ISSN 00310301, Paleontological Journal, 2010, Vol. 44, No. 7, pp. 839–850. © Pleiades Publishing, Ltd., 2010

Life Conditions on the Early Earth after 4.0 Ga A. Yu. Rozanov Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997, Russia email: [email protected] Received September 25, 2009

Abstract—The organization level of Precambrian fossils is the most reliable indicator of the state and param eters of the biosphere, such as the atmosphere composition, average temperature of the earth’s surface, and others. At present, cyanobacteria, unicellular and multicellular eukaryotes, and coelomates are considered to appear in the geological history of the Earth much earlier than it was supposed previously. Our knowledge and ideas of the early Earth are very important for considering the problems of the origin of life. A key boundary of the earliest period was probably about 4 Ga. This boundary is between the periods documented and undoc umented by the geological record. The Earth history and probable surface conditions before 4 Ga are consid ered by L.M. Mukhina, A.V. Vityazeva, G.V. Pechernikova, and L.V. Ksanfomaliti in this volume. Key words: Early Earth, atmosphere composition, life conditions. DOI: 10.1134/S0031030110070129

INTRODUCTION Earth environments after 4 Ga have been discussed in many publications. The models of Earth evolution after 4 Ga are also numerous (Dobretsov, 2003, 2005; Dobretsov et al., 2006). Principal reconstructions, particularly those of the initial stage from 4 Ga to the Riphean (1.6 Ga), are based mostly on geochemical, petrologic, and tectonic–geophysical data. Records from general geology and lithology (more precisely, sedimentology) are taken into account to a lesser extent. To date, it has become common opinion that the history of the atmosphere is divided into two stages, the stages of reducing (oxygen free) and oxygen atmo sphere, which was formed about 2.6 Ga. In addition, many researchers are convinced of high subaerial tem peratures (100°C and higher). This is highly improba ble, particularly with reference to the average temper ature of the Earth’s surface (at present, it is 15°C). The upper limit of the period under consideration is defined by the well known and repeatedly described events, i.e., expansion of the skeletonless Ediacaran Fauna (0.6 Ga), which is recorded in all continents, the appearance of the Cambrian skeletal fauna, when many organisms acquired skeletons of different com position (the socalled “Cambrian explosion,” about 530–540 Ma), and appearance of plants on land (about 400 Ma) (Fig. 1). Records of the Archean–Proterozoic history are less reliable, although Earth environments of the last 400 m.y. were essentially the same as in the Recent,

except for changes in relative configurations of land masses and seas and climatic fluctuations. But for zircons older than 4 b.y. (see below), the oldest metasedimentary rocks are dated 3.7–3.85 Ga. Thus, Earth history has been documented by the geo logical record beginning from that level, which coin cided with the termination of the last intense meteor ite bombardment. The summaries on the oldest rocks (De Wit and Ashwal, 1997; Rozen et al., 2006) indicate a high pro portion of metasedimentary formations in many tens of greenstone belts, which occurred on the Earth (Fig. 2). This is reliable evidence of the presence of many basins of sedimentation over the Earth’s surface at that time, i.e., a great volume of water. A consider able increase in the volume of sedimentary rocks about 3–3.5 Ga apparently indicates a very rapid increase in water volume, as schematically shown in Fig. 3. Thus, it seems plausible that, before 4 Ga, the Earth’s surface was free of significant water masses, which could have provided the formation of sedimen tary rocks and wide colonization by microorganisms. The possibility of appearance and existence of the 1

RNA World at that time remains an open question. It is possible to propose that intense meteorite bombardments resulted in the disappearance of water, which was brought by some way to the Earth’s surface. The Archean greenstone belts contain a set of sed imentary and volcanic rocks, which “do not demon strate any noticeable systematic distinctions from 1 See the paradoxes of Spirin (2007).

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ROZANOV Earth history documented geologically Metazoa Metaphyta Protobionta

RNA World

Fungi Protoeukaryota

Eukaryota Archaea Cyanobacteria Bacteria

Prokaryota

4.6

1.8

2.7

4

0.6 0.54 0.4

biomarkers

Skeletal fauna "Cambrian explosion"

Ediacaran Fauna

Time of last intense meteorite bombardment

Eukaryota (2.7 Ga) Metazoa (Spongia) (1.8 Ga)

0

Transition of plants on land

ArcheanEarly Proterozoic evaporites glaciations

Kokdveni, Hapvaal, S. Africa

25 Appalachians, USA

Murchison Yilgarn Australia

20 15 10

Isua, Greenland

5

Stratigraphic thickness, km

100 Cape Smythe, Ungada, Canada

75 50 25 0

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1.5

1.0

0.5

0

Sedimentary rocks, % volume of sum of magmatic and sedimentary rocks

Fig. 1. Some biotic and abiotic events on the Earth after 4 Ga.

