What sparked the Cambrian explosion?

Feb 19, 2016

Photo credit: John Sibbick/Natural History Museum

By Douglas Fox

A series of dark, craggy pinnacles rises 80 metres above the grassy plains of Namibia. The peaks call to mind something ancient — the burial mounds of past civilizations or the tips of vast pyramids buried by the ages.

The stone formations are indeed monuments of a faded empire, but not from anything hewn by human hands. They are pinnacle reefs, built by cyanobacteria on the shallow sea floor 543 million years ago, during a time known as the Ediacaran period. The ancient world occupied by these reefs was truly alien. The oceans held so little oxygen that modern fish would quickly founder and die there. A gooey mat of microbes covered the sea floor at the time, and on that blanket lived a variety of enigmatic animals whose bodies resembled thin, quilted pillows. Most were stationary, but a few meandered blindly over the slime, grazing on the microbes. Animal life at this point was simple, and there were no predators. But an evolutionary storm would soon upend this quiet world.

Within several million years, this simple ecosystem would disappear, and give way to a world ruled by highly mobile animals that sported modern anatomical features. The Cambrian explosion, as it is called, produced arthropods with legs and compound eyes, worms with feathery gills and swift predators that could crush prey in tooth-rimmed jaws. Biologists have argued for decades over what ignited this evolutionary burst. Some think that a steep rise in oxygen sparked the change, whereas others say that it sprang from the development of some key evolutionary innovation, such as vision. The precise cause has remained elusive, in part because so little is known about the physical and chemical environment at that time.

But over the past several years, discoveries have begun to yield some tantalizing clues about the end of the Ediacaran. Evidence gathered from the Namibian reefs and other sites suggests that earlier theories were overly simplistic — that the Cambrian explosion actually emerged out of a complex interplay between small environmental changes that triggered major evolutionary developments.

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5 comments on “What sparked the Cambrian explosion?

  • 1
    Cairsley says:

    The caption to Figure 7.4 in Chapter 7 of Evolution: What the Fossils Say and Why It Matters (2007) by Donald R. Prothero states:

    “A detailed examination of the stratigraphic record of fossils through the late Precambrian and Cambrian shows that life did not “explode” in the Cambrian, but appeared in a number of steps spanning almost 100 million years. The large soft-bodied Ediacaran fossils … appeared first at 600 million years ago, in the late Precambrian (Vendian). Toward the end of their reign, we see the first tiny shelly fossils, including the simple conical Cloudina and Sinotubulites. The Nemakit-Daldynian and Tommotian stages are dominated by the “little shellies” … plus the earliest lamp-shells, or brachiopods, and the conical sponge-like archaeocyathans, and many burrows showing that worm-like animals without hard skeletons were also common. Finally, in the third stage of the Cambrian (Atdabanian, around 520 million year ago), we see the radiation of trilobites, and huge diversification in total number of genera … . Thus the Cambrian explosion took over 80 million years to develop, and was no “sudden” event, even by geological standards.”

    In the main text, Prof. Prothero continues this theme thus:

    “Thus we have seen that the Cambrian explosion is a myth. It is better described as the Cambrian slow fuse. It takes from 600 to 520 million years ago before the typical Cambrian fauna of large shelly organisms (especially trilobites) finally develops. Eighty million years is not explosive by any stretch of the imagination! Not only is the explosion a slow fuse, but it follows a series of logical stages from simple and small to larger and comples and mineralized. First, of course, we have microfossils of cyanobacteria and other eukaryotes going back to as far as 3.5 billion years ago and spanning the entire fossil record since that ancient time. Then, about 600 million years ago, we get the first good evidence of multicellular animals, the Ediacara fauna. They are larger and multicellular but did not have hard shells. The earliest stages of the Cambrian, the Nemakit-Daldynian and Tommotian stages, are dominated by the little shellies, which were just beginning to develop small mineralized skeletons. Only after several more steps do we see the full Cambrian fauna. In short, the fossil record shows a gradual buildup from single-celled prokaryotes and then eukaryotes to multicellular soft-bodied animals to animals with tiny shells, and finally by the middle Cambrian, the full range of large shelled invertebrates. This gradual transformation by logical advances in body size and skeletonization bears no resemblance to an instantaneous Cambrian explosion that might be consistent with the Bible but instead clearly shows a series of evolutionary transformations.”

