The origin of life and the early biosphere

Planet Earth is believed to have formed from the coalescence of dust particles at some time close to 4.55 Ga. While this accretion and the ensuing phase of catastrophic impacts would have caused a molten surface, the crust appears to have been cool by about 3.85 Ga. If any life forms were synthesized before this date they must have been hyperthermophile heat-tolerant bacteria, similar to those found living around volcanic vents or deep in the Earth's crust today. The oldest rocks on the Earth are found in Western Australia and northern Canada dated at ~4 Ga and the Isua Group from western Greenland, dated at ~3.8 Ga. The Isua rocks are a mix of abiogenic limestones, sandstones and pillow lavas. These rocks formed under water and indicate a crust had stabilized and oceans were present (Fig. 6.1).

Origins of life

The Oparin-Haldane hypothesis for the origins of life (Fig. 6.2) envisaged that the primitive atmosphere was reducing and contained CO2, CO, H2 NH3, CH4 and H2O but no O2. It is now thought that NH3 and CH4 would have been unstable in the early atmosphere. A scarcity (but not a lack) of oxygen is a reasonable assumption given the existence of pyrite conglomerates before 2 Ga (Figs 6.1, 6.2) and the derivation of nearly all O2 in the modern atmosphere from photosynthesis. Experiments by Miller & Urey (Miller 1953) showed that amino acids may be synthesized from a mixture of these gases and water, through which ultraviolet light or electric discharge (cf. lightning) has passed, especially if temperatures are kept below 25°C. In fact, temperatures close to freezing can conserve nucleic acids much better, and it has been suggested that nucleic acids and ultimately DNA could have been synthesized in as little as 10,000 years. It is difficult to reconcile glacial conditions, however, with other indications for a very warm greenhouse world at this time.

The panspermia hypothesis (Fig. 6.2) suggests that prebiotic materials in space seeded the surface of the planet during the phase of massive meteorite bombardment until about 3.8 Ga. Simple organic compounds such as HCN, formic acid, aldehydes and acetylenes are certainly abundant in a group of meteorites known as carbonaceous chondrites, as well as in the 'heads' of comets and in some interstellar dust clouds. An extreme version of this hypothesis is that DNA may also be found in space. Experiments certainly suggest that DNA can tolerate high radiation doses when desiccated and at low temperatures.

The hydrothermal hypothesis (Fig. 6.2) argues that amino acid to DNA synthesis took place around hot, alkaline hydrothermal vents, possibly like the 'black smokers' associated with modern mid oceanic ridges (Russell & Hall 1997). Further support for this model is provided by molecular sequence evidence.

Did life originate on Mars?

In August 1996 Dr David McKay and a team from NASA announced to the world that they had found possible microfossils and geochemical evidence consistent with life in ALH 84001, a martian meteorite, confirmed by a distinctive 15N/14N isotopic ratio. Full details of this exciting discovery can be found in Treiman (2001). The orthopyroxene minerals in the meteorite crystallized ~4.5 Ga ago, it had suffered

MaBP 0-

1000

1600

2000

3000

3800 4000

4550

ENVIRONMENTAL EVOLUTION

Glaciations CO2

BIOLOGICAL EVOLUTION

High 02

Mammals diversify c. 60Ma

First land vertebrates c. 250 Ma

First land plants c. 410 Ma

Skeletal fossils diversify c. 543 Ma Ediacara fauna c. 564-543 Ma Animals diversify

Stepwise increase in oxygen owing to tectonics?

First red algae, brown algae, c. 1100 Ma Eukaryotes diversify Evolution of sexual reproduction

Stepwise increase in oxygen owing to tectonics?

- Earliest eukaryotic biomarkers

Oldest known sedimentary rocks: Isua Group, Greenland

— Oldest prokaryotes: Warawoona,

Australia, c. 3450 Ma

- First carbon isotopic indicators for life, Greenland, c. 3700 Ma

Impact frustration of the origin of life Interval of massive meteorite bombardment

Prebiotic organic synthesis widespread through solar system

Formation of the Earth and Solar system

Fig. 6.1 The main succession of events inferred for the evolution of the biosphere alongside geological evidence for changing levels of atmospheric oxygen and carbon dioxide during the Precambrian. (Modified from Brasier et al. 2002.)

