Cambrian Explosion

Abstract

The evolutionary phenomenon popularly known as the “Cambrian Explosion” refers to the synchronous first appearance of numerous different animal fossils in the great Cambrian lagerstätten, most famously Chengjiang and the Burgess Shale. Often and misleadingly, this phenomenon has been interpreted literally, or at least is presented literally, as if to imply that the numerous, different animal groups actually evolved from some unknown common ancestor, within a very short space of time, during the Early Cambrian. That is almost certainly not the case. Despite the rapid proliferation of evolutionary novelties which apparently occurred at this time, at least some of the phenomenon is attributable to the acquisition of preservational characteristics – “hard parts” – and multiple lines of evidence reveal that life was already highly diversified prior to the so-called “Explosion”.

Keywords: Cambrian, Cambrian explosion, Cambrian radiation, fossil record, evolution, Metazoan radiation

Introduction

The “Cambrian Explosion” – what, when, how fast, if at all – is one of those perennially fraught issues in evolutionary science; there are passionate advocates for a variety of positions on the subject and, if I am any judge, none of the extremists are likely to be even remotely close to the truth. It is only fair to point out that I am an extremist myself: I do not believe anything much happened in the Cambrian that wasn’t a “business as usual” unfolding of evolutionary trends that had been set in motion long before.

In an effort to do justice to the debate, however, this page will endeavour to present simple observations, what we may call facts, in the first two sections, then outline one of the main points of view – rather an old-fashioned one, it seems to me – that the fossil record is a good guide to what actually happened in the Cambrian, and should be interpreted more-or-less literally. Finally, I will lay out my own preferred alternative to the literal interpretation model, and wind up with a few concluding prejudices.

There are other views around, and the interested reader will readily find a plethora of informed and uninformed opinions on the internet, not least the “God did it” variety. The internet being what it is, however, or more specifically what it is not – a source of reliable, objective information – I have suggested some further reading in the box to the right. These offerings will not always be correct in every detail, of course. Some I disagree with vehemently. On the other hand, I can promise that they will all have the advantage of sound scholarship, and be written by people knowledgeable on the subject, as opposed to the fevered rantings of internet opinionists.

What is it?

The Earth is something like 4,600 million years old, and life is thought to have originated quite soon after the crust solidified. The evidence supporting this belief, and the exact dating of it, are themselves controversial topics which are addressed elsewhere. Suffice to say that the oldest uncontroversial fossils occur in the early Archean Eon, approximately 3,500 million years ago. However, the fossil record from these very ancient rocks is extremely meagre, and it remains so until about 600 million years ago. At this time, a series of strange ecosystems arose, characterised by creatures that appear quite unlike those we know and understand well today. These creatures are collectively known as the Ediacarans, although it is now considered unlikely that the various individual Ediacaran species were members of a single closely related group. (That, too, was a long-running controversy, and it has not yet entirely abated.) Ediacaran fossils are found around the world and, although usually only a few different kinds are found at each fossil locality, when they are all added together there is quite a large number of different sorts. But then, at the beginning of the Cambrian Period, approximately 540 million years ago, fossil Ediacarans seem to disappear entirely. Or almost. Whether or not there are a few stragglers is another debated topic but, essentially, they disappear from the known fossil record and their place is taken by … very little. The few fossils known from the earliest Cambrian Period are mostly very small shelly remains, probably including a few actual molluscs but more commonly the dispersed parts of larger organisms. It is not until some 20 million years into the Cambrian, in an interval that used to be known as the Atdabanian but is now called by the depressingly unimaginative designation “Age 3”, that fossils become common.