    We do need to stop using misleading phrases like ‘Cambrian explosion’, especially when ignorant and unprincipled purveyors of religion can use them to misinform others of the facts and promote their own superstitions.

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  • Cairsley #1
    Feb 20, 2016 at 12:10 am

    Eighty million years is not explosive by any stretch of the imagination!

    We do need to stop using misleading phrases like ‘Cambrian explosion’, especially when ignorant and unprincipled purveyors of religion can use them to misinform others of the facts and promote their own superstitions.

    We need to remember that geological time where diversity of multicellular life “exploded” over tens of millions of years, is beyond the comprehension of YECs who struggle with the digital count of thousands of years when they run out of fingers and thumbs!

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  • The term explosion merely points to an explosion in numbers of different higher and lower taxa coming into existence and the opportunities given by the occupation of new ecological niches, in what is indeed a short timespan. The 2000 million years that life existed on earth leading up to the Ediacara fauna and the Cambrian explosion seems to me a long time compared to the 80 million years the explosion lasted. Compare it to the technological explosion since the start of the industrial revolution some 250 years ago. 6250 years ago mankind hadn’t even entered the bronze age, we were still in the neolithic!

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  • 5
    Cairsley says:

    I get the impression from Donald R. Prothero’s occasional casual use of the expression ‘Cambrian explosion’ that there was a time when he too held the general view that there was something metaphorically explosive about the diversification of life in the Cambrian, a view based on the fossil evidence then had of that period. One of the points he makes several times in his book Evolution: What the Fossils Say and Why It Matters (2007) is that great advances have been made in palaeontology in the last two (now three) decades, thanks to the increasing range of fossil discoveries. Although he knows that ‘Cambrian explosion’ has long been part of the fossil story, he takes the view that, in light of all the evidence now available, that expression is no longer accurate and may well be misleading. That eighty million years is a short time compared with two thousand million is beside the point here; what is relevant is that the gradual evolution that took place in the radiation of life in the Cambrian was in fact too slow to be called an explosion, however much it may have looked as though there had been some such evolutionary explosion, when the fossil record was much less complete.

    Notably, Prof. Prothero’s title for the thirteenth chapter of the same book is “Mammalian Explosion”, and there is not a hint or murmur of apology on his part in this case for using the word ‘explosion’ thus. Let me quote this passage therefrom to give a sense of how or why he is using the word here:

    “Within a million years after the end of the Cretaceous, mammals began an explosive evolutionary radiation, with many new groups appearing for the first time in the fossil record, and the mostly shrew-sized Mesozoic mammals evolving into much larger dog-sized and even cow-sized animals. By the middle Eocene, only 15 million years after the nonavian dinosaurs had vanished, almost all the living orders of mammals (rodents, rabbits, bats, whales, carnivores, primates, and so on) had appeared, although they were very primitive members of those families that look nothing like their living descendants. Paleontologists frequently point to the evolutionary radiation of Cenozoic mammals as a classic example of what life can do when competition is suddenly removed, and there are many new ecological resources and adaptive zones left vacant.”

    Bear in mind that a real explosion goes bang and moves stuff in all or many directions at once, and this can serve as a suitable metaphor for an event that happens suddenly and gives rise to a wide range of developments. Prof. Prothero denies the Cambrian its erstwhile distinction of being a time of explosive evolution but bestows that distinction upon the Cenozoic. It depends, I suppose, on how much evolutionary radiation and change occurs in how few millions of years whether it can all be deemed to have occurred “suddenly” enough to merit being dubbed an ‘explosion’.

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