The sudden appearance of complex marine life in the Cambrian is perhaps the most striking and perplexing phenomenon in the whole fossil record. One authority describes it thus:
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).
Despite huge resources expended on the problem, it remains unsolved, and we do not claim to have a complete solution either. But it is at least a smaller problem: one of having to explain why so many disparate forms of marine life appear all at once, rather than having additionally to explain why, at the beginning of their history, organisms that were supposedly all related should be so disparate.
The Cambrian period is not the beginning of things; it 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 other phyla besides that do not fit into any modern group must have existed long before. Since the scientific approach must be to suppose that creation occurred only at the beginning, is there an explanation that can make sense of this subsequent history in purely natural terms?
During the Archaean, magma welling up from below replaced the land that was destroyed at the end of the Hadean. Since no pristine crust from that time remains, geology can tell us little about what kind of world existed before that time. Genesis suggests that the seas created on the third day of creation were primarily underground seas (The antediluvian world). There may have been lakes at ground level, but since all terrestrial surfaces were destroyed in the Cataclysm, the only aquatic organisms likely to have survived would have been organisms used to living in the dark. 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, very small and lacking calcitic shells.
Sea temperatures in the Archaean were high – some estimates have them averaging in excess of 60° C. Except for heat-loving bacteria, organisms close to the mid ocean ridges where new seafloor was being generated could not have survived but were probably confined to the poles and deep-water regions far removed from these 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 late Proterozoic. Prior to that stage almost the only signs of life on the seafloor were stromatolites – layered mats produced by microbes. As the seas became more oxygenated, stromatolites 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 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 the later 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 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 and 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. They were dependent on them, and 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’ in structure to modern food webs. Fundamentally, marine food chains changed little over time, just as, fundamentally, the organisms that composed them changed little. By the Middle Cambrian the phyla of the marine realm had nearly all made themselves known. The subsequent history of marine life was, by comparison, merely fugues and variations on themes already introduced at the beginning. The disparate organisms that appeared in the Cambrian 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.