Cometary showers containing HCN, formic acid, aldehydes, \ panspermia hypothesis acetylenes x

Fig. 6.2 Hypothesis for the origins of life on Earth (from various sources).

Amino acids, proteins andDNA? Bombardment by meteorites, e.g. from Mars or Asteroid Belt

Oparin-Haldane hypothesis

Amino acids, proteins andDNA? Bombardment by meteorites, e.g. from Mars or Asteroid Belt

Amino acids, proteins and DNA

UV light or electric discharge

Alkaline hydrothermal vents Hydrothermal hypothesis

Amino acids, proteins and DNA

Fig. 6.3 Reported biogenic structures from ALH 84001. (a) Carbonate globule. (b), (c) Scanning electron micrographs of elliptical and rod-like structures. The specimen in (c) is approximately 2 |m long. (Photographs courtesy of the Lunar Planetary Institute.)

impact shocks at 4 Ga and 15 Ma ago and landed in Antarctica 13,000 years ago. In addition to the many lines of evidence proposed by McKay et al. (1996) in support of life, zoned carbonate globules (Fig. 6.3a) were thought to provide evidence for the existence of liquid water essential for life. The geochemistry of these globules suggested bacteria-like metabolism and the presence of organic compounds thought to have been derived from microbial degradation. More provocatively, they described bacteria-like microfossils (e.g. Figs 6.3b,c). Some have suggested that this is direct evidence that life originated on Mars, though others have strongly criticized this interpretation (e.g. Grady et al. 1996; Bradley et al. 1997).

Evidence for the earliest biosphere

The fossil evidence for life on Earth gets increasingly scarce as age increases. This is because older rocks have suffered more exposure to erosion and a greater chance of alteration by metamorphism. The rules for accepting microfossil-like objects as evidence for life include them being demonstrably biogenic and indigenous to the formation of the rocks of known provenance. Biogenicity is the most difficult to demonstrate, as with the martian objects. Whilst the oldest sedimentary rocks on Earth have been too heavily metamorphosed to yield preserved microfossils, molecular and biochemical evidence indicates life may have existed when these rocks were deposited. Evidence no longer indicates that life was already established on the Earth by 3.5 Ga (Brasier etal. 2002).

Evidence from molecular sequences and biogeochemistry

Comparisons of the rRNA sequences and ultrastructures of diverse bacteria, protozoa, fungi, plants and animals have recently resulted in two contrasting views on the origin and early evolution of life. The currently widely accepted hypothesis, based largely upon rRNA phylogeny (Woese et al. 1990), views life on Earth as three primary domains: the Archaebacteria (or Archaea), which includes the autotrophic methano-genic and sulphur bacteria; the Eubacteria (or Bacteria), including cyanobacteria and the Eukaryota (or Eukarya), including all the protozoa, fungi, plants and animals (Fig. 6.4). The following represents the chronological appearance of grades within autotrophic prokaryotes (that used carbon dioxide as their sole

Fig. 6.4 The threefold branches of the tree of life, in which all the deep seated branches are taken by hyperthermophilic bacteria (shown in bold). The approximate times of branching are shown. Time increases along the branches, but not necessarily in a linear fashion nor at the same rate in each branch. Longer branches relate to faster evolution. The times of branching are speculative and are hotly contested. (Adapted from Woese et al. 1990, Sogin 1994 and Nisbet & Fowler 1996; Brasier 2000.)

Fig. 6.4 The threefold branches of the tree of life, in which all the deep seated branches are taken by hyperthermophilic bacteria (shown in bold). The approximate times of branching are shown. Time increases along the branches, but not necessarily in a linear fashion nor at the same rate in each branch. Longer branches relate to faster evolution. The times of branching are speculative and are hotly contested. (Adapted from Woese et al. 1990, Sogin 1994 and Nisbet & Fowler 1996; Brasier 2000.)

source of carbon) based upon the traditional phylo-genetic interpretation:

1 anaerobic chemolithotrophic bacteria, which mainly use H2 produced from inorganic reactions between rock and water as their main electron source;

2 anaerobic anoxygenic bacteria such as green and purple sulphur bacteria, which use photosynthesis to reduce CO2 to form organic matter, with H2S as the electron source, in the absence of oxygen;

3 oxygenic cyanobacteria, use photosynthesis to reduce CO2 to form organic matter, with H2O as the electron source, in the presence of oxygen.

