The Cambrian Explosion: a mystery solved

We have seen that the sudden appearance of complex marine life in the Cambrian is acknowledged to be perhaps the most striking and most perplexing phenomenon in the whole fossil record. Another authority has described it as follows:

At the beginning of the Cambrian Period, within a span of a mere 10 million years, all the major groups of complex animal life – all the phyla – appeared. Ten million years may seem like a vast stretch of time: by most criteria it is a lot of time. But consider that nearly 3 billion years had already gone by since life had left its first traces in the fossil record. And consider, too, that no new phyla are known to have originated since the early Cambrian.

Here again we find a familiar pattern – on a truly grand scale: relatively suddenly, the whole spectrum of invertebrate life – including sponges, brachiopods, arthropods (trilobites, chelicerates and crustaceans), mollusks, plus spineless chordates in the same phylum as the vertebrates – burst on the scene, the world over. By the end of the Cambrian we have records for all the major groups of hard-shelled invertebrate organisms – and some evidence that vertebrates had appeared as well.

… What could have caused such a proliferation?

Niles Eldredge, Fossils: The Evolution and Extinction of Species, 1991 (p 189).

Eldredge suspects the key factor was the continuing rise of atmospheric oxygen. The level of oxygen that was dissolved in sea water

passed a critical threshold, creating an exploitable environment – and evolution produced its closest possible approximation of an immediate response.
    Opportunity knocks, the door is opened and life proliferates into forms that have never existed before.

In the introduction to this section (Recolonisation in a nutshell) it was suggested that one of four possible ways of interpreting the fossil record was to suppose that the start of every new evolutionary lineage represented a fresh act of creation. Really this is what we are given here, except that the agent whose magical powers initiated these fresh acts of creation is called Evolution rather than God. It is a ‘Hey presto!’ kind of explanation, with ‘evolution’ the shibboleth that makes it sound naturalistic and scientific.

The new approach

Maybe the Cambrian Explosion cannot be explained without such an agent, but if so, it seems better to restrict his acts of creation to the beginning of things, and to try to explain subsequent history in purely natural terms. Is there, in fact, a better explanation?

During the Archaean magma welling up from below replaced the land that was shattered in the Hadean and plate tectonics brought about the replacement of the seafloor. Since no pristine land or seafloor remains, it is an open question what kind of world existed before the Hadean. The prehistoric tradition of Genesis indicates that complex marine and terrestrial life already existed – that there existed an antediluvian world which was already fully created.

Bombardment by asteroids would have devastated the seas and released enormous amounts of energy, heating some almost to boiling point. Except for bacteria, marine life would have survived only at the poles and in deep water. Recolonisation of the seas nearer land would therefore not have been possible until:

• the seas had cooled sufficiently
• marine oxygen levels had recovered, as a result of the seas cooling
• animal stocks had begun to recover and spread out from their cold-water refuges
• there were sufficient nutrients in near-shore environments

These conditions developed in the course of the Proterozoic. For most of that time almost the only signs of life on the seafloor were stromatolites – layered mats formed by microbes, which as the seas became more oxygenated became more common and more diverse. Higher in the water column plankton and microplankton were also beginning to recover.

Then towards the end of the Proterozoic a strange array of soft-bodied organisms known as the Ediacaran fauna appeared. Some of them were fixed to the seafloor by holdfasts, like modern seapens, and, where fossilised, were commonly buried in life position. They occur at this point in the fossil record in many parts of the world, including Charnwood Forest, England. None seems to have been ancestral to later, more complex organisms, and most forms became extinct before the Cambrian. Then, at the very end of the Proterozoic, worms began penetrating the sediment, leaving the first trace fossils in the form of horizontal meanderings and vertical burrows.

In seeking to understand what led to the Cambrian Explosion, palaeontologists speak of the ‘Cambrian substrate revolution’, in which barren seafloors (substrates) were turned into habitable space for almost the full range of seafloor-dwelling organisms. Worms, molluscs, sponges, corals, echinoderms, trilobites, crustaceans were benefiting from the recovery of marine animal life further down the food chain, as increasing amounts of organic matter from algae and cyanobacteria were processed by zooplankton, accumulated on the seafloor and attracted other microbes. The microbes, in turn, attracted grazers and burrowers to churn up the sediment, aerate it and fertilise it, so that it became possible for organisms to live in, and feed off, considerable depths of sediment. A positive feedback loop was initiated, limited only by the range of organisms in existence to take advantage of the new opportunities.

Instead of supposing that the organisms which brought about this revolution all evolved into existence, unseen, the simpler explanation is to suppose that they pre-existed. They appeared successively in the fossil record – cyanobacteria first, then zooplankton, then worms, then other seafloor-dwellers – because those higher up the food chain depended on those lower down and could not reproduce themselves as quickly as those lower down.

Thus it was not that the earliest-appearing organisms just happened to be those which ever afterwards, right up to the present day, would occupy places near the bottom of the food chain. Food chains and evolutionary chains are quite different things. Food chains are composed of disparate kinds of organism. There is no evidence – or likelihood – that zooplankton evolved from bacteria, or that burrowing animals (themselves a very heterogeneous group) evolved from zooplankton.

Darwin’s theory of evolution requires the evidence of ‘numerous, fine, intermediate fossil links’. He imagined that in the vast ages before the Cambrian the world must have ’swarmed’ with living creatures. What we find, however, is revolution, not evolution: an explosion of life forms as shelves and platforms began to be colonised by recovering populations from far out to sea. Starting with bacteria and climaxing with sharks, it was an ecological progression – something that occurred over thousands of years, not three thousand million.

The Cambrian period was also associated with large-scale transgression, when rising sea levels caused much of the land to be submerged. The marine communities which we know about occur on the flooded continents, not ocean crust. But conditions around the ever-changing coasts did not suit all types of organism. Many remained ‘behind the scenes’ until their appearance later in the Palaeozoic and in the Mesozoic.