The billion years missing from Earth’s history

Host: Nick Petrić Howe

Welcome back to the Nature Podcast. This week, we have something slightly different for you. We’ve got an extended Briefing chat, so we’ll be covering three recent stories from the world of science that have been featured in the Nature Briefing. Joining me for our chat this week are Shamini Bundell and Dan Fox. How are you both doing?

Host: Shamini Bundell

Hi, good, thank you.

Host: Dan Fox

Yeah, great, thanks. Glad to be making my Nature Briefing debut.

Host: Shamini Bundell

This is double excitement. We have three hosts and we have Dan Fox as Nature Podcast host. I’m very excited for this.

Host: Nick Petrić Howe

Well, it’s good to have you both here. And I thought I’d go first this week, and I’ve been looking into a geological mystery about a missing billion years of time.

Host: Shamini Bundell

That seems very careless. The geologists have mislaid a billion years? It’s decreasing my faith in geology a little bit if they’ve only just noticed that.

Host: Nick Petrić Howe

Well, they actually noticed it around 150-or-so years ago, so it’s something they’ve been working on for quite a while, but it’s been really hard to work out why this period of time is missing. And well, I should clarify that there’s not actual time missing. It’s more archaeological record of it. So, this was an article I was reading in BBC Future, and you might already know that when we look back in the past using geology, you look at different layers of rock, and by going down the layers you can see further and further back in time. And well, around 150 years ago, it was noticed in the Grand Canyon that as you went down the layers, something weird happened, and there were lots and lots of horizontal layers and then suddenly there was a shift and there were vertical layers. And one of the geologists who observed this also observed that there should be more of these layers than there actually were, and so there was a missing chunk of geological time.

Host: Shamini Bundell

Wait, so was the bit where the layers went vertical associated with the bit where some layers were missing?

Host: Nick Petrić Howe

Yeah, so the bits where the layers went vertical are a harder type of rock that is older, and these rocks are around 1.7 billion years old. Whereas the horizontal line, the oldest one we had there was around 500-or-so million years. So, there’s something that’s happened between those two which has meant that that rock is no longer present in those places. And I should say as well, this varies in different regions. So, within the Grand Canyon, there’s more or less of this missing time and across the world it sort of varies as well. But regardless, there is always a bit of time that is missing.

Host: Dan Fox

So, what are the theories for where these rocks that represent a billion years have gone to?

Host: Nick Petrić Howe

Well, ‘theories’ is the right word for it because no one is really sure, and the reason we’re talking about this now is recently there’s been a new theory. But before we get onto that, I’ll take you through the two existing theories. One is that these rocks were simply eroded away by fast-moving glaciers during a time when the Earth was frozen – Snowball Earth as it’s known. This is the reason for the missing time. Another theory is that heat from the centre of the Earth caused an ancient supercontinent called Rodinia to expand and rise up into the air and the top layer again got eroded away.

Host: Dan Fox

So, what’s the newest theory?

Host: Nick Petrić Howe

Well, the newest theory is that actually this gap wasn’t a huge gap at all. What we’re seeing are lots of little gaps that we’ve sort of confused together and think is one big gap in the geological record. So, this was a recent analysis that was done, and I talked a little bit about the Grand Canyon where you can see this, but there are other places as well, and in this study, they compared the Grand Canyon to this place called the Canadian Shield, where there’s another one of these gaps. And according to their analysis, it looks like the gap in the Grand Canyon happened before the one in this Canadian Shield, probably before the Snowball glaciation. And so, actually, it could be several different events that have come together and it’s actually several smaller gaps that have happened in a similar sort of few hundred million years of time and have ended up being sort of confused together. Because the further back in time you go, the harder it is to sort of place these events. There are greater degrees of uncertainty to work out actually when things happened, and so that’s the latest theory.

Host: Shamini Bundell

So, if it’s not all one big, huge, global, missing billion years of geological layers, then I suppose that means you don’t need one unifying theory to explain all the gaps. It could be both of the ones you mentioned. It could be lots of different things.

