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Figure 8.5 Postulated prokaryotes from the Apex Chert of Western Australia (c. 3465 Ma) showing filament-like microbes preserved as carbonaceous traces in thin sections. All are examples of the prokaryote cyanobacterium-like Primaevifilum, which measures 2-5 |im wide. (Courtesy of Bill Schopf.)

Brasier and colleagues (2002) had argued the month before that Schopf's "microfossils" were found in a chert that had not formed in shallow seas, but at high temperature in a hydrothermal vein. Any microbes in the solidifying rock would have been roasted. So the "microfossils", said Brasier, must be inorganic structures. Brasier and his colleagues then examined the original specimens, and found that many had been selectively photographed, so that the full complexity of some shapes was not seen in Schopf's published photographs. Many of the "filaments" were extensions of more complex blobs and cavities in the chert, and some showed branching and other features unlikely in a simple prokaryote. Further, the 11 supposed species could not be distinguished, and all kinds of intermediate shapes were found. Brasier believes the "microfossils" are traces of graphite in hydrothermal vein chert and volcanic glass. At high temperature the graphite flowed, forming black, carbon-rich strings and blobs.

Schopf and colleagues (2002) countered that the carbon traces were formed from living material, and they applied a new technique, laser Raman spectroscopy, to prove it. They noted that the spectral bands of the Apex Chert fossils matched signals from known biological materials. But Brasier rebutted this by suggesting that the Raman spectra cannot uniquely identify biological carbon, but simply match color and grain size between areas of a specimen. Their Raman spectra suggested that the "microfossils" and the rock matrix consisted of graphite and silica.

Read more about the dispute at http://www.blackwellpublishing.com/paleobiology/. The debate is renewed in articles by Brasier, Schopf and other commentators in a special issue of the Philosophical Transactions of the Royal Society in 2006 (Cavalier-Smith et al. 2006).

Figure 8.6 Stromatolites, a Precambrian example from California, USA (magnification x0.25). (Courtesy of Maurice Tucker.)

Figure 8.7 The oldest fossils on Earth? A mass of thin thread-like filaments found in a massive sulfide deposit in Western Australia dated at 3.2 Ga. The fact the threads occur in loose groups and in tight masses, and that they are not oriented in one direction, suggests they are organic. The filaments are lined with minute specks of pyrite, showing black, encased in chert. Field of view is 250 |im across. (Courtesy of Birger Rasmussen.)

Figure 8.7 The oldest fossils on Earth? A mass of thin thread-like filaments found in a massive sulfide deposit in Western Australia dated at 3.2 Ga. The fact the threads occur in loose groups and in tight masses, and that they are not oriented in one direction, suggests they are organic. The filaments are lined with minute specks of pyrite, showing black, encased in chert. Field of view is 250 |im across. (Courtesy of Birger Rasmussen.)

a massive sulfide deposit produced in an environment like a modern deep-water black smoker, with temperatures up to 300°C. The fossils show evidence of recrystallization by the influx of hydrothermal fluids, and then progressive replacement by later sulfides. The fossils are thread-like filaments (Fig. 8.7) that may be straight, sinuous or sharply curved, and even tightly intertwined in some areas. The overall shape, uniform width and lack of orientation all tend to confirm that these might really be fossils, and not merely inorganic structures. If so, they confirm that some of the earliest life may have been thermophilic ("heat-loving") bacteria. Other tubes and filaments of similar age have been reported, but many of these are highly controversial.

There is then a long gap in time until the next generally accepted fossils. These are diverse fossils of cyanobacteria from the Campbellrand Supergroup of South Africa, dated at 2.5 Ga (Altermann & Kazmierczak 2003). The fossils include cell sheaths and capsules that can be identified with modern orders of cyanobacteria. There is then a further long time gap before the next assemblage of prokaryote fossils, from the Gunflint

Chert of Ontario, Canada, dated at 1.9 Ga. The Gunflint microorganisms include six distinctive forms, some shaped like filaments, others spherical, and some branched or bearing an umbrella-like structure (Fig. 8.8). These Precambrian unicells resemble in shape various modern prokaryotes, and some were found within stromatolites. Most unusual is Kakabekia, the umbrella-shaped microfossil (Fig. 8.8b); it is most like rare prokaryotes found today at the foot of the walls of Harlech Castle in Wales. These modern forms are tolerant of ammonia (NH3), produced by ancient Britons urinating against the castle walls; so were conditions in Gunflint Chert times also rich in ammonia?

