Our knowledge of the fossil record has changed immensely since 1859, when Darwin felt obliged to explain why his hypothesis of gradualism was not confirmed by the study of fossil successions. His argument was, as is well known, that the fossil record exhibits extreme imperfection. The abrupt appearance of macrofossils at the base of the Cambrian was recognized and Darwin deduced that the evolutionary origins of those animals must have extended well back into the Precambrian.
"One-and-a-half centuries of subsequent research have revealed a vast microscopic fossil record of unicellular protists and bacteria extending, some would argue, as far back as there are sedimentary rocks from which they could be recovered. But although fossils of millimetre- to metre-scale multicellular organisms characterize the 90 million years of the Ediacaran period that precedes the Cambrian, pre-Ediacaran macroscopic fossils are exceedingly rare."
The newly reported fossils from Gabon were laid down in shales (Source here)
Over the years, various attempts have been made to vindicate Darwin's approach. The Ediacaran fauna shows that soft-bodied animals were preserved in the Precambrian, even in coarse sandstone beds, suggesting that fossils are not found because they were not there. In any case, there is no gradualist understanding of the Ediacaran fauna or any gradualist connection with the fossils of the "Cambrian Explosion". Prior to this, there is a unicellular fossil record with very few suggestions of macroscopic fossils. The newly reported finds are from sediments dated at 2.1 billion years old. They are interpreted by the researchers "as highly organized and spatially discrete populations of colonial organisms" with growth patterns "commonly associated with multicellular organization". If correct, this makes them the earliest known multicellular life.
"We have these macrofossils turning up in a world that was purely microbial," says Stefan Bengtson, a palaeozoologist at the Swedish Museum of Natural History in Stockholm and a co-author on the report. "That's a big deal because when you finally get big organisms, it changes the way the biosphere works, as they interact with microbes and each other." (Source here)
If confirmed, this discovery are exciting, although it "raises more questions than it answers". We consider below whether Darwin's expectations are met. However, some cautionary comments are appropriate in the interests of promoting multiple working hypotheses. Donoghue and Antcliffe's News & Views article considers the arguments put forward by the research team and declare:
"The null hypothesis, however, has to be that these remains represent bacterial colonies. Future work must determine whether the sterane signature, a hallmark of eukaryotes, is derived from soluble organics generated within the sediments, or whether they migrated into these sediments from younger rock sequences. [. . .] Although the fossils are macroscopic, they do not seem to represent anything other than the basic type of multicellularity, which occurs earlier in time in the form of stromatolites."
We should recognize that some scholars with expertise in this field are skeptical. R. Ford Denison gives several reasons, including: "A consistent size and shape is another criterion for true multicellularity, met by Volvox, for example. The fossils don't look any more consistent in size and shape than one would expect from bacterial colonies." In a Nature News item, Maxmen draws attention to another:
"Calling the Gabon specimens "pseudo-fossils", palaeontologist Adolf Seilacher at Yale University in New Haven, Connecticut, instead interprets them as aggregations of the mineral pyrite that grew in different shapes depending on the changing state of the surrounding sediment. In 1998, Seilacher reported finding a fossil eukaryote 1.1 billion years old. Referring to that find, he says: "I now firmly believe that my own so-called first animals were pseudo-fossils too.""
But let us allow that these putative fossils represent genuine multicellular organisms. Does it support a Darwinian perspective on evolution? Donoghue and Antcliffe think that it does:
"But within the confines of a very patchy global record of Proterozoic and Archaean rocks, which extend from about 3.8 billion years ago to the beginning of the Cambrian, these remains contribute to a fossil record that belies the dated caricature painted in the Origin. It was Darwin's view that absence of organisms in these early intervals of Earth's history would prove his theory of biological evolution wrong. The discovery and continuing elucidation of the Precambrian fossil record has met Darwin's predictions on the extent and structure of evolutionary history."
The "absence of organisms" problem persists for Darwinism, as the abrupt appearance of animal phyla in the Cambrian Period is not preceded by any evidence of gradual transformation. It is preceded by multicellular organisms in the Ediacaran - but largely unrelated to the Cambrian fauna. It is preceded by unicellular life forms, but there is no convincing Darwinian perspective on these (see here and here). The discovery of an isolated early multicellular organism does nothing to support gradualism, because there is no gradual succession of ancestral or descendant forms. The find should be regarded as enigmatic until further insights into context are obtained.
Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago
Abderrazak El Albani, Stefan Bengtson, Donald E. Canfield, Andrey Bekker, Roberto Macchiarelli, Arnaud Mazurier, Emma U. Hammarlund, Philippe Boulvais, Jean-Jacques Dupuy, Claude Fontaine, Franz T. Fursich, Francois Gauthier-Lafaye, Philippe Janvier, Emmanuelle Javaux, Frantz Ossa Ossa, Anne-Catherine Pierson-Wickmann, Armelle Riboulleau, Paul Sardini, Daniel Vachard, Martin Whitehouse & Alain Meunier
Nature, 466, 100-104, (01 July 2010) | doi: 10.1038/nature09166
First para: The evidence for macroscopic life during the Palaeoproterozoic era (2.5-1.6 Gyr ago) is controversial. Except for the nearly 2-Gyr-old coil-shaped fossil Grypania spiralis, which may have been eukaryotic, evidence for morphological and taxonomic biodiversification of macroorganisms only occurs towards the beginning of the Mesoproterozoic era (1.6-1.0 Gyr). Here we report the discovery of centimetre-sized structures from the 2.1-Gyr-old black shales of the Palaeoproterozoic Francevillian B Formation in Gabon, which we interpret as highly organized and spatially discrete populations of colonial organisms. The structures are up to 12 cm in size and have characteristic shapes, with a simple but distinct ground pattern of flexible sheets and, usually, a permeating radial fabric. Geochemical analyses suggest that the sediments were deposited under an oxygenated water column. Carbon and sulphur isotopic data indicate that the structures were distinct biogenic objects, fossilized by pyritization early in the formation of the rock. The growth patterns deduced from the fossil morphologies suggest that the organisms showed cell-to-cell signalling and coordinated responses, as is commonly associated with multicellular organization. The Gabon fossils, occurring after the 2.45-2.32-Gyr increase in atmospheric oxygen concentration10, may be seen as ancient representatives of multicellular life, which expanded so rapidly 1.5 Gyr later, in the Cambrian explosion.
Origins of multicellularity
Philip C. J. Donoghue & Jonathan B. Antcliffe
Nature, 466, 41-42, (01 July 2010) | doi: 10.1038/466041a
Interpreting truly ancient fossils is an especially tricky business. The conclusion that 2.1-billion-year-old structures from Gabon are the remains of large colonial organisms will get palaeobiologists talking.
Maxmen, A. Ancient macrofossils unearthed in West Africa, Nature News (30 June 2010) | doi:10.1038/news.2010.323
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