Life’s big bang?

by Harriet Drage and Scott Billings

You may have heard of the Cambrian Explosion, an ‘event’, starting roughly 540 million years ago, when all the major animal groups suddenly appear in the fossil record, an apparent explosion of life and evolution.

But was there really an evolutionary explosion of all these animal groups, or is the lack of evidence from earlier periods due to some peculiarity of the fossilisation process? The debate has rumbled on for a number of years.

Now, a new study from our research team, the University of Oxford’s Department of Zoology, and the University of Lausanne, claims that the early Cambrian saw the origins and evolution of the largest and most important animal group on Earth – the euarthropods – in a paper which challenges two major pictures of animal evolution.

Euarthropoda contains the insects, crustaceans, spiders, trilobites, and a huge diversity of other forms alive and extinct. They comprise over 80 percent of all animal species on the planet and are key components of all of Earth’s ecosystems, making them the most important group since the dawn of animals over 500 million years ago.

Exceptionally preserved soft-bodied fossils of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis from the Burgess Shale, Canada. Top left: Frontal appendage showing segmentation similar to modern-day euarthropods. Bottom right: Full body specimen showing one pair of frontal appendages (white arrows) and mouthparts consisting of plates with teeth (black arrow) on the head. Images: A. Daley.

A team based at the museum, and now at Lausanne, conducted the most comprehensive fossil analysis ever undertaken on early euarthropods, to try and establish whether these animals really did emerge in the early Cambrian period, or whether fossilisation just didn’t occur in any earlier periods.

In an article published today in the Proceedings of the National Academy of Sciences they show that, taken together, the total fossil record does show a gradual radiation of euarthropods during the early Cambrian, 540-500 million years ago, challenging other ideas that suggest either a rapid explosion of forms, or a much slower evolution that has not been preserved in the fossil record.

Each of the major types of fossil evidence has its limitation and they are incomplete in different ways, but when taken together they are mutually illuminating
Professor Allison Daley

Reconstruction of the Cambrian predator and stem-lineage euarthropod Anomalocaris canadensis, based on fossils from the Burgess Shale, Canada. Reconstruction by Natalia Patkiewicz.

By looking at a huge range of fossil material the researchers ruled out the possibility that Pre-Cambrian rocks older than around 541 million years would not have preserved early euarthropods. The only plausible explanation left is that the origins of this huge animal group didn’t evolve until about 540 million years ago, an estimate which also matches the most recent molecular dating.

The timing of the origin of Euarthropoda is very important as it affects how we view and interpret the evolution of the group and its effects on the planet. By working out which groups developed first we can trace the evolution of physical characteristics, such as limbs.

Exploring all the evidence like this allows us to make an informed estimate about the origins of key animal groups, leading to a better understanding of the evolution of early life on Earth.

Model of the Cambrian stem lineage euarthropod Peytoia, based on fossils from the Burgess Shale. Top left: Closeup of the mouth parts and frontal appendages. Bottom right: Overall view of the body. Model and image: E. Horn.

Death, decay and fossilization

By Duncan Murdock, Research Fellow

Our oceans, rivers and lakes teem with life of all shapes and sizes, and have done so for hundreds of millions of years. We can get a glimpse of the wonderful diversity of life deep in the Earth’s past from fossils that can be found in the rocks beneath our feet. But the fossil record is as much a history of death as it is of life.

All animals die, in huge numbers every day, but the sea beds and forest floors of the Earth are not filling up with their remains. Decay is as inevitable as death. This is good news for those left behind, but bad news for fossil hunters.

Being preserved as a fossil is very much the exception, not the rule, and the chances of anything surviving the various processes by which the component parts of an animal are lost forever are vanishingly small, even for hard parts like shells, teeth and bones. For the ‘soft’ parts of animals, such as the muscles, eyes, guts and nerves, it is nearly impossible.

But ‘nearly impossible’ is good enough when you can consider every animal that ever lived, or more importantly, died. In exceptional circumstances ‘soft’ tissues do become fossils, and when they do they invariably give an unrivalled view of an otherwise completely lost world.

Although exceptionally well preserved, this fossil of a jawless fish is not entirely complete. Some features have been preserved, like the prominent dark eye spot and gill supports just beneath, but others, such as the guts and fins, have rotted away before they could be preserved. Image: Mark Purnell, Sarah Gabbott, Robert Sansom (University of Leicester)

We know from these exceptional fossils that the path from death to fossil is not random. Yes, you have to be lucky, but the odds are very much stacked towards certain combinations of who, what, where and when.

Furthermore, decay is not the whole story. Not only does anatomical information have to survive decay, it has to undergo parallel (but distinct) processes of preservation – conversion into materials that are stable over millions of years as part of a rock. It is the balance between the loss and retention of information that seals the fate of an organism’s remains.

Three hundred million years ago, a small worm gulped its last breath and died. Its body began to rot and, were it not for the peculiar conditions of the sediments it was laid to rest in, would have been lost forever. Fortunately for us, what remained was preserved in rock – a rotten fossil. But how much rotted away before it was fossilized? By decaying modern relatives in the lab we can model this missing history, and build better-informed reconstructions of extinct animals. Image: Duncan Murdock

Left with only the lucky few, the parts of animals where retention exceeded loss, the fossil record is profoundly biased. One way to unravel this lost history of loss is it to conduct experiments, replicating decay and preservation. However, trying to make fossils in the lab, by contriving one particular set of conditions, is fiendishly complex – there are simply too many variables to set.

I have been working with researchers from the Universities of Leicester, Bristol, Manchester and University College Cork, and together we have described an alternative approach to unpack the ‘black box’ of fossilization and take each variable in turn, individually examining the different processes that result in retaining information as potential ‘fossils’ and, crucially, those that result in loss.

This cartoon illustrates the difference between experiments that attempt to replicate fossilization, treating the process as a black box, and the approach we are taking. The black box approach reveals little about the processes of information loss and information retention, the cumulative effects and interactions which ultimately results in a fossil (or, more often, not). Image: Purnell et al. 2018.

Ultimately this approach will allow more and more complex experiments to be designed, to unpick the interactions between the who, what, where and when in the lost history of death.

The techniques described here are published in Palaeontology today as ‘Experimental Analysis of Soft-Tissue Fossilization: Opening the Black Box‘, Purnell et al. 2018.