The sudden appearance of complex marine life in the Cambrian is perhaps the most striking and most perplexing phenomenon in the whole fossil record. One authority describes 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 that 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 (A record of Earth’s recolonisation) we 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 Eldredge offers here, except that the agent whose magical powers initiated these fresh acts of creation is called Evolution rather than God: a ‘Hey presto!’ kind of explanation, with ‘evolution’ the shibboleth that makes it sound naturalistic and scientific.

The new approach

It could be, of course, that the Cambrian Explosion cannot be explained without such an agent – that is, after all, what the sudden appearance of so many different types of organism implies – but if so, the last thing we should be doing is confusing natural causes with supernatural ones. The Cambrian period is not the beginning of things, but comes after a long stretch of time when other forms of life, predominantly bacteria, algae and ‘acritarchs’ (unidentifiable plankton), already existed. Although they did not appear until the Cambrian, sponges, brachiopods, branchiopods, gastropods, myriapods, trilobites, chelicerates, crustaceans, molluscs, cnidarians, comb jellies, echinoderms, graptoloids, flatworms, roundworms, segmented worms, radiolarians, chordates and many more phyla that do not fit into any modern group must have existed long before the Cambrian. Thus the scientific approach is to restrict all acts of creation to the beginning of things, and try to explain subsequent history in purely natural terms. Can, then, a better explanation be found?

During the Archaean, magma welling up from below replaced the land shattered at the end of the Hadean. Since no pristine land from that time remains (extant ocean floors are much younger), it is an open question what kind of world existed before the Hadean. Complex marine and terrestrial life might already have existed.

In The antediluvian world it is argued that the seas created on the third day of creation were primarily underground seas. There may have been lakes at ground level, but since all surfaces were destroyed in the Cataclysm (many craters on the Moon, remember, exceed 300 km in diameter) the only aquatic organisms that could have survived would have been organisms used to living in the dark. Bear in mind that a great variety of creatures still live in the unlit bathyal zone (below 200 m), many retreating into the depths during the day and rising into the moonlit shallows at night. In view of the failure of such life to make any mark on the Precambrian record, we should probably be visualising organisms that were geographically restricted, mostly small and lacking calcitic shells.

Sea temperatures in the Archaean were high – some estimates have them averaging in excess of 60 °C. Except for bacteria, organisms could not have survived close to the actively erupting spreading centres (the mid ocean ridges) but are likely to have been confined to the poles and deep-water regions from removed from the spreading centres. Recolonisation of the seas nearer land, as it emerged, would 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. As the seas became more oxygenated, these 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, from the Ediacara Hills of Australia to 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. The story is one of colonisation rather than evolution. Appearing as if from nowhere, worms, molluscs, sponges, trilobites, crustaceans were all benefiting from the recovery of marine animal life further down the food chain, as increasing amounts of organic matter from algae, cyanobacteria and phytoplankton were processed by zooplankton, fell to the seafloor and attracted other microbes. The microbes, in turn, attracted grazing and burrowing animals, which fed on the microbes and, as they did so, churned up the sediment, aerated it and fertilised it. Their actions, in turn, prepared the ground for other burrowers to live in, and feed off, still greater 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.

It is unnecessary to suppose that the organisms which brought about this revolution evolved into being ex nihilo, unseen and as if by magic. If they had arisen as products of the great ‘struggle for existence’, acquiring new capabilities and new levels of organisation as they out-competed their rivals, sediments should have abounded with their fossils. The simpler explanation is that in some shape or form they already existed, albeit undergoing radical transmutations. They appeared in the fossil record as they increased in number and as environmental conditions (the stability of substrates, oxygen levels, the availability of nutrients) favoured their increase; and they appeared successively – cyanobacteria first, then plankton, then worms, then other seafloor-dwellers – because organisms higher up the food chain took longer to recruit than those lower down, being dependent on them. They were not programmed to reproduce as quickly.

Nor is it necessary to suppose that the earliest-appearing organisms just happened to be those which, as primary producers, would later be essential for more complex life. As a recent analysis has demonstrated, the food webs of the Cambrian were ‘remarkably similar’ to modern food webs. Fundamentally, food chains changed little over time, just as, fundamentally, the organisms of which they were composed changed little. By the end of the Cambrian nearly all the phyla of the marine realm had made themselves known. The subsequent history of marine life was, by comparison, merely fugues and variations on themes already introduced at the beginning. These disparate organisms were linked by food chains, not evolutionary chains. There is no evidence that zooplankton evolved from bacteria, or that worms, molluscs, sponges and so on 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 continental 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.