Earliest Metazoans And Darwins Dilemma

The history of complex forms of life begins around the dawn of the Paleozoic Era, some 575 million years ago, when recognizable ancestors of many of today's phyla made their debut. Thirty million years later, a great diversity of new forms of life appeared just before and after the beginning of the Cambrian Period — so many that this burst of evolutionary branching is often referred to as the "Cambrian explosion."This was the great expansion of the metazoans, animal life-forms characterized by their advancement over primitive single cells in several important ways. Metazoans are all multicelled creatures —including your-self—in which cells have differentiated to serve different functions, by forming tissues such as muscles and nerves, and which have an interior digestive cavity or gut. These basic qualities distinguish metazoans from the numberless and mostly unknown single-celled organisms —prokaryotic bacteria and blue-green algae, and later eukaryotic protozoans—which had the seas of the Earth to themselves for billions of years before the Cambrian Period, which began 543 million years ago.

What came before these metazoans that demonstrated such adaptability and evolutionary potential in the Cambrian seas? The earliest multicellular organisms remain invisible to the body fossil record, but trace fossils provide a crucial clue to their existence. Up until 560 million years ago, fossilized prehistoric sea floors show even, regular deposition of fine layers. After that point, however, the layers begin to show disorder and mixing of layers, which is interpreted as bioturbation: disturbance by burrowing seafloor animals. Fossil burrows begin to be found, preserved as wormlike casts twisting through the prehistoric sediments, made by burrowing wormlike creatures. Multicellular life had already appeared by the Late Precambrian, although molecular techniques in paleontology push the date for complex life as far back as 900 million years. However, we are fortunate to have extraordinary evidence of several early communities on the ancient sea floor, the Ediacaran biota.

The sudden appearance of the Ediacaran biota in the Precambrian Period heralded the beginning of large complex life. Because these animals possessed only soft tissue, their fossil remains were unknown until a discovery in the Ediacara Hills of southern Australia in 1946 by a mining geologist. The Australian Ediacaran fossils date from 550 million years ago, but since their discovery older localities have been found. The earliest representatives of the Ediacaran biota can be found at Newfoundland's 575-million-year-old Mistaken Point.

The Mistaken Point fossils, called rangeomorphs, represent a group of organisms at or near the base of animal evolution. This is truly where the evolution of animals began on Earth, albeit on the sea floor. The 580-million-year-old Gaskiers Glaciation helps date the Mistaken Point fossils, literally underpinning their stratum. It seems that life got complex immediately following the meltdown of a "snowball earth." Up to this point life had been microbial. What caused the change?

Prior to and including the Gaskiers stratum, sediment chemistry seems to indicate that oxygenless environments prevailed, but in the overlying Drook, Briscal, and Mistaken Point rocks the ratios of reactive to total iron show more oxygenated sediments. This has led Guy M. Narbonne and his colleagues to suggest that complex multicellular life evolved due to a sudden infusion of oxygen into the oceans 580 million years ago. But how the oxygen penetrated to the depths of the oceans and where it came from is still the subject of debate.

A major problem with envisioning these unique Ediacaran fossils is that they are flattened two-dimensional representations of soft-bodied animals that were once three-dimensional. How do you unsquish them? These early animals have been extinct for more than a half-billion years, making their reconstruction very complicated. Such attempts have led to a brisk debate on the evolutionary relationships of these early life-forms. Yet their sudden appearance in the fossil record begs an even bigger question.

Did the Cambrian explosion actually take place? This was one of Darwin's dilemmas. In true Monty Python fashion, Professor Martin Brasier of the University of Oxford would say, "Nobody expects the Cambrian explosion." On a recent trip to Newfoundland I met Brasier and Duncan Mcllroy, of Memorial University, to hear about their latest findings on the Ediacaran fossils. Brasier, with a big smile, said much of the activity in this particular area of science was a function of the Mofaotyof principle: "My oldest fossils are older than your oldest fossils." Sometimes science can be driven by the more earthly desires of fame. Nonetheless, the fossils of Mistaken Point have attracted some of the finest minds in paleontology to resolve their mystery.

Brasier has been using state-of-the-art laser scanners that can elucidate the finest details of these squished fossil structures, with many looking like the proverbial Mandelbrot inkblot test. The new laser scanning method has allowed Brasier and his team to unpick the folded structures, reinflating and reconstructing these long-extinct animals in glorious 3-D.These powerful visualization tools seem to be the way of the future. Once we can examine a reconstructed organism, we can also start comparing one fossil to another, and even identify growth stages. Quoting Darcy Thompson, Brasier reminded me with a wry smile, "If you do not understand how an organism grows, you do not understand the organism." The same could be said for many fossil groups that are constructed upon often fragile evidence.

Mcllroy, who has been working on the Ediacaran fossils since 1992, was so captivated by the fauna that he eventually moved to Newfoundland in 2005. He started working his way through the literature, local sites, and many Mistaken Point Ediacaran fossils in the collections of Memorial University Mcllroy has made himself at home in the scenic wilds of Newfoundland, with hunting, fishing, and berry collecting a part of everyday life. He is a remarkable chap whom I first met when he was an undergraduate and I a post-grad student at the University of Manchester in 1990. Mcllroy has had a meteoric rise in academia because of his direct, well-thought-out, often firm approach to paleontology and geology, based upon his extensive field skills and scrupulous attention to detail when recording data. Mcllroy would say, "You have to appraise all the data that are laid before you in the rocks." Specific scientific disciplines have the nasty habit of "cherry-picking" a single component of a problem (the fossils, sediments, geochemistry, etc.) to fit a hypothesis. Mcllroy's approach is to use all of the available data. If the data do not support the hypothesis, it's time to rethink the problem and come up with a new solution. In the words ofThomas Kuhn, a paradigm shift is required.

As they work on the Ediacaran fossils of Mistaken Point, Mcllroy and Brasier are picking their way through the complex shadows of life that were preserved in the windswept rocks of Newfoundland. They both have realized that these "early" life-forms are actually quite complex. This supports that the theory that the beginning of complex life was much earlier than the 575-million-year-old fauna they are currently studying. Somewhere in the fossil record lies the answer to this particular question. However, without the Mistaken Point fossils, we would not even know that we have to search deeper in geological time for the moment that life first became complex.

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