Info

then from the carbon isotopes. The carbon atom has two stable isotopes, carbon-12 and carbon-13, usually written as 12C and 13C. The ratio of 12C to 13C, usually written 513C, can indicate the presence or absence of organic residues of previously living organisms: enrichment in 12C relative to 13C is characteristic of photosynthesizing organisms, and the organisms that eat them. Rosing and Frei (2004) reported values of 513C in organic matter from the Isua Group rocks that match those of modern living organic matter, and these might have come from plankton in the oceans that were photosynthesizing. This is a dramatic claim, and it has been disputed, but if true, this is the first evidence for life on Earth.

The Archaean world was anoxic: when did oxygen become a part of the atmosphere, and why?

The "great oxygenation event"_

The Proterozoic Eon, from 2.5 Ga to 542 Ma (Fig. 8.3), represents a very different world from the Archaean. Archaean atmospheres contained volcanic gases, but no oxygen. Oxygen levels are maintained in the atmosphere today by the photosynthesis of green plants and cyanobacteria, and the latter were the source of the initial buildup of oxygen during the first part of the Precambrian. Then, 2.4 Ga, atmospheric oxygen levels rose to one-hundredth or one-tenth of modern levels, not much perhaps, but an indicator of a complete change in the global system that has been dubbed the "great oxygenation event" (GOE). What caused this dramatic rise in oxygen?

The first organisms had anaerobic metabolisms, that is, they operated in the absence of oxygen. Indeed the first prokaryotes would have been killed by oxygen. This is a shocking fact that is confirmed by living microbes: some can switch from anaerobic to aerobic respiration depending on oxygen levels. Others, though, are obligate anaerobes that have to respire anaerobically and cannot survive even the smallest amount of oxygen. Did living things generate sufficient oxygen to change the Earth's atmosphere? Early photo-synthetic bacteria did not produce oxygen, and some have argued that modern styles of photosynthesis that liberate oxygen arose about 2.4 Ga. However, there is evidence from biomarkers (see below) that this had happened by 2.7 Ga. Others have proposed that the oxygen built up after a dramatic reduction in volcanic activity; however, there is no compelling evidence for this. Perhaps the secret lies in methane.

David Catling and colleagues at the University of Washington in Seattle proposed that methane was much more abundant in the Archaean atmosphere than today. Methane (CH4) is a key product of the activities of anaerobic microbes that use a form of anaerobic respiration called methanogenesis to breathe. Today, methane is consumed by oxygen in the atmosphere, but in the absence of oxygen Archaean methane levels might have been 100-1500 times as much as today. Methane is also a potent greenhouse gas, which would help explain why the early Earth did not freeze over, given that the 4.0 Ga sun was about 25-30% less luminous than today. Methane can diffuse up to the outer fringes of the atmosphere, where it is decomposed by ultraviolet light and the liberated hydrogen atoms are lost into space. In a world without the escape of hydrogen, Catling and Claire (2005) have calculated that oxygen would be mopped up continuously by gases released by volcanism and metamorphism, as well as by soluble metals in hot springs and seafloor vents, and the world would remain forever anaerobic. With high Archaean methane levels, hydrogen atoms were transferred out of the Earth's atmosphere, and the oxygen was not all locked up in water molecules but eventually flooded out as an atmospheric gas. The collapse of the methane greenhouse 2.4 Ga may have triggered glaciation worldwide.

The rise of oxygen in the atmosphere had a profound effect on life and the planet. New aerobic organisms arose that exploited the atmospheric oxygen molecules in their chemical activity. The oxygen also built up a stratospheric ozone layer that blocks out solar ultraviolet radiation. The ozone layer has been hugely important since this point in the earliest Proterozoic in blocking solar rays harmful to life, which allowed diverse life to colonize the land surface

After the GOE, oxygen levels remained low, perhaps 1-5% of present levels, for as much as 1 billion years. In the Archaean, banded iron formations occurred worldwide; these consist of alternating bands of iron-rich (magnetitic/hematitic) chert and iron-poor chert (chalcedony). In the Archaean, iron released from vents in the seafloor was mobile in the deep ocean and welled up onto the continental shelves. This is unlike today, where oxygen extends to the bottom of the sea and iron is immediately deposited as an oxide on the flanks of mid-ocean ridges. The banding in banded iron formations may reflect seasonal plankton blooms that released a great deal of oxygen into the surface ocean, which combined with upwelling iron ions to produce the iron-rich layers. About 1.9 Ga, banded iron formations largely disappear. Continental red bed sediments had first appeared at approximately 2.3 Ga, following the rise of oxygen. These red beds indicate higher oxygen levels because the red color comes from weathering of the iron in the rocks in the presence of atmospheric oxygen. A second rise of oxygen around 0.8-0.6 Ga is indicated by increased levels of marine sulfate. Oxygenated rainwater reacts with pyrite on the continents and washes sulfate through rivers to the oceans, so an increase in oceanic sulfate suggests an increase in oxygen.