The Atdabanian fossils, most famously represented by the Chengjiang fauna, are not only different from the older fossils in being vastly more numerous and vastly more diverse, but in being recognisably “modern”. I need to explain this remark. Although trilobites (for example) are virtually poster children for extinction, nobody doubts that they are arthropods; they belong to the modern phylum Arthropoda. Similarly, the numerous sponges, cnidarians, molluscs, annelids, brachiopods, echinoderms, and even chordates from the Chengjiang and similarly aged deposits: they all belong to modern phyla. With one or two exceptions (and Stephen Jay Gould’s Wonderful Life notwithstanding) even the exotic “Cambrian problematica” can be readily assigned to a modern phylum.

It is said that all modern animal phyla that could be expected to leave fossils, do indeed have a fossil record extending back into the Cambrian, although not necessarily all the way to Chengjiang.

This phenomenon – the approximately synchronous, first known appearance of numerous different animal fossils in the great Cambrian lagerstätten – is what is called the “Cambrian Explosion” (or the “Cambrian radiation” for those of a less histrionic disposition).

Exploring the phenomenon

Although the particular phrase “Cambrian Explosion” only came into common usage in the early to mid 1970s, the phenomenon itself has long been recognised as demanding some accommodation from evolutionary theory. As early as 1859, Charles Darwin (1809-1882) drew attention to the matter in Origin, and no doubt he had long pondered the matter prior to that.

Darwin could not have been aware of any of the great Cambrian lagerstätten – Chengjiang, Sirius Passet, Emu Bay, the Burgess Shale – because none of them had been discovered during his lifetime. Yet he was still aware the earliest fossils known to him, mostly the trilobite faunas of Wales, were already quite advanced organisms. Clearly, if his then-new theory of evolution were true, they must have had precursors that had not yet been found. For many years afterward, new fossil discoveries, including the Burgess Shale discovered by Charles Walcott in 1909, not only appeared to reinforce this conundrum, but to exacerbate it.

Some of Darwin’s missing precursor fossils had, in fact, already been found: The first of the enigmatic Precambrian fossils that would eventually become known as “Ediacarans” was described from the Newfoundland by E. Billings in 1872, although it would take another sixty years for the first diverse assemblage of these fossils to be described (from Namibia; Gürich 1933). Over time, various Precambrian fossils have come to light from all over the world, some dating back to possibly as much as 3,700 million years (Nutman et al. 2016).

Darwin himself, though he was unaware of any Precambrian fossils, assumed that Precambrian organisms existed but had simply not been found, or perhaps that their fossils had been lost to time; the so-called “imperfection” of the fossil record. He appears to have believed that, if the fossils were to hand, they would show a more or less regular pattern of simplification trailing back into the past. That position is no longer tenable, now. Today, even after the discovery of innumerable Precambrian fossils, the same perplexing pattern persists: The fossil record is meagre and lacks diversity from the oldest fossils right through to the Atdabanian, when complex, diverse, and obviously highly evolved fossils representing most of today’s living animal phyla, “suddenly” become common.

A variety of opinions purporting to explain this pattern has appeared over time. They span quite a range, and I’ll attempt to outline the two endpoints fairly, while reminding the reader that I personally have a very strong preference for the second one, so I am anything but objective.

Differing views: 1. Literal interpretation

One extreme view is that the fossil record should be interpreted quite literally. This is not a view held in opposition to the “imperfection of the fossil record” – most paleontologists today accept that the fossil record is a good guide to the creatures that were alive at the time (at least the biggish ones, especially those with hard parts); there is little disagreement about that – but it is a statement about the meaning of the fossil record. Perhaps the most famous exposition of the literalist view is Stephen Jay Gould’s book, Wonderful life, in which he opines that life was furtive and homogeneous up until the Cambrian, when suddenly, for reasons unknown, it burst forth into an explosion of different forms, the like of which has never been seen again. Even among adherents to the literal view, Gould was something of an extremist. Today it would be hard to find a serious biologist who believed that the wonderful cast of characters in Gould’s book were anything other than stem-group arthropods. His belief that the Cambrian biota was more diverse and more disparate than that of the present day, while trivially true for any extinct organisms, can safely be dismissed as a general description of biodiversity.