In the traditional scenario the deepest roots of the tree of life are occupied by hyperthermophilic bacteria (Fig. 6.4). At the present day these are adapted to life in hot hydrothermal springs or life deep in the Earth's crust, at temperatures of >80°C or more and are seldom able to grow below 60°C. This fact has been taken as evidence to suggest that the last common ancestor of all living organisms was a hyperthermophile (Nisbet & Fowler 1996). This proposal is however inconsistent with the fundamental ultrastructural differences to be found within the prokaryotes (i.e. monoderms having a single cell membrane and diderms with a double cell membrane) and phylogenetic trees based on signature protein sequences or 'indels'. A second hypothesis (Gupta 1998, 2000) recognizes the uniqueness of the Eukaryota and Prokaryota but points to fundamentally different divisions and evolution within prokaryotes. Specifically, a close relationship is envisaged between the Archaebacteria and the gram-positive bacteria (Eubacteria), both of which are monoderm prokaryotes and distinct from the rest of life. This hypothesis postulates the earliest prokaryote was a gram-positive bacterium from which the Archaebacteria and diderm prokaryotes evolved in response to selection pressures exerted by antibiotics produced by some grampositive bacteria. Accepting this hypothesis (and the underlying phylogenetic methodology) allows for the evolution of the early forms of life from a common ancestor through gram-positive (Low G + C; Archaebacteria), gram-positive (High G + C; Archaebacteria), Deinococcus Group, Green non-sulphur bacteria, Cyanobacteria, Spirochetes, Chlamydia-Green sulphur bacteria to Proteobacteria.

The gram-positive (Low G + C) bacteria appear to be the earliest Eubacteria and include anoxygenetic photosynthetic organisms (e.g. Heliobacterium). The phylogenetic inference of this is that the common ancestor of all life on Earth was a photosynthetic anaerobe. If this hypothesis is correct then the major evolutionary changes have, and will continue to have, a linear line of descent.

Geochemical proxies for early life

Biomarkers

The three domains of life contain within their cell walls diagnostic molecules called lipids which turn into hydrocarbons in sediments. 2-Methyl-bacteriohopane-polyols (2-methyl-BHP) are characteristic of the cell walls of cyanobacteria and are found in cyanobacterial mats. These are converted to 2a-methylhopanes in sediments found in high abundance in bitumens in the ~2.5 Ga Mt McRae Shale of the Hamersley Basin in Western Australia (Summons et al. 1999) and indicate that oxygenic photosynthesis was important by this time.

Stable carbon isotopes

The carbon isotopic record of the Archean is still poorly known and there are large ranges of values for specific time intervals (Fig. 6.5a). Carbonates in the Isua Group rocks, as old as 3.8 Ga, have S13Ccarb signatures close to 0% comparable to modern marine bicarbonate (Fig. 6.5b). Organic matter of this age yields S13Corg values of -15% lighter than those of associated carbonates, comparable to the light isotopic values found in modern living organic matter. Some scientists have argued this as evidence for oxygenic photosynthesis during the deposition of the Isua sediments but this is highly contentious. The Isua rocks have both more negative S13Ccarb and less negative organic §13Corg values than those in later sediments, indicating values from the Isua may be entirely due to metamorphism. Similar data have also led to claims that organic matter within phosphatic grains from metasediments in the Itsaq Group of Greenland (~3.85 Ga) provides evidence for a biological origin (Mojsis et al. 1996). This claim is controversial as the sedimentary origin of the phosphate is questionable

Archean Organic Carbon Isotope Record

flowering plants

Fig. 6.5 (a) Changes in stable carbon isotope values through the history of life. Values are expressed as 813Ccarb from carbonates and S13Corg from kerogen samples. (b) S13C values of modern autotrophs and recently oxidized inorganic carbon. This figure is commonly described as the Schidlowski diagram. (From Schidlowski 1988, figure 4, permission from Nature. Copyright © 1988 Macmillan Magazines Ltd.)

