Dr Ken McNamara, Sedgwick Museum of Earth Sciences, Cambridge
Life on Earth evolved in the oceans around 4 billion years ago, but it's not life that we could see with the naked eye. The first complex life we see in the fossil record turns up around 630 million years ago, but they still don't bear much resemblance to anything we see today. Sarah spoke to Ken McNamara of the Sedgwick Museum in Cambridge, to find out more about the evolutionary experiments of the Ediacaran and Cambrian eras...
A fossilised stromatolite:
Dickinsonia, an example of the bizarre Ediacaran biota:
A reconstruction of the Burgess Shale fossil Wiwaxia:
Ken - This rather unprepossessing lump of rock doesn’t looking very exciting at all, but in terms of what it actually is, is the earliest evidence of life on earth. So it’s a lump of rock that’s about 3.5 billion years old and the strange structures are actually made by the activity of microbial communities.
You’ve got the same sort of organisms today – cyanobacteria, that we call blue-green algae – making these same sort of structures in lakes and shallow seas today. They were doing in 3.5 billion years ago.
And really that’s the only evidence of life in the oceans for a phenomenal period of time – for about 3 billion years. Then at some stage we had the evolution of more complex cells of which animals and plants are make – eukaryotic cells. But it wasn’t until you’re getting much closer to the modern day, 6, 7, 800 million years ago that you had these cells coming together producing multicellular, more complex organisms.
Sarah – So these other fossils that I can see here on the table, are they the first evidence of complex, multicellular life?
Ken – They are. You find evidence there are trace fossils, trails of who knows what made them, worm-like things. Seemingly relatively quickly just under 600 million years ago you start finding really quite large fossils, and complex-looking fossils. This fauna that’s known as the Ediacaran, well I shouldn’t say fauna, Ediacaran biota, are either round jellyfish-type things or they’re more frond-like structures. You can break them down into these two groups.
They have a resemblance to later forms but there’s something not quite right about them. I like to call them the gutless wonders, because you can’t see any sign of a mouth, any sign of an anus, no sign of a gut. So there’s this big problem: what were these things? Where they animals? If so, how did they get their nutrients? How did they feed? So the big question is, are they the ancestors of all alter animals, or are they something completely different. And that’s one of the suggestions that have been put forward a couple of times. Maybe they were a great evolutionary experiment?
Sarah – So these were pre-Cambrian. I think a lot of people will have heard of the Cambrian explosion of life where we saw this massive explosion of all different types of body forms, of complex organisms. These are earlier than that?
Ken – Yes they are. They immediately pre-date the Cambrian explosion. Then you had this amazing change around about 540 million years there’s a relatively sudden change in the nature of fossils. At this time there were probably sponges and things around but at the beginning of the Cambridge there was the relatively rapid appearance of whole groups of organisms that we’re familiar with today, like clams, worms, even very primitive early fish, early vertebrates. Much more familiar-looking organisms that had recognisable mouths, they had guts that we even have the gut contents from some of them, we know what they fed on. And so we can see that they developed this ability to feed off plants, but also to feed off other animals. So it’s a completely different change in lifestyle.
Sarah – And I believe you have some examples here in the museum of probably the most famous examples of the Cambrian explosion which are from the Burgess Shale in Canada.
Ken – Yes, we have some really good specimens here in the Sedgwick Museum because this is where a lot of the original research was done in the last 30 years. So we can go and have a look at some of those if you like.
Sarah – So what are we looking at here then?
Ken – What characterises the Burgess Shale is their soft parts are preserved. Not only that but in rocks of this age we start finding animals develop the ability to produce minerals – hard shells or spines – which is something that the Ediacaran animals just didn’t do.
The advantage of this is there’s a greater chance of it being fossilised. But surprisingly there are a lot of deposits of this age where you have the soft tissue preserved. So we can learn a lot about the whole biology of the animals.
One that I really like is this strange slug, worm like thing here. It’s called Wiwaxia and it has what looks like a whole row of steak knives sticking out of its back. You would look at them and say it’s got those because they are protective. If something tried to eat it, it would get a mouthful of those spines. It wouldn’t taste very nice. It’s also got armoured plates. This is what characterises what was going on the Cambrian explosion. We seem to have the evolution of predators and prey. Animals develop this ability to feed on other animals and this is what basically drives a lot of evolution: predation pressure. And so the predators are trying to outcompete the prey and the prey are coming up with new strategies to stop being eaten.
So Wiwaxia probably did it really quite successfully for a while.
There’s another thing here called Halceria, which is like a huge slug but it’s covered in amazing armour. It’s like medieval armour of tiny, really hard plates.
But we also have animals that are quite similar to modern day things. We have crustaceans there that really look quite like modern day crustaceans. We have a reconstruction that we’ve done of one of these.
So there are some groups of organisms that evolved at this time, that hit on the right body form and haven’t changed dramatically over a long period of time. Their interspersed with some of these weird things like Wiwaxia, with steak knives, that last for a short period of time and then became extinct.
We know there were predators around because we have examples of these and the most spectacular is this amazing thing called Anomolocaris, which is a sort of arthropod, so it’s sort of related to lobsters and crabs and so on, and they grew up to nearly a metre long. Huge claw-like structures at the front and an amazing mouth part.
And one finds lots of other fossils of things like trilobites, segments animals that are now extinct, but with a big armoured head, and many of these you find with bits broken off them, bite marks, and they’ve survived and they’ve re-grown part of the bite mark. So there was a lot of predation going on. Sometimes it was successful, sometimes it wasn’t successful.
So as I said, this predation pressure which is driving a lot of evolution, and that’s what really marks the beginning of the Cambrian explosion, and that’s then continued on to the present day.
Sarah – So this is the first real example of things that we might recognise today but also some pretty wacky stuff as well, that perhaps wasn’t as successful. It’s really an example of evolution in action – things that have been successful and things that haven’t?
Ken – That’s right. It’s really what you’d expect. Early on you’d have these evolutionary experiments, some will work, some won’t.
There’s another one that’s not shown here called Opabinia, which is a bizarre animals that had 5 huge eyes on its head, and this long thing coming out the front that looked like a vacuum cleaner.
So there were some really quite bizarre experiments that didn’t work. But a lot of standard ones that did and so we have these crustacean-like animals that are quite similar to things that are around today.
Sarah – and I believe also some things that are perhaps related very distantly to ourselves, as well?
Ken – Indeed. It’s become realised in relatively recent times both in this deposit but also in other similar deposits, there’s one in China, slightly older, called the Chengjiang fauna, there are worm-like animals but they have distinctive structures in them, particularly a notochord, precursor of a neural spine, that do indicate that chordates, that vertebrates – part of – were around at this time. So these were the earliest ancestral type fish, which evolved into amphibians, reptiles and us. So our long-lost ancestors were lurking around in these muds in Cambrian times.
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Sedgwick Museum of Earth Sciences, Cambridge University