Earth's first animals

Alex Liu explains what the fossil record tells us about how Earth's first animals developed from bacteria...
06 May 2014

Interview with 

Alex Liu, University of Cambridge


So Alex, tell us a bit about your work and how life began. How come we study that?

Alex - Well, in terms of when life began, we're looking around 3.8 billion years ago. So, the Earth is around 4.6 billion years ago and for almost a billion years, there's no evidence in the fossil record of any life at all on the planet. But as around 3.8, we get the first evidence of fossilised cells. So, things looking like modern bacteria. It seems that for the next almost 3 billion years, we don't get any fossils of anything that's bigger than those bacteria-sized organisms. Slowly, they become a little bit more complex but generally, there's nothing at all. It's only in the period I particularly look at around 500 to 600 million years ago that we see the first large fossils - things that you could see without the use of a microscope. The earliest ones are called the Ediacaran biota because they come from the Ediacaran period which is very recently named the same as the Jurassic called the cretaceous. It's very similar.

Chris - What about when you say, 'life got started after a billion years or so' but there's not really any fossil record of it. How do we know life got started 3.9 or 4 billion years ago?

Alex - Well, it could've got started earlier and we're just not seeing the record of it. We say that it started around 3.8 because that is the time where we see the first fossils of these bacterial cells.

Chris - Well, you can see fossilised bacteria.

Alex - Yes.

Chris - What do they look like? I mean obviously, bacteria presumably, but what do they actually look like? Where do you find them?

Alex - The oldest ones are in Greenland and in western Australia. The two main forms are either small rod-shaped things like drain pipes, but on a very microscopic scale or ball-shaped spheres.

Chris - How do we know that they're microorganisms though and not just say rock formations?

Alex - The way we look at this is to look at the chemistry of what they're actually made of and the chemistry of the surrounding rock as well. And so, a real cell, say, will have been made of carbon or at least that have had some carbon within them and biological carbon has a very distinct signature that we can see that it is very different to any carbon that is formed non-biologically. And so, if we look at the chemistry of the carbon we see that makes up these fossils, we can determine whether they are really biological or whether they're not.

Chris - Any questions let's come to you. What's your name?

Liz - My name is Liz and I'm from Longstanton. My question is, you must go looking on purpose to find fossils that small because you wouldn't possibly find them by accident. So, do people look everywhere? Why would they look in Western Australia or in Greenland?

Alex - There's only certain places around the world that have rock types of particularly ages. Generally, the older the rocks get, the more they've been subjected to pressure and temperature. And therefore, the original fossils that you might see in them and the chemical signatures you might get from them, they become less reliable the further you go back. And so, when we're looking for rocks about 3.8 billion years old, most of the ones that we find on the planet are either buried under other rocks or igneous or metamorphic, so they're not suitable environments, they're not places where the organisms would've lived in the first place. So, we need very specific sedimentary rocks say, sands and mudstones, and things like that. And we also need them to have not being crushed or heated, to the extent that all the fossils are lost. There's very few places in the world that that happens which is why people focus on let's say, northern Greenland. And they will go and look specifically for evidence of fossils in those regions.

Chris - Any other questions?

Neil - Hi. I'm Neil from Cambridge. So, when you find these, how do you actually know how old they are?

Alex - So, in rock sequences, the rocks are all made up of different types of minerals and there's a particular mineral called zircon which isn't a very large component of rocks, but they're very, very useful because they form in volcanoes generally. When the volcanoes erupt, these zircons are deposited in the ash. But as they crystallise and go from a liquid state to a solid state, they trap within them various elements. And so, uranium is one that is regularly trapped. But uranium has various states or isotopes that decay. And so, we know the rate at which they decay and so, each of these zircons is like a tiny clock. If we can measure the amount of uranium in those zircons compared to the amount of lead which is what the uranium decays to and we know exactly how long it takes for that ratio to occur, we can measure lots of different zircons from around the fossils and compare all of those ages. We should get a very definite single one that dates the rocks.

Chris - So, how do you then turn those tiny microorganisms into the big life that then you have beautiful examples of in front of you? Tell us about these.