marine bicarbonate cyanobacteria flowering plants

green sulphur bacteria al9ae purple atm. CO

sulphur methanogens , bacteria , anoxygenic photosynthetic bacteria living autotrophic organisms

Fig. 6.5 (a) Changes in stable carbon isotope values through the history of life. Values are expressed as 813Ccarb from carbonates and S13Corg from kerogen samples. (b) S13C values of modern autotrophs and recently oxidized inorganic carbon. This figure is commonly described as the Schidlowski diagram. (From Schidlowski 1988, figure 4, permission from Nature. Copyright © 1988 Macmillan Magazines Ltd.)

and the age of the phosphate grains may also be significantly younger, around 3.7 Ga (Kamber & Moorbath 1997).

The mean S13Corg for 3.5-Ga-old sediments is —26%o, falling within the range of S13Corg values for living anaerobic autotrophic bacteria. A major negative S13Corg excursion of —50% has been found at around 2.7 Ga with a further negative excursion of similar scale at 2.1 Ga. The causes of these excursions are not known. It has been suggested that large amounts of carbon burial during this time brought about a stepwise increase in the oxygen levels in the atmosphere (Karhy & Holland 1996). It may be coincidence that it is only after this event that unequivocal eukaryote organization is found.

Sulphur isotopes

Sulphur isotopes can also be used to trace the history of sulphate reduction. In this case, 32S is preferentially taken up by sulphate-reducing bacteria, leaving the water column enriched in the heavier isotope 34S. Studies of 34S/32S (S34S) ratios in sedimentary pyrite and in gypsum and anhydrite have shown that sulphate reduction may not have taken place before 2.8 Ga. This may be because there was insufficient free oxygen in the atmosphere to form the sulphate ions needed for sulphate reduction or that surface water temperatures were too high to produce a measurable fractionation.

Banded iron formations (BIFs)

BIFs are deeper water sediments that show millimetric laminations of Fe2O3-rich (hematitic) chert and iron-poor chert (chalcedony). These laminations can be remarkably continuous - some have been traced for up to 300 km. BIFs were particularly common in Archean and Palaeoproterozoic marine basins between about 3.5 and 1.8 Ga. Their presence suggests that some kind of seasonal 'rusting' of the oceans took place, in which oxygen released by blooms of photosynthetic microbes was mopped up by Fe2+ ions in solution. These ions were widely available in the water column owing to the reducing chemistry of the early oceans and widespread hydrothermal exhalation. Settling of hematite precipitates through the water column formed laminae on the ocean floor. This interpretation does not require the production of oxygen by photosynthesis, since this oxygen may have a source from photodissociation of water or even from volcanic oxidized mineral species.

After about 1.8 Ga, BIFs seldom appear in the rock record and continental red beds begin to become widespread. This suggests that the ferrous iron oxygen sink had become saturated, and that oxygen was now able to accumulate in the atmosphere, leading to the oxygenic weathering of terrestrial rocks.

Archean fossils

Stromatolites (Fig. 6.6)

These sedimentary structures (see Chapter 8) are known to occur in carbonate rocks as old as 3.5 Ga in the Pilbara Supergroup of Western Australia (Fig. 6.6r) and 3.4 Ga in the Swaziland Supergroup of South Africa. Although an origin from the growth of cyano-bacterial mats has often been inferred, they do not