Host: Nick Petrić Howe

Yeah, exactly, and what’s happening now is there’s something called thermochronology, which is a new sort of technique that is allowing geologists to look into the thermal history of rocks. So, typically, when you talk about dating of rocks, you look at radiometric dating, and it’s a similar sort of idea, looking at the decay of various elements. But with this, you are actually able to see the sort of history of the rock over time and the sort of thermal history. You can see like how it came from the Earth, not just looking at the rock when it was fully formed and solid. And this new technique could help answer some of these questions and maybe finally get an answer to this sort of 150-year-old mystery of what happened with this great, big gap in geological time.

Host: Shamini Bundell

Well, I think, actually, it’s going to be quite a geology heavy week in a way because I’ve got some exciting rocks and layers to talk about later. But Dan, is your story vaguely geology related maybe?

Host: Dan Fox

I mean, I guess in the sense that petrol comes out of the ground, then, yes, it is geology related, sure.

Host: Shamini Bundell

That’ll do. Yeah, the geology special. There we go.

Host: Dan Fox

So, yeah, this is a story that was originally published in Chemistry World, and a UN environment programme has declared the era of leaded petrol over. That’s because in July, Algeria became the last country in the world to stop selling leaded petrol.

Host: Shamini Bundell

So, at petrol stations, there used to be leaded and unleaded and you had to make sure you went to the right pump and get the right one. But basically, now, if I went to a petrol station anywhere in the world, would I basically not be able to get leaded petrol anymore?

Host: Dan Fox

You shouldn’t be able to get leaded petrol anymore. So, I think most high-income countries started to ban the sales of leaded petrol around the end of the twentieth century because, despite adding a little tetraethyl lead to petrol improving engine performance, it does cause a host of health problems, so heart disease, cancers, and harms cognitive development in children. So, as I said, high-income countries started to phase it out around the end of the last century, but most low- and middle-income nations weren’t able to just sort of switch the switch and get rid of leaded petrol. And if you think about the sort of process involved in getting everyone in a country to stop using older engines, you can see why that might be difficult. So, in 2002, the UN established a campaign to end the sale of leaded petrol worldwide, and with Algeria’s ban coming into effect, they’ve succeeded.

Host: Nick Petrić Howe

Okay, so, is this sort of the end of lead pollution? You said there was obviously a host of health effects, and are we going to start to see a healthier, happier world?

Host: Dan Fox

Well, well, unfortunately, not immediately. It’s not going to just be an off switch. So, a study in London, where we haven’t had leaded petrol in cars for over 20 years, showed that half the lead in the air still came from leaded petrol that was being used 20 years ago because lead sort of sticks around in the environment, in dust, and we still have 50 times more lead in the air than you would expect naturally.

Host: Shamini Bundell

But overall, getting rid of it must be a good thing. The UN must be pretty happy about their campaign having been successful.

Host: Dan Fox

Yeah, completely. They estimate that there’ll be more than 1.2 million premature deaths per year avoided, increase in IQs for children, and they think we’ll save US$2.45 trillion for the global economy because of these benefits, and that could even lead to a decrease in crime rates. They also see this successful campaign as something that could inspire new efforts to reduce vehicles’ environmental impact in the future.

Host: Shamini Bundell

Definitely something where there’s still a lot of work that I’m sure we can do in terms of pollution and burning fossil fuels and all of that.

Host: Nick Petrić Howe

Yeah, it seems like a lot of stories that we have on the Briefing end with that sort of note, but there is more work to be done on the Briefing as well. So, Shamini, I’m intrigued by your story that you alluded to earlier, so can you tell us a little bit about it?

