This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In the first edition of "On the Origin of Species" (1859) Darwin only briefly addresses the earliest known fossils, or better the lack thereof:
"If the theory [of evolution] be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed . . . and the world swarmed with living creatures. [Yet] to the question why we do not find rich fossiliferous deposits belonging to these earliest periods . . . I can give no satisfactory answer."
The supposed lack of fossils older than 500 to 1.000 million years was not only the result of the "imperfection of the geological column", as Darwin argued, nor the yet incomplete geological survey of most areas. At the time nobody could really imagine how the earliest organisms on earth - and especially their petrified remains - would look like, so what to search for ?
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The Cambrian was considered one of the oldest geological formations with supposed certain signs of life. One fossil found in Cambrian rocks was Oldhamia - a strange, branched something, described by geologist Sir John William Dawson (1820-1899) as polyp-like animal.
Fig.1. A diagram of 1888, showing the sequence of strata and their characteristic fossils - as supposed most ancient fossils are shown Oldhamia (Cambrian) and Eozoön (instead of “Precambrian” this scale uses the term “Laurentian”, since the studies of Precambrian rocks had made the most progress in the Laurentian region of the Canadian Shield), image in public domain.
However the Irish geologistJohn Joly (1857-1933) suggested, after the accidentally observation during a cold morning on a muddy trail, that Oldhamia was in fact not a fossil, but a cast formed by growing ice-crystals. Later he noted that one Oldhamia species - O. antiqua - was found always in relief, another Oldhamia species - O. radiata - was found always as depression. It seemed strange that an accidental, inorganic mechanism, like freezing, would show such a distinct pattern. In fact Oldhamia is not a true fossil, despite being somehow organic in origin - Oldhamia is today reclassified as an ichnofossil, feeding tracks made by unknown organisms.
Just one year before Darwin published his book, a strange green-white-layered limestone of "Precambrian age", collected along the Ottawa River, was brought to the attention of William E. Logan (1798-1875), Director of the Geological Survey of Canada. He considered the layers to be more than just a geological coincidence. Dawson shared this view and described the strange rock in 1864 as Eozoön canadense, the "dawn animal of Canada."
Fig.2. Magnified and Restored Section of a Portion of Eozoon Canadense, image from DAWSON (1888): The Chain of Life in Geological Time (image in public domain).
Dawson reconstructed Eozoön as sort of gigantic, shelled microorganism. This interpretation was soon challenged, but only in 1894 the debate was settled by the discovery by geologists J.W. Gregory and Hugh Johnston-Lavis of similar rocks in material erupted from Mount Vesuvius. It became now evident that the strange pattern was formed by chemical alteration of limestone by the heat of the subterranean magma.
It seemed that Precambrian rocks were indeed "life-less".
In 1878 Charles Doolittle Walcott (1850-1927), a self-taught geologist working at the U.S. Geological Survey, was introduced for the first time to Cryptozoon, "the hidden life" (not to be confused with the Cryptozoologicon). Nobody was sure if these strange layered rocks, almost cabbage-like in section, were formed by fossilized microbial mats, but it seemed the best explanation, as the fossil Cryptozoon resembles modern calcareous tufa (a freshwater limestone formed by deposition of carbonate in algal mats).
Fig.3. Cryptozoon, today better known as stromatolite (Gannat, France).
Fig.4. Microscopic section of 1,5 - 2,5 mm thick tufa layers, the outer layer is characterized be still living green algae.
Walcott and other geologists started to find Cryptozoon all over the United States and Walcott described it for the first time from Precambrian rocks at the Grand Canyon. Walcott also described Chuaria, small black discs, which he interpreted first as compressed shells of a sort of clam, but are in fact the remains of planktonic algae. He continued to find Precambrian fossils and described Cryptozoon, fossil bacteria and even invertebrates from various outcrops in the Canadian Rockies. Walcott's discoveries seemed to prove without doubt that fossils were abundant even in such ancient rocks.
However Sir Albert Charles Seward (1863-1941), Professor of Botany at the University of Cambridge, studying some of the supposed algae-fossils collected by Walcott questioned their organic origin. He dismissed also the notion that Cryptozoon was formed by algae, and in his "Plant Life Through the Ages" (published 1935) he remarks that no preserved cell-structures could be found in the rock layers.
His conclusions, that Cryptozoon like Eozoön was just another fake fossil, will delay further research on early terrestrial life for almost half a century.
To be continued...
Bibliography:
JACKSON, P.N.W. (2011): History of Ichnology: John Joly (1857-1933) on Oldhamia: Poetic and Scientific Observations. Ichnos 18(4): 209-212
SCHOPF, J.W. (1999): Cradle of Life - The Discovery of earth's Earliest Fossils. Princeton University Press: 367
SCHOPF, J.W. (2009): The hunt for Precambrian fossils: An abbreviated genealogy of the science. Precambrian Research, Nr. 173: 4-9