Fig. 6.6 Pseudofossils, stromatolites and microfossils from the Archaean and Proterozoic. (a) Corner of the 'microfossiliferous' clast reported by Schopf (1993) from 3.46 billion year old Apex chert, reinterpreted by Brasier et al. (2002) as a shard within a subsurface hydrothermal dyke. (b—1). Detailed views of Earth's oldest supposed 'microfossils' (shown at white arrows in (a)), regarded by Brasier et al. (2002) as carbonaceous pseudofossils. (k) Pseudofossil Primaevifilum delicatulum. (n-o) Similar pseudofossils from resampled cherts. (p) Pseudofossil Eoleptonema apex from the 3.46 Gyr Apex chert, showing angular morphology caused by wrapping around crystal margins, shown alongside original interpretation as a beggiatoan bacterium (inset, photographs and drawing at right). (q) Pseudofossil Archaeoscillatoriopsis disciformis from the 3.46 Gyr Apex chert, showing branched morphology and proximity to crystal growths (arrowed) alongside original interpretation as an oscillatoriacean cyanobacterium (inset, photographs and drawing at right). (r) 3420 Myr 'stromatolite' from the Strelley Pool chert of Western Australia, controversially claimed as earliest evidence on Earth for microbial entrapment of sediment. (s, t) Proterozoic (1900 Myr) filaments of unquestioned biogenic origin (probable iron bacteria), Gunflint chert of Mink Mountain, Ontario, Canada. Black scale bar: (c) = 10 cm. White scale bar: (a) = 400 |m; (l, t) = 100 |m; (b-k, m-q, s) = 40 |m.

contain microfossils and they have simple rotational symmetry and isopachous sedimentary laminae. It has also been shown that many Archean stromatolites may have formed by the direct precipitation of aragonite from sea water. The evidence from stromatolites is therefore less than conclusive. Even though the size, shape and millimetre scale laminations within these structures are a little bit like those of younger, both fossil and modern, stromatolites, some of which may also be abiogenic.

Silicified microbiotas

Early diagenetic silica has preserved the cells of pro-karyotic and even eukaryotic microorganisms at a number of localities with latest Archean and Paleo-proterozoic rocks (c. 2.7-1.8 Ga; Fig. 6.6(s, t)). These microfossils, which can be well preserved in three dimensions, are usually studied by means of standard petrographic thin sections at high magnification. Most of these chert microbiotas are associated with stroma-tolitic carbonates in evaporitic settings.

One of the oldest cherts to have yielded a supposed bacterial microflora is associated with basalt lava flows in the Warrawoona Group of Western Australia, dated at 3.465 Ga (Schopf 1992) and from the Barberton mountains of South Africa. Eleven species of bacterial cells and cyanobacterial filaments have been described from the Apex Chert within the Warrawoona Group and were once taken as the oldest morphological evidence for life on Earth. The structures are nearly 1 Ga older than putative cyanobacterial biomarkers. Recent reanalysis of these 'microfossils' has led to questions about their authenticity (Brasier et al. 2002), and further work shows they are pseudofossils formed by recrystallization of the chert.

An even more famous microbiota preserved in the Gunflint Chert (~1.9 Ga) comprises about 12 species, some of which closely resemble modern coccoid and filamentous cyanobacteria, while others resemble iron bacteria (Schopf & Klein 1992; see Fig. 8.2).

Palynology of shales

The techniques of palynological maceration (see Appendix) have been applied to organic-rich Archean and Proterozoic shales, with interesting results. In rocks older than about 1.8 Ga, the macerations consist largely of very small (10-20 |im) and relatively simple compressed spheres which have been called cryptarchs, owing to their uncertain biological affinities. These could be the remains of either benthic or planktonic cyanobacterial spores. After about 1.8 Ga, there was a slow increase in size and complexity, suggestive of the progressive development of morphologies relating to eukaryotic protozoan organization.

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'nanofossils' in Martian meteorite. Nature 390, 454-456. Brasier, M.D. 2000. The Cambrian explosion and the slow burning fuse. Science Progress 83, 77-92. Brasier, M.D., Green, O.R., Jephcoat, A.P., Kleppe, A.K., Van Kranendonk, M.J., Lindsay, J.F., Steele, A. & Grassineau, N.V. 2002. Questioning the evidence for Earth's oldest fossils. Nature 416, 76-81. Grady, M., Wright, I. & Pillinger, C., 1996. Opening a Martian can ofworms? Nature 382, 575-576. Gupta, R.S. 1998. Protein phylogenies and signature sequences: a reappraisal of evolutionary relationships among Archaebacteria, Eubacteria and Eukaryotes. Microbiology and Molecular Biology Reviews 62, 1435-1491. Gupta, R.S. 2000. The natural evolutionary relationships among prokaryotes. Critical Reviews in Microbiology 26, 111-131.

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