Host: Shamini Bundell

Yeah, so it’s not really geology, it’s archaeology, but they’re friends. The archaeologists and the geologists, they love digging in the ground and looking at layers, and this particular story is based on a Nature paper and there was a News and Views article in Nature that was linked to from the Briefing, and it’s all about archaeological work in the Arabian Peninsula. And the reason that this is sort of particularly interesting is that if we were doing this podcast ten years ago, pretty much all of the sort of archaeological studies relating to humans came from the very recent past, so 10,000 years ago, after the last Ice Age, after sort of Neanderthals were extinct. But in the last ten years, they’ve been working hard and have really expanded what we know about humans in Arabia over the last several hundred thousand years.

Host: Nick Petrić Howe

Well, colour me intrigued then. What do we know about humans in Arabia in the past hundred thousand years?

Host: Shamini Bundell

Well, one of the sort of key findings is that Arabia wasn’t always so dry and arid. This particular paper was looking at studies from ancient lake beds, actually sort of multiple lakes that had ended up stacked on top of each other, again several hundreds of thousands of years apart. And they found things like evidence of hippopotamuses and freshwater molluscs, and they believe that there were sort of periods of time between when it went back to sort of desert-like and arid where Arabia was probably green and lush, maybe more like the sort of current African savanna, and would have been a really nice place for ancient humans to live.

Host: Dan Fox

So, an interesting thing, I guess, geographically, about Arabia is that it connects to Africa, and connects Africa to Asia, and historically to Australia, so during these periods, is that when perhaps people were coming out of Africa and spreading out into Asia through the Arabian Peninsula?

Host: Shamini Bundell

Yeah, absolutely. And I think that that’s why this is sort of so key because we know obviously that different human species, including Homo sapiens, did come out of Africa and spread across the world, and they must have come through the Arabian Peninsula to do it at some point we think, but it wasn’t quite known sort of when and how. Did they sort of hug the coastline so that they would have access to water and food? But what this paper has found is five distinct periods, going back 400,000 years, in which in each of these periods, not only was it damp and green and lovely, but there is evidence of humans, some kinds of humans, in each of these five periods. So, we know now that humans were coming out of Africa and potentially back in again during these warm wet periods. What we don’t know yet, what the researchers don’t know, is whether the expansions from Arabia into the rest of the world, you mentioned humans spreading across Asia, spreading all the way to Australia, we don’t know if that was happening from Arabia during those periods as well, which would be really key to understanding the still quite sort of confusing pattern of humans spreading across the world.

Host: Nick Petrić Howe

So, you mentioned as well other human species spreading out of Africa. Do we know much about them spreading through the Arabian Peninsula?

Host: Shamini Bundell

Yeah, well the sort of oldest lake from 400,000 years ago, that was before modern humans evolved in Africa. Later on, in more recent layers, there is evidence of Homo sapiens, and there’s some bits in the middle where you’re not really sure what kind of species it was. But there are, for example, flake tools, that are associated with modern humans, but also, interestingly, in the Arabian Peninsula, at the same time, were the kinds of tools that were associated with Neanderthals. And one of the reasons that that’s kind of intriguing is that, well, you have Neanderthal DNA in you and so do I because when Homo sapiens left Africa, scientists think they mixed with Neanderthals and now Neanderthal DNA is all over the world. So, it could be that Arabia is one of the places where that sort of mixing happened. So, given that Arabia was a little bit of a bottleneck to the rest of the world, it kind of makes sense that maybe that’s the place that modern humans and Neanderthals were sort of meeting up and mixing, and there’s obviously, as always, lots more work to be done to figure out exactly what was going on.

Host: Nick Petrić Howe

Well, I look forward to hearing more about this in the future, but I think this is all we’ve got time for on this week’s show. Dan and Shamini, thank you so much for joining me. And listeners, if you’re interested in more stories like these, why don’t you try out the Nature Briefing? We’ll put a link of where you can sign up, along with the stories that we discussed, in the show notes. We’ll be back again next week with a regular show. Thanks for listening. I’ve been Nick Petrić Howe.

Host: Shamini Bundell

I’m Shamini Bundell.

Host: Dan Fox

And I’m Dan Fox.