5. Recovery of marine communities

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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 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).

Despite huge resources expended on the problem, it remains unsolved. Nor do we claim to have a complete solution. But it is at least a somewhat smaller problem when Darwinism is abandoned: that of having to explain why so many disparate forms of marine life appeared all at once, rather than additionally having to explain why, at the beginning of their history, organisms were already 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 that do not fit into any modern group must have existed, somewhere and in some form, 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?

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). Lakes may have existed 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 and very small. They certainly did not have hard, fossilisable shells.

Sea temperatures in the Archaean were high – some estimates have them averaging in excess of 60° C. Regions of submarine volcanism spewed large volumes of dissolved iron, manganese and sulphur into the oceans, making all but the thin surface layer, where oxygen-producing cyanobacteria were active, poisonous to animal life. The oxygen produced by the cyanobacteria escaped into the atmosphere, for the high temperatures and high concentrations of iron stopped oxygen from diffusing or circulating downward. Bacteria and algae flourished in low-latitude seas, but animals – the ancestors of those that appeared in the Cambrian Explosion – were probably confined to the poles. Recolonisation of the seas nearer land would not have been possible until:

  • the seas had cooled sufficiently
  • oxygen levels at the seafloor were high enough
  • the coastal waters were no longer toxic (e.g. because of hydrogen sulphide)
  • animal stocks had begun to recover and spread out from their cold-water refuges

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 less common. Higher in the water column, plankton and microplankton were also beginning to recover.

Click on image for larger viewThen 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 time 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 to penetrate 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 as much as evolution. Appearing as if from nowhere, worms, molluscs, trilobites and so on were all benefiting from the recovery of marine animal life further down the food chain. Faecal pellets dropped by zooplankton attracted the grazing and burrowing animals that now had sufficient oxygen to live on the seafloor. As they consumed the droppings, they 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.

Evidently the organisms which brought about this revolution did not come into being ex nihilo, unseen and as if by magic. Nor did they arise as products of the great ‘struggle for existence’, acquiring little by little new capabilities and levels of organisation as they out-competed their rivals, for then sediments should have abounded with their ancestors. The simplest explanation is that in some shape or form they already existed. They appeared in the fossil record as they increased in number and the increasingly benign environments around the coasts allowed for their increase. In a jump shift analagous to the miraculous way in which many fossilisable creatures – barnacles, for instance – start out as unfossilisable larvae, the new conditions triggered radical transmutations.

In relation to the Precambrian-Cambrian transition as a whole, marine organisms appeared successively – first primary producers, then zooplankton, seafloor-dwelling herbivores and immobile filter-feeders, then swimming and seafloor-dwelling carnivores and deposit-feeders, finally large predators. This was the nature of the order of fossils. Organisms higher up the food chain depended on those lower down and were not programmed to be as numerous, to reproduce as quickly. In all its diversity marine life was designed as a complex community.

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 platforms began to be colonised by recovering populations from elsewhere – presumably, from shallow-marine polar regions. Starting with bacteria and climaxing with sharks, it was an ecological progression, something that occurred over thousands of years, not three thousand million.

Next: The first steps from sea to land