from the rest of the foot, like ever so slightly, and it would be positioned

slightly differently from step to step along the length of this trackway. And so that kind of variation in placement of the big toe gives us a hint that maybe there would be some evidence of that in the foot skeleton of Paranthropus 2. Are there other sites where now you might revisit to see if maybe all of the footprints are Homo erectus or maybe there might already be kind of documented evidence of these two species walking alongside each other?

We actually went back to datasets from other fossil footprint sites that are about the same age, that are known from the same area. And we reanalyzed the data from those and we ended up finding that this pattern, it actually repeats itself across multiple sites.

And so that gave us a little bit more confidence in our hypothesis in this paper when we saw this is not a sort of one-off thing. It's not that something unusual seems to be happening here at this particular site. It seems to be a pattern that occurred multiple times over the course of more than 100,000 years. Do you have your eyes set on another location to look at? Or are you interested in kind of looking at maybe footprints from other early...

early hominem species as well? Yes to both. I mentioned this area is kind of exceptional for how it seems to preserve these sites. So there are plenty more that we know of that are in this same vicinity and plenty of other sites that might hold more data that could help us address questions like how are these two species walking differently? The question of

What do these environments look like that seem to be attracting both species? Or we might be able to figure out if there's something that attracts one species more than the other. So there are lots of questions that can be pursued in the same geographic area and time period. At the same time, I'm very interested in what happened before this too. So it's a matter of, you know, finding the right kind of environment where these kinds of fossils are likely to be preserved.

I really appreciate you taking the time to speak with me. Thank you so much for talking about your paper. Yeah, you're welcome. Kevin Hatala is an associate professor of biology at Chatham University. You can find a link to the paper we discussed at science.org slash podcasts.

And that concludes this edition of the Science Podcast. If you have any comments or suggestions, write to us at sciencepodcasts at aas.org. To find us on podcast apps, search for Science Magazine, or you can listen on our website, science.org slash podcast.

This show was edited by me, Megan Cantwell, and Sarah Crespi. We had production help from Megan Tuckett-Podigy. Our music is by Jeffrey Cook and Nguyen Coi Nguyen. On behalf of Science and its publisher, AAAS, thanks for joining us.