Before complex animals could evolve on Earth, there had to be an environment favourable for their survival. Researchers have examined a number of environmental factors that might have been instrumental in the evolution of new body plans, but the two strongest contenders are a rise in oxygen levels and the end of extreme glacial conditions.
Multicellular animals use oxygen to fuel their metabolism. At low oxygen levels, they don’t function well … without it, they cannot survive. Photosynthesis could have caused a rise in the amount of oxygen in the seas and atmosphere near the beginning of the Cambrian, allowing the evolution of larger, more complex animals with respiratory and circulatory systems.
However, there does not seem to be much variation in oxygen levels across the Ediacaran-Cambrian boundary. Earlier increases might have triggered the evolution of large Ediacaran metazoans prior to the explosion, and a subsequent post-explosion rise in oxygen levels may have allowed animals to adopt more active, energy-intensive lifestyles such as swimming and hunting.
Another possible environmental explanation for why the explosion occurred when it did involves glaciers. Some researchers have suggested the entire Earth was covered with ice before the Cambrian explosion. (This is known as the “Snowball Earth” hypothesis.)
The ice would have limited the number of evolutionary niches for life in the sea, and blocked most of the sunlight on which cyanobacterial mats and algae depend. But once the glaciers receded, huge expanses would suddenly be opened for life: an ideal situation for experiments on different body plans. Unfortunately for the hypothesis, the last worldwide glaciation seems to have ended around 635 million years ago – nearly 90 million years before the first signs of the Cambrian explosion in the fossil record (which was followed by another major regional glaciation around 580 million years ago). Even if there is no direct triggering link between Precambrian glaciations and the Cambrian explosion, the post-glacial period was a crucial time in evolution. The appearance of the first large and complex multicellular organisms shortly after the return to a warmer global climate suggests that environmental conditions had become ripe for them to evolve.
Some scientists have argued there was nothing in the environment during the Precambrian-Cambrian transition that was particularly unique. For these researchers, the answer to the question “why did the Cambrian explosion take place when it did?” can be found within the organisms living at the time. According to this approach, life first had to evolve the ability to develop new and diverse body plans.
Developmental genes in animals regulate how and when other genes operate to “build” the organism through its earliest life stages. Many important developmental genes are shared between widely-divergent animal groups. They are so closely shared that control genes from a lab mouse work perfectly well in a fruit fly. This conservation means those control genes must also have been present in the last common ancestor to both the fruit fly and the mouse. Very small changes in developmental genes can have a surprisingly large impact on the resulting organism. For instance, changes to the so-called hox genes in fruit flies can cause a fly to sprout an extra set of wings, or to grow legs where the antennae should be. From this, it could be argued that the fuse setting off the Cambrian explosion may have been ignited when the genome in the ancestor of all modern animals reached a level of complexity (including the evolution of hox-like developmental genes) sufficient to create radically new body plans. This would have provided more raw material for natural selection to act upon.
Such genomic changes might have been in the making long before the Cambrian, perhaps giving rise to the Ediacarans through a novel developmental pathway that was never repeated after their extinction. Following the Cambrian explosion and the evolution of the body plans that we know today, large-scale developmental change seems to have been “locked” into place, and no new body plans have appeared since.
The explanation of the Cambrian explosion may lie not in the wider environment, or in the controlling genes, but in the complex ecological interactions between animals. The explosion may be a result of co-evolution, with different inhabitants of the Cambrian ecosystem being “pushed” to evolve by changes within that ecosystem.
For example, the emergence of predators might have stimulated the evolution of skeletalization (including mineralized plates) for protection, or swimming as a means of escape. Before predation became widespread, early “experiments” in different body plans could have briefly thrived because species interactions were probably more limited.
Moving into previously-unexploited environments would allow even a poorly-adapted animal to survive, perhaps with one of the “exotic” body plans seen in the Cambrian. New ecological niches – particular spaces in the ecosystem occupied by species – would have been created by organisms interacting with the environment.
Examples might include the evolution of zooplankton making organic material available for bottom dwellers, or the evolution of burrowing making new interactions with the sediment possible. But over tens of millions of years, as more organisms moved into these niches, the resulting competition would remove the less-fit body plans, leaving only the ancestors of today’s phyla.
In 1990, noted palaeontologist Stephen Jay Gould spoke at the Royal Ontario Museum about the fossils of the Burgess Shale. While many of Gould’s interpretations have been challenged, his talk provides a snapshot of how the organisms were viewed then. (6:20)
So this is Marrella. I should say that arthropods are classified primarily by numbers of segments and patterns in their various body parts.
And here’s Marrella, it’s an arthropod that doesn’t fit into any group. It has these two sets of spines… there it is. It doesn’t have any allegiance.