More moderate versions from the “explosion is real” camp have been put forward by most of the big names in the field in the years since then. Most of the softening from Gould’s extreme interpretation has come from two directions: (1) near universal acceptance that even the weird Cambrian “problematica” with odd numbers of eyes and mouths on stalks, are mostly just unfamiliar arthropods; and (2) concessions that the earliest metazoans – several or maybe many phyla – appeared far earlier than the Chengjiang horizon, and probably even before the beginning of the Cambrian Period, although they remained inconspicuous and with low diversity until something – whatever it was – happened, and dramatic, near simultaneous, within-phylum radiations began.

Some of this softening to the literalist stance from the “old guard” has been disingenuous: Suggestions that the born-again moderate literalists of today had been moderate all along are nothing more than dishonest attempts to rewrite history. Any cursory glance through the literature of the day makes it obvious that most were happily in Gould’s camp until the cracks started to appear, when they suddenly remembered they’d left something on the stove. But that’s enough of that.

However, much of the new generation of literature is genuinely aimed at an objective understanding of the pattern we see in the fossil record, in the light of current knowledge.

Differing views: 2. Business as usual

At the other extreme, the “BAU interpretation” of the Cambrian radiation – at least my version of it – is that the event was nothing out of the ordinary. Yes, there is strong circumstantial evidence that there was a radiation of some groups of animals happening about this time, but, even assuming there was, it was just an ordinary, run of the mill radiation such as we see many times in the fossil record. Multiple, distantly related phyla did not “suddenly” evolve during this interval. The event does not require novel explanations and it certainly does not justify the hand-wringing existential angst we have seen from many supposedly grown-up paleontologists over the years.

This view is based on multiple strands of reasoning. In no particular order:

1. The greatest genetic disparity of all living things was established billions of years ago; long before the Cambrian, when the archaean and bacterial lineages diverged. Since then, all genetic variation has occurred within that envelope. For anybody seeking a single, fundamental, evolutionary event to fixate upon, this is it. (Also see David & Alm 2011.)
2. Only metazoans, and only some of them, appear to have been involved in the Cambrian radiation. Plants did not diversify significantly until the late Silurian or Devonian, and unicellular organism diversity actually appears to have suffered one or more setbacks in the late Ediacaran and early Cambrian. Animals may seem very important to us, but they are far from being representative of all life; most organisms alive today, or that have ever lived, are bacteria.
3. The supposed proliferation of taxa at the phylum level is arbitrarily chosen (for example, to exclude the major radiations of insects in the Carboniferous and Cretaceous) and finesses the obvious criticism that phyla are arbitrary, human constructs to begin with.
4. But if we are going to count phyla, molecular phylogenies (and, increasingly, fossils too) indicate that many, and probably most, animal phyla were already established well before the beginning of the Cambrian. It doesn’t matter which particular phylogeny you choose (Peterson et al. 2008 is my preferred model for now); they all indicate the same pattern with a consistency only the wilfully bloody-minded can ignore.
5. The Chengjiang, Qingjiang, and Burgess Shale lagerstätten are unequalled in the whole of geological time. Perhaps that means something; perhaps not. If a similarly well-preserved fossil trove from the late Ediacaran existed, what would it show? We have little idea so, even today, we still cannot say whether a metazoan radiation really took place, or if we are still being fooled by the vagaries of the fossil record. Despite having a great deal more data than was available to Darwin, we are still not sure. (See Anderson et al. 2023 for an interesting perspective on this aspect.)
6. Possession of “hard parts” (spikes, shells, even the crunchy exoskeleton of a shrimp) greatly improves the likelihood that an organism may become fossilised. Of all the things that may or may not have happened in the Cambrian, I am willing to bet my best bottle of wine that numerous groups of marine invertebrates were busily acquiring hard parts – for whatever reason. (There are several theories.) Whatever was happening in the way of explosions at that time, there was certainly a quantum increase in the abundance of hard things to become fossils.
7. There have been numerous radiations of various groups of living things since the Atdabanian. One of the greatest was in the Ordovician: the “GOBE” (Great Ordovician Biodiversification Event; Servais & Harper 2018). These demonstrate that radiations, per se, are not unique events.