The two rises in oxygen levels, at the beginning and end of the Proterozoic, respectively, mark the beginning of modern-style biogeo-chemical cycles, in which oxygen and carbon are exchanged continuously between living organisms and the Earth's crust.

The universal tree of life

There used to be a quiz show on British radio called Animal, vegetable or mineral? in which a team of scientists had to identify mystery items. Each week, members of the public would send packages of strange tubers, dried internal organs and other revolting fragments for the experts to consider. The division of natural objects into two living (animal, vegetable) and one non-living (mineral) category reflects the common view that life may be divided simply into plants (generally green, do not move) and animals (generally not green, do move). To these two might be added microbes (for all the microscopic critters).

The three-kingdom view was expanded to four by the division of "microbes" into two kingdoms, Protoctista for single-celled eukary-otes and Monera for prokaryotes. Four kingdoms became five in 1969 when Robert Whittaker recognized that Fungi (mushrooms and molds), classed by chefs as plants, are fundamentally different from all other plants.

This five-kingdom picture of life was blown out of the water by a series of revolutionary papers by Carl Woese and colleagues from the University of Illinois from 1977 onwards. Woese and George Fox had been working on molecular phylogenies (see p. 133) of pro-karyotes, and they realized that prokaryotes fell into two fundamental divisions, the domains Archaea (named Archaebacteria by Woese and Fox in 1977) and Bacteria (or Eubacteria). The third domain is Eucarya (or Eukaryota), for all eukaryotes. In this view, animals, plants and fungi are then distant twigs within Eucarya. Woese had generated the first universal tree of life (UTL). It is likely that the Archaea and Bacteria split first, and then the Eukarya split from the Bacteria, but the root of the UTL is still uncertain.

Further work since 1990 has confirmed Woese's insight, although alternative schemes talk of two domains or six kingdoms, and other subdivisions. With the power of modern gene sequencing, it should have been relatively easy to build the UTL with progressively more detail. One of the largest versions of the UTL consists of 191 organisms for which complete genome sequences have been established (Ciccarelli et al. 2006). However, molecular biologists had not at first contemplated the notion of jumping genes: simple organisms seem to be prone to exchanging genes in a process called horizontal gene transfer. Genes can be transferred between eukaryotes, but the process is commoner among prokaryotes. Horizontal gene transfer occurs in bacteria today that take up DNA directly from their surroundings, through infection from a phage virus, or through mating. Jumping genes make the task of the phylogenetic sequencer difficult: parts of the genome may show linkages to one group, while jumping genes may link the organism to another. Once a jumping gene has been identified, however, it may become locked into the genome of all descendants, and so provide evidence for the affiliation of all organisms that possess it.

Bacteria

Archaea

Eucarya

Green non-sulfur bacteria

Purple bacteria

Cyanobacteria Flavobacteria

Methanosarcina acetivorans thermoautotrophicum Aeropyrum (L6 Mb) solfaticarus

Animals Slime

-Plants

Bacteria

Archaea

Eucarya

Green non-sulfur bacteria

Purple bacteria

Cyanobacteria Flavobacteria

Animals Slime

-Plants

Microsporidia

Figure 8.4 The universal tree of life, based on molecular phylogenetic work. The major prokaryote groups are indicated (Bacteria, Archaea), as well as the major subdivisions of Eucarya. Among eukaryotes, most of the groups indicated are traditionally referred to as "algae", both single-celled and multicelled. The metaphytes (land plants), fungi and metazoans (animals) form part of a derived clade within Eucarya, indicated here near the base of the diagram. Mb, megabase (= 1 million base pairs). (Courtesy of Sandie Baldauf.)