So Whittington was puzzled when he first published on Marrella in 1971 but he went on and the next creature he studied was Yohoia.
Looked like a shrimp, had been called one by Walcott, and again, as Whittington studied it with care, it just didn’t fit into any modern group. It looks like a shrimp superficially, but when you start counting the segments you don’t have anything like the crustacean body plan.
For instance, up in the head you have this unique set of frontal appendages which have no homologue anywhere else in the arthropods. Whittington ended up calling them simply “the great appendages” because he didn’t know what to do with them.
This is Odaraia, a creature that swims on its back and has a tail fluke that looks more like a whale than an arthropod, but again, not allied to anything.
Looked vaguely like a swimming crustacean, but isn’t when you look at the segments and their patterns of the tail.
This is Sidneyia, which was described by Walcott as a chelicerate, that is a member of the horseshoe crab, eventually the spider-scorpion group. And in some superficial sense that’s what it looks like. But in detail it isn’t.
All chelicerates have six pairs of appendages on their head. Sidneyia has one pair. It’s not like anything… just these antennae… it’s not like anything else… it is just is what it is.
This is Habelia, an odd creature…
… with tubercules all over its body.
This is Leanchoilia, my personal favourite for elegance, but not among the survivors.
Again, these odd great appendages, as Whittington calls them, with their whiplash endings.
This is Aysheaia.
Now, this creature is probably an onychophore, that is it is a member of a modern group symbolized by the genus with the wonderful name Peripatus, which is a not very well known group, but it’s thought to be possibly intermediary between annelids and arthropods and may be the ancestor of the insect group. So here we may have a creature that is truly related to one of the surviving groups of arthropods.
And here is a form that Des Collins found and initially gave a field name, following paleontological tradition…
… he called it “Santa Claws”. And eventually named it Sanctacaris, which means much the same thing. Now again, does it look any different than the ones I just showed you?
Would you have picked out this creature for success? Could you have predicted that this, by virtue of superiority would go on? Yet it looks as though Sanctacaris really is a chelicerate.
There are six pairs of appendages in the right place on the head so this animal may be at least a cousin to one of the successful lineages. Again, would you have known? Could anyone have known?
This is Opabinia. Opabinia, I think, should stand as one of the great moments in the history of human knowledge.
Because Opabinia, which was described as an arthropod, a shrimp-like creature, by Walcott, who shoehorned it into modern groups as he always did. Opabinia was the first creature re-studied by Whittington that broke the conceptual dam, so to speak, and gave insights into this new world.
Because Whittington began his studies in the early 1970s on Opabinia thinking it would be an arthropod. He realizes, as Walcott did not, that there was some three-dimensionality in these creatures, that they were not just films on the rock.
That he could therefore dissect through and find structures underneath. So he said “Now I can resolve this, I’ll dissect through the body and find the appendages underneath which will prove its arthropod nature. He dissected through and he found nothing. There are no appendages.
And as he reconstructed Opabinia, he came to understand it is not an arthropod, it is some bizarre creature of its own unique anatomy. And in publishing a monograph on Opabinia in 1975 I think you have the breakthrough point in the new interpretation of the Burgess Shale.
Here is Marianne’s picture of Opabinia, a bizarre creature with five-count them, five-eyes, this vacuum-cleaner like nozzle with a food-collecting device in front, this bellows-like apparatus behind, followed by a tail. I don’t know what it is. It’s just weird.
This is Nectocaris, a peculiar creature that looks like a chordate behind, combined with a fin ray…
… and more like an octopod in the front. Who knows?
This is Dinomischus, a peculiar, stalked, stemmed creature…
… with no known affinity to anything else.
This is Odontogriphus, literally meaning “the toothed mystery” a good name.
A flat, gelatinous, annulated creature with a row of tooth-like structures surrounding a mouth and a pair of sensory palps.
Walcott described three separate genera which he allocated, as was his wont, according to the shoehorn, into three conventional groups.
This animal he called a jellyfish and called Peytoia.
This creature he called a sea cucumber and called Laggania.
And this, which had been described before and looks like the body of an arthropod, he called (it had been named before) Anomalocaris, meaning “the odd shrimp”. Well I think that you’ve guessed it already.
It turns out that all three go together. They form a single creature which is one of the weirdest of all the odd animals of the Burgess.
It’s also the largest Cambrian organism. Some specimens are almost a metre in length.
The so-called jellyfish is the mouth of this creature, working on a circular, nutcracker principle rather than the jaw of vertebrates principle.
The Anomalocaris itself turns out to be one of a pair of feeding appendages, and the so-called sea cucumber is the body of the whole animal.