I believe that all of these observations support, or are at least consistent with, the idea of the Cambrian as simply part of a stately unfolding of biodiversity, beginning whenever the first cell divided for the first time, as far back as 3 Ga or so, and proceeding through to the present in a manner that was not necessarily smooth, but closer to “steady” than “wildly fluctuating”, with the exception of mass extinctions. At many steps along the way, various thresholds were crossed – both environmental (free oxygen, the “snowball” glaciations) and evolutionary (origin of the eukaryote cell, multicellularity, sex) – all of which were unique in some respects, assuming the word even has a meaning in such a context. When every event in a trajectory is unique, it seems to me that none of them are.

Conclusion

Contemporary articles by the high priests of the Cambrian explosion tend to be relatively measured, as the cheerleaders of yesterday’s hysterical claims scramble to revise history, although much popular writing is still presented in the breathless rhetoric of 1980 to 2010.

Professional offerings from a newer generation of researchers – Wood et al. 2019 is a thoughtful example – mostly take an intermediary position which, I suspect, is genuine, rather than attempting to have a dollar each way. Typically, these writers acknowledge deep, Precambrian roots for most metazoan phyla, while conceding that something rather special did, in fact, occur in the early to middle part of the Cambrian Period.

Of course, the real truth – if we ever find it – will probably be much more interesting.

References

Anderson, R.P.; Woltz, C.R.; Tosca, N.J.; Porter, S.M.; Briggs, D.E.G. 2023: Fossilisation processes and our reading of animal antiquity. Trends in Ecology and Evolution.

Briggs, D.E.G.; Erwin, Douglas H.; Collier, Frederick J.; Clark, C. 1994: The fossils of the Burgess Shale. Smithsonian Books: 1-256.

Conway Morris, S. 1998a: The crucible of creation. Oxford University Press: 1-242.

Darwin, C.R. 1859: On the origin of species by means of natural selection [first edition]. John Murray, London: 1-490.

David, L.A.; Alm, E.J. 2011: Rapid evolutionary innovation during an Archaean genetic expansion. Nature 469 (7328): 93-96.

Erwin, D.; Valentine, J. 2013: The Cambrian explosion: The construction of animal biodiversity. Freeman: 1-416.

Fedonkin, M.A.; Gehling, J.G.; Grey, K.; Narbonne, G.M.; Vickers-Rich, P. 2007: The rise of animals: evolution and diversification of the Kingdom Animalia. Johns Hopkins: 1-326.

Gould, S.J. 1989: Wonderful life. Penguin: 1-347.

Gürich, G. 1933: Die Kuibis Fossilen der Nama-Formation von Sudwestafrika. Paläontologische Zeitschrift 15: 137-154.

Nutman, A.P.; Bennett, V.C.; Friend, C.R.L.; Kranendonk, M.J.V.; Chivas, A.R. 2016: Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures. Nature 537: 535-538.

Peterson, K.J.; Cotton, J.A.; Gehling, J.G.; Pisani, D. 2008: The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records. Phil. Trans. R. Soc. Series B 363: 1435-1443.

Servais, T.; Harper, D.A.T. 2018: The Great Ordovician Biodiversification Event (GOBE): definition, concept and duration. Lethaia 51: 151-164.

Wood, R.; Liu, A.G.; Bowyer, F.; Wilby, P.R.; Dunn, F.S.; Kenchington, C.G.; Cuthill, J.F.H.; Mitchell, E.G.; Penny, A. 2019: Integrated records of environmental change and evolution challenge the Cambrian Explosion. Nature Ecology and Evolution 3: 528-538.