Microsporidia

Figure 8.4 The universal tree of life, based on molecular phylogenetic work. The major prokaryote groups are indicated (Bacteria, Archaea), as well as the major subdivisions of Eucarya. Among eukaryotes, most of the groups indicated are traditionally referred to as "algae", both single-celled and multicelled. The metaphytes (land plants), fungi and metazoans (animals) form part of a derived clade within Eucarya, indicated here near the base of the diagram. Mb, megabase (= 1 million base pairs). (Courtesy of Sandie Baldauf.)

The broad patterns of the UTL are not completely resolved (Fig. 8.4) because of jumping genes and other problems: the three domains branch equally, and it is not clear which split came first, between Bacteria and Archaea, or Archaea and Eucarya (Baldauf et al. 2004; Doolittle & Bapteste 2007; McInerney et al. 2008). Until the order of branching is resolved, if it can be, there will be many mysteries about the origin of life. The Domain Bacteria includes Cyanobacteria and most groups commonly called bacteria. The Domain Archaea ("ancient ones") comprises the Halobacteria (salt digesters), Meth-anobacteria (methane producers) and Eocytes (heat-loving, sulfur-metabolizing bacteria). The Domain Eucarya includes a complex array of single-celled forms that are often lumped together as "algae", a paraphyletic group. Among the "algae" are green algae, flagellates and slime molds, and a crown clade consisting of multicellular organisms. Perhaps the most startling observation is that, within this crown clade, the fungi are more closely related to the animals than to the plants, and this has been confirmed in several analyses. This poses a moral dilemma for vegetarians: should they eat mushrooms or not?

Precambrian prokaryotes_

The question of the oldest fossils on Earth has always been controversial. Paleontologists are understandably keen to identify that very first fossil (it is a sure-fire way to attract attention and secure tenure), but that very first fossil is going to be pretty tiny and pretty featureless. How then can the Precambrian paleontologist be sure to identify the fossils correctly, and not be fooled by some whisker or bubble on a microscope slide? The first Archaean fossils were identified only in the 1950s, and over the last decades each new announcement is actively challenged to ensure the specimens are genuine. The latest furor has concerned the reputed microfossils from the 3.5 Ga Apex Chert of Australia (Box 8.1).

The first traces of life occur in rocks dated from 3.5 to 3.0 Ga. These include structures identified as possible stromatolites from various parts of the world. Modern stromatolites are constructed by cyanobacteria and other prokaryotes (Fig. 8.6). Cyanobacteria live in shallow seawater, and they require good light conditions to enable them to pho-tosynthesize. The cyanobacteria form thin mats on the seafloor in order to maximize their intake of sunlight, but from time to time the mat is overwhelmed by sediment. The microbes migrate towards the light, and recol-onize the top of the sediment layer, which may again be swamped by gentle seabed currents. Over time, extensive layered structures may build up. In freshwaters, and sometimes in the sea, stromatolites build up by precipitation of calcite. In most fossil examples, the construct ing microbes are not preserved, but the layered structure remains. Many early examples have proved controversial, but the oldest that are generally accepted come from Australia, and are dated as 3.43 Ga (see p. 290).

Perhaps the oldest currently accepted fossils other than stromatolites date from 3.2 Ga. They were found in Western Australia by Birger Rasmussen, and reported in 2000, from

Box 8.1 The Apex Chert: oldest life or hot air?

There was a sensation when Bill Schopf announced the world's oldest fossils in 1987 (Schopf & Packer 1987). He later reported a diverse assemblage of 11 species of bacteria and cyanobacteria from the Apex Chert of the Warrawoona Group in Western Australia, dated as 3465 Ma (Schopf 1993). All specimens are filament-like microbes, ranging in length from 10 to 90 |im; some are circular single cells, while most are filaments consisting of several compartments (Fig. 8.5). These were widely accepted as genuine fossils, and they featured in all the textbooks and web sites as real examples of the earliest cyanobacteria and bacteria.

But their validity was challenged in April 2002. At the second Astrobiology Science Conference held at NASA's Ames Research Center in Moffett Field, California, there was a bombshell. As reported in Nature:

It was the academic equivalent of a heavyweight prizefight. In the red corner, defending his title as discoverer of the Earth's oldest fossils, was Bill Schopf of the University of California, Los Angeles (UCLA). In the blue corner, Martin Brasier of the University of Oxford, UK, who contends that Schopf's "microfossils" are merely carbonaceous blobs, probably formed by the action of scalding water on minerals in the surrounding sediments.

0 0

Post a comment