Fig. 2. Diagram showing the thickness of deposits of the greenstone belts and the proportion of the sedimentary rocks included (Rozen et al., 2006): (a) total thickness of deposits of the greenstone belts (representative estimation from stratigraphic sections and geophysical data provided by De Wit and Ashwal, 1997); (b) proportion of sedimentary rocks in the magmatic and sedimen tary rocks taken together in the greenstone belts (44 belts described by De Wit and Ashwall, 1997). PALEONTOLOGICAL JOURNAL

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Phanerozoic analogues” (Rozen et al., 2006). This set comprises all types of rocks, including carbonates, evaporites, and others. A distinctive character of the oldest strata is a wide distribution of ferruginous quartzites. However, they occur even in the Upper Riphean of Canada, Brazil, and Namibia (Windley, 1999). Thus, in terms of rock associations, the atmosphere and hydrosphere of the Late Riphean did not differ in essence from those of later periods. Suppositions about considerably differences in the atmosphere composition are based on certain facts which are not reliable enough. The presence of water or, more exactly, its supply to the Earth’s surface about 4.6–4.0 Ga was a subject of many discussions. The ideas of Marov (Marov and Ipatov, 2001, 2005) that water, the volume of which is commensurable with the modern ocean, was brought to the Earth with comets seem highly probable. Appar ently, such a great volume of water had never been on the Earth before 4 Ga and, more probably, before 2

1.3 Ga. It is only possible to suppose that water was accumulated in small ponds and subsurface cracks. The question of where such a great volume of water occurred is a subject of special discussion. A few words should be said about the oldest zircons dated 4.2 Ga and even 4.4 Ga. The oxygen isotopic study of zircon grains cannot evidence their clastic origin or rounding in water; the same is true of the acuteangled shape and size (up to 300 µm) of the grains reproduced. Their volcanic (ash) or eolian ori gin is more probable. Let us turn to the Archean reducing atmosphere and uraninites. The described and reproduced uraninite grains are treated as rounded clasts and compared with rounded monocytes (Figs. 4a, 4b; Schopf, 1983). The reproductions of grains less than 0.15 mm in size may speak of an origin other than rounding, because they are smaller than rounded clasts. For the purpose of com parison Fig. 4c shows modern beach sediments from the same area as rounded monocytes. All grains are acute angled, while the rounded forms are foraminifers. The uraninite grains may be uncrystallized amorphous col loid structures or even fossilized bacterial clusters. Additional studies under an electron microscope at high magnification are required. Different signs of reducing atmosphere have received ambiguous interpretations. A very indicative example is provided by curves in the book edited by Schopf (1983, p. 284, textfigs. 11, 12). They are built based on geological and geochemical records, which 2 Zharkov

(2005) is probably correct to proposed, based on the distribution of evaporites in time, that stabilization of the oce anic salt regime and corresponding water volume of the World Ocean began about 1.3 Ga. PALEONTOLOGICAL JOURNAL

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Vwater, % 10

0 4.6

4

3.85

3

2

1.3

1

0 Ga

Fig. 3. Scheme of a presumable increase in water volume.

the authors regarded as indicators of reducing or oxi dizing conditions. Because of contradictory character of available data, the same localities were referred to different areas based on different features. If records of uraninites are taken away from the curves, nothing confirms reducing conditions. Naturally, traditional ideas of the reducing atmo sphere influence the treatment of the organizational level of Archean–Lower Proterozoic fossils. They also give rise to fantastic discussions (Brasier et al., 2002) on fossils from the Archean rocks of Australia, which were dated approximately 3.5 Ga and described previ ously by Schopf and other researchers. We believe that the biosphere parameters, such as oxygen content and subaerial temperature, should be estimated proceeding primarily from the organiza tional level of fossils found in the Archean–Protero zoic strata. For instance, the occurrence of cyanobac teria determines the presence of atmospheric oxygen, and the occurrence of Metazoa, particularly coelo mates, suggests that the oxygen content is comparable with the modern level. The Archean–Proterozoic records, which were reviewed by Schopf, Hoffman, Noel, and others in the late 20th century, were recently supplemented by Rus sian researchers (Rozanov, 2004, 2006; Fedonkin, 2006; Sergeev et al., 2007). Some key characteristics indicative of the following events are discussed below: (1) occurrence of the oldest fossils of bacterial organi zation level; (2) occurrence of these fossils in the rocks of continental genesis, which indicate the time of land colonization; (3) occurrence of the earliest cyanobac teria, which are unambiguously indicative of the atmospheric oxygen; (4) occurrence of eukaryotes indicative of oxygen and temperature parameters of the biosphere; (5) occurrence of multicellular organ isms; (6) coelomates; (7) skeletal organisms of differ ent groups, which are capable of building skeletons

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ROZANOV

0

0.1 mm

0

0.1

0.2 mm

Fig. 4. Shape and size of (a) uraninite structures and (b) rounded monocyte grains (Schopf, 1983); (c) monocyte siltstone from modern beaches of Australia; pay attention to dimensions; rounded grains are living foraminifers.

varying in composition; and (8) expansion of multicel lular organisms onto the land. The characteristics of these levels and events is based on fossilized organisms, trace fossils (particu larly traces of movements), biomarkers, isotopic records, etc. Before considering individual finds, I would like to remind that, as has become clear recently, fossilization of bacteria and all other micro organisms is an ordinary and very rapid process. Therefore, bacteria are easy to detect at high magnifi cation in all sediments of any