Earliest animals could have driven Cambrian explosion

During the Cambrian explosion, roughly 530 million years ago, in a very short time we went from a small number of animal species to nearly every body plan that exists today. It could well be the most important period in the history of animal life. So how did it happen? To find out, scientists have modelled the habits of the animals that existed in the period just before this explosion, known as the Ediacaran, and, incredibly, the activity of these first animal life forms may have helped set the conditions necessary for this explosion of life to occur. To find out how, our colleague Will Tingle went to meet Emily Mitchell who is the curator of the Cambridge University Zoology museum’s non-insect invertebrate gallery…

Emily - The animals in the study are some of the very first animals to have ever existed. So they predate the ability to move around, they predate predation, they can't swim. They lived attached to the sea floor, upright in the water column, feeding on nutrients in the water a little bit like sponges or corals.

Will - How do we know this then?

Emily - We have the most amazing fossil record for the Ediacaran in terms of preservation. We have what's known as exceptional preservation because what we have is hundreds of square metres of fossils preserved under volcanic ash. So it's a little bit like Pompeii in that this underwater ash came down and smothered them all where they were living.

Will - It seems to be quite the anomaly when it comes to being able to study the fossil record because, in general, let's call it what it is, it's a bit rubbish. But it sounds like you've hit the jackpot, as it were, being able to look at these with such fantastic detail.

Emily - I'm not sure I'd call the fossil record rubbish, but I do think that the preservation in the Ediacaran is really, really special. What's particularly interesting about it and useful for it is, because we have this census of Ediacaran life at the time, we can use all sorts of techniques that normally are only really applicable today where we have these full communities and apply them back to the Ediacaran and work out what's going on within the communities and with evolution.

Will - So how have you done it then? How have you gone from admittedly very well preserved, very old fossils, to be able to say that these were similar to sponges, you can almost talk about how they lived their lives?

Emily - Something that I've used that we've utilised within this particular study is looking at how the fossils are positioned on the rock surface. So, because they're immobile and because they're preserved where they lived, the position of the fossil on the rock surface captures all their life history traits: how they got there, how they reproduced, how they interacted with their local environment and how they interacted with each other. So then we can use different sorts of spatial statistics to analyse these patterns. By comparing to different sorts of known reproductive competitive environmental patterns today, we can work out what's driving their patterns and processes. That's relevant in terms of this particular study because we used another technique, which is known as computational fluid dynamics, coupled these spatial patterns to reconstruct the communities.

Will - Now we come to the interesting bit, which is that, as you say, these organisms are churning up the sediment and the fluids around them. What kind of effect is that having on the wider ecosystem?

Emily - So this is really interesting to think about. Before the Ediacaran, the oceans were what's known as rather stratified in that, nowadays, you have very well mixed oceans, whereas back before the Ediacaran it was very, very stratified. You had layers of oxygenated water and layers of anoxic - water without oxygen - and so what these organisms may have potentially been able to do is start to mix up the water. So, beyond their immediate environment, the water was starting to mix and mix and mix, potentially helping to spread the nutrients round to a greater range of areas.

Will - And the big question, then, as you've alluded to this whole time, is that obviously this is the point just before the Cambrian explosion, just before the biggest, perhaps the most important radiation of life on earth. We've now realised that we've got these organisms being able to churn out all these nutrients, destratify the oceans. Do you think there's a link between these organisms and then suddenly this huge burst of life?

Emily - What I think is going on is you have many different processes, potentially, during the Ediacaran, and they all contribute to the radiation we see in the Cambrian. And so things like mixing the water column are the first time organisms have started to have a really strong impact on their local environment beyond just themselves. We think of microbial colonies, it's not really going to affect the large area. Then, through time and throughout the Ediacaran, the animal's ability to impact their local environment changes. So it starts with this potential increase of vertical mixing of the water, then, as they start to move, they start to be able to churn up the sediment, start mixing the substrate, the seafloor beneath them. Then, just at the end of the Ediacaran, before the Cambrian, you start to see biomineralisation and reefs, they start building hard parts. Then, they have the ability to impact their local environment, not only during their lives, but further. All these different processes and these different ways that animals are changing their environment and potentially making it easier for other organisms to live, potentially creating new niches and new potential for new organisms and animals to live in.

Will - Not to be dramatic then, but all of life on Earth has some strange looking cabbages 550 million years ago to thank for existing?

Emily - I think that might be taking it a bit far, if I'm honest, but life on earth, most of life on Earth has been microbial and is microbial. So life has existed on Earth for, you know, pretty much 4 billion years, but it's only around 600 million years ago that we start seeing complex life in animals. So bacterial life would continue very happily. But in terms of other animals, I think that these were very much the first animals and how they lived and how they impacted the environment undoubtedly would have impacted the rest of early animal evolution.