The Burgess Shale

Environmental Explanations

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.

Oxygen

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.

Glaciation

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.

Developmental Explanations

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.

Ecological Explanations

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.