Biomarkers

Even if the oldest fossils are controversial, paleontologists have been able to identify another source of information on early life. These are so-called biomarkers, organic chemical indicators of life in general, and of particular sectors of life. Most biomarkers are lipids, fatty and waxy compounds found in living cells. For a long time, the oldest accepted biomarkers dated from 1.7 Ga, but Brocks et al. (1999) reported convincing examples from organic-rich shales in Australia dated at 2.7 Ga. The biomarkers they identified were not only 1 billion years older than previous examples, they also proved a wider diversity of life at that time than anyone had suspected.

The 2.7 Ga biomarkers were of two types. First were indicators of cyanobacteria, as might be expected. Brocks and colleagues identified 2-methylhopanes, which are known to be breakdown products of 2-methylbacteriohopanepolyols, specialized lipids that are only found in the membranes of cyanobacteria. The investigators also, unexpectedly, identified C28-C30 steranes, which are sedimentary molecules derived from sterols. Such large-ring sterols are synthesized only by eukaryotes, and not by pro-karyotes. Moreover, the biochemical synthesis of such large sterols requires molecular oxygen, so that the eukaryotes likely lived in proximity to oxygen-producing cyanobacte-ria, strengthening the interpretation of the 2-methylhopanes. So, this biomarker evidence confirms the existence of cyanobacteria at

(a)

Figure 8.8 Prokaryote fossils from the Gunflint Chert of Ontario, Canada (c. 1.9 Ga): (a) Eosphaera, (b) Kakabekia, and (c) Gunflintia. Specimens are 0.5-10 |im in diameter. (Redrawn from photographs in Barghoorn & Taylor 1965.)

least 2.7 Ga, but it is also the oldest hint of the occurrence of eukaryotes, long before any fossils of that major life domain.

LIFE DIVERSIFIES: EUKARYOTES

Eukaryote characters_

Evidence about the earliest evolution of the three domains is scant. It has long been assumed that prokaryotes (i.e. Archaea and Bacteria) were the sole life forms for a billion years or more, and that eukaryotes came much later. This evidence is much more blurred now (Embley & Martin 2006), and the fossils, biomarkers and molecular evidence suggest that eukaryotes might be as old as one or other of the prokaryote domains. The appearance of eukaryotes was important, whenever it happened, because they are complex and include truly multicellular and large organisms.

Eukaryotes are distinguished from pro-karyotes (Fig. 8.9a, b) by having a nucleus containing their DNA in chromosomes (pro-karyotes have no nucleus, and they have only a circular strand of DNA) and cell organelles, that is, specialized structures that perform key functions, such as mitochondria for energy transfer, flagella for movement and chloro-plasts in plants for photosynthesis. There are also many major biochemical differences between prokaryotes and eukaryotes.

The origin of eukaryotes is mysterious because they are in many ways so different from prokaryotes. The most attractive idea for their origin is the endosymbiotic theory, proposed by Lynn Margulis in the 1970s. According to this hypothesis (Fig. 8.9c), a prokaryote consumed, or was invaded by, some smaller energy-producing prokaryotes, and the two species evolved to live together in a mutually beneficial way. The small invader was protected by its large host, and the larger organism received supplies of sugars. These invaders became the mitochondria of modern eukaryote cells. Other invaders may have included worm-like swimming prokaryotes (spirochaetes) that became motile flagella, and photosynthesizing prokaryotes that became the chloroplasts of plants.

The endosymbiotic model is immensely attractive, and some aspects have been confirmed spectacularly. Most notable is that the mitochondria and chloroplasts in modern eukaryotes are confirmed as prokaryotes, the mitochondria being closely related to a-proteobacteria and the chloroplasts to

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