Alex - The microfossils as I said for about 3 billion years, we don't see very much else other than these bacterial forms. Suddenly, around 700 million years ago, you get the first evidence of things that were getting slightly larger. There's lots of different ideas for why this might have taken place - a rise in oxygen levels is one big idea. There's also various glaciation events that may have covered the whole planet in ice for tens of millions of years at that time at about this time and they may have played a role in preparing the planet for larger organisms, making it more habitable for them. And so, those organisms, very rapidly diversify into lots of different forms. But they're very unusual and although we can tell that they're probably complex organisms, we can't say for certain that they're animals because they don't look anything like any modern animal that we know of. I've got a few here. A lot of these fossils resemble leaves or fern fronds. So, this is one from Leicestershire in Charnwood Forest - if any of you are familiar with that - and it's called Charnia. It's one of the earliest macrofossils, large fossils the first to be found from rocks of this age and secondly, to have actually lived on Earth at any point.

Chris - How do you know it's an animal and not a plant?

Alex - We don't. That is, the main focus of my research is trying to find out exactly what these creatures were. They could've been fungal. They could've been animals. They probably weren't plants just because of the environments that we find them in. So, these are all found in marine rocks, marine sediments that were laid down under the sea at a depth of around a kilometre or so. The important thing about that is that light can't penetrate that deep into the oceans. So, there's no way these could've photosynthesised in the way that modern plants do.

Chris - Kate...

Kate - I've got a question that's come in Facebook that's related to that and Jo has asked, "Is it frustrating studying creatures that you'll never get to see?"

Alex - Slightly, it does mean that you'll never be able to know whether you are absolutely right about the ideas that you've come up with. But it also does mean that no one can prove you wrong very easily. That does help.

Chris - What's your name?

Meluka - Meluka and I'm from Cambridge. My question is, with the microorganisms that you find, roughly how big are they - are they tiny or can you see them with the naked eye?

Chris - Are they the same sort of sizes we see microorganisms today or are they much smaller in history? How do they relate?

Alex - They're comparable to microbes that we find today. If you think of a millimetre, typically, bacterial cells, you're looking at hundredth to a thousandth of a millimetre in length. And so, you can't see them with your naked eye. You'd need a microscope. Luckily, universities have lots of microscopes and say it's very, very easy for us to study them if we prepare them in the right way.

Chris - There's a big jump therefore between turning from something that is a thousandth of a millimetre across to these specimens you've got in front of you which are roughly the size of your hand. So, how do you think that happened?

Alex - That's one of the biggest questions in palaeontology and it's very difficult to work out. It's the switch from organisms being single-cells to becoming multicellular and grouping together, not only to just survive as a colony, but to actually start to differentiate all of the tissues to do certain roles. So some of the cells will start feeding, some cells will be just for respiring or breathing and then that's a very complex biological problem. It is really the biologists who are looking into that rather than the palaeontologists because we can't see changes on that scale from the fossils that we can find in the fossil record. The biologists are the only ones who can actually look at how those processes might be taking place.

Niko - My name is Nikko and I'm from Longstanton. My question is, do you know how long the creatures lived?

Alex:: We know that they lasted as a species for millions of years, but each individual one, we don't know at all. Some of Ediacaran fossils I look at, the way that they're preserved is underneath volcanic ashes. So, we know exactly what they look like when they died because it's almost like Pompeii. You had a big volcanic eruption that's completely smothered lots of living organisms all together on a seafloor. And so, we get this replica of the communities and we can see what the whole seafloor looks like and all the organisms on it. But we can't tell how old any of those individual organisms were.

Chris - Any other questions?

Arushon - Hello. My name is Arusha and I'm from Cambridge. My question is, what you research is fascinating. Is it useful?

Chris - Yeah. So a taxpayer is saying, "Look! I'm paying for this."

Alex - Yes. Firstly, it is a very curiosity-based project - I have to admit - in terms of, we all want to know as humans where we come from. Some people look to religious sites for guidance on how that might have happened. As a scientist, I look for how we might have evolved and how life might have evolved. The key questions I look at is some of the more fundamental ones in explaining the diversity of the planet we see around us. But you're right. In the end, it is just a curiosity-driven question. The thing that we do that does help society - one of the things - is the techniques we need to look at some of these fossils really are pushing the boundaries of science. The machinery and the equipment is often developed in scientific labs, looking at questions like these that then gets farmed out into companies and turned into beneficial equipment for the medical industry or even to end up in your own home. So, indirectly science as a whole actually, it is not just curiosity-driven. Often, that's what gets us started on some of the questions, but it can lead to a very useful societal impacts.


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