Fossil fats reveal world's oldest animals

25 September 2018

Interview with 

Jochen Brocks, Australian National University

dickinsonia-fossil.jpg

Fossil of Dinckinsonia

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The Earth’s about 4 and a half billion years old. And we’ve got evidence that life started here pretty rapidly - it was up and running within 500 million years. But then, things stayed very small and very simple for the next few billion years - it was just microbes. Then something special happened because, about 600 million years ago, large, complex multicellular life as we know it suddenly appears in the fossil record.  But the question is: are these fossils the remains of animals, plants or some other bizarre evolutionary offshoot? No one could tell from their appearance alone. But now scientists in Australia have nailed it by achieving the incredible feat of extracting from one of these ancient fossil species called Dickinsonia the fats and cholesterols that would have been in the tissue when it was alive. So is it an animal, a vegetable, or a mineral. Chris Smith heard from Jochen Brocks...

Jochen - Dickinsonia is an oval shaped creature that was lying flat on the seafloor in relatively shallow water. It looks a little bit like a big coffee bean with lots of ribs and the smallest we analyse about 1 centimetre; the biggests 6 centimetres, but there were some true giants that came up to 1 metre 40. It is a 558 million year old creature, and the fat tells us it was the earliest animals in the record.

Chris - And they’re important, of course, because if these are big animals then they are some of the earliest big animals and effectively they’re what gave rise to the life that turned into us?

Jochen - I think those fossils are the most important fossils in the entire geological record. If you have a time machine and you go back to 580 million years ago - go scuba diving - you would need a microscope to see anything at all. Life was microscopic. And about 570 million years ago, those Ediacaran creatures appeared and they became enormous quite quickly - up to 2 metres. That’s when life became big and that’s why it’s important to know what these creatures actually were.

Chris - How did you actually decide to pursue this in terms of looking at the fats and how did you get at the fats? It’s extraordinary to think there are fats there which are more than half a billion years old.

Jochen - The idea for this project, in fact, comes from a PhD student Ilya Bobrovsky. He contacted me from Russia; he’s a Russian student, in 2013 and said “well, Jochen, I’ve found these Ediacaran fossils and they are almost mummified. They are preserved organically. I want to extract fat molecules from them and that should tell me if this creatures were or were not our earliest animals”. And I thought it was the most stupid idea I’ve ever heard. I thought it was totally crazy. But he was a very very smart student so I thought well, he should try it for himself. So I hired him as a PhD student; he extracted these things and it simply worked. It was completely stunning.

Chris - How do you get the fats out of the fossils?

Jochen - You can’t try this at home. It’s a) very dangerous and b) difficult. What Ilya does is first he drips hydrofluoric and hydrochloric acid on them so that the organic matter is lifted up from the rock underneath and then we analyse the molecules using chemical techniques.

Chris - And those fats have definitely come from the fossil? They’re the vestige of the fossil when it was in life?

Jochen - That’s right. So you could think ah, I know, if we actually touch these fossils with our fingers we would introduce cholesterol, which is the hallmark of animals. And immediately “oh look, we found an animal” but actually it’s our own fingerprints. But we went into extraordinary lengths to exclude continents and look at exactly where these molecules came from.

Modern cholesterol from us humans is a modern living molecule, but what we found is actually a fossil molecule that has changed its structure. Where we can estimate approximately how old it is and the structure of the molecule fitted perfectly the age and the maturity of the rock we found it in.

Chris - So you’re saying because we can see this slightly different form of cholesterol that is the signature of complex animal life and it’s in the context of this fossil, we think it’s come from the fossil? But could there not be, for instance, microorganisms living on the fossil that themselves made this funny form of cholesterol or other organisms that have come along since and lived around the fossil and they put the cholesterol there and you’re saying well, it’s from the fossil but it’s not it’s from something else?

Jochen - Alright. That’s a very very good question. What we found is a little bit more. We can imagine a slab of rock in the middle of this beautiful fossil. Surrounding the fossil is a fossilised microbial mats because Dickinsonia was living on the seafloor, living on these microbial mats that are full of sand, bacteria and algae, and these mats were also fossilised around the Dickinsonia.

So what we did is we analysed the molecules in Dickinsonia, but also the molecules from the mats surrounding Dickinsonia, then we compared the two. There was a huge difference. Dickinsonia was full of fossil cholesterol, which is typical for animals. And the surround was typical of a different type of molecule which is produced by green algae.

Chris - Now you have got this, you can say at this moment in time we’ve got what looks like this animal. It’s not a plant, it’s not a fungus, it’s not some of these other possibilities. How does that change our view of what was going on almost 600 million years ago and, ultimately, how that line led to us?

Jochen - It really changes the story of how we perceive our earliest animal ancestors when and how they evolved. Now that we know that Dickinsonia actually was an animal, and probably many of those Ediacarans were animals, we know that there was already an enormous animal ecosystem between 570 and 540 million years ago. But they were very peaceful animals. They were mostly vegetarians. None of these fossils have bite marks or predation marks.

And then about 540 million years ago those creatures died out and modern type animals appeared. It’s actually quite possible that the modern type animals drove those Ediacarans to extinction by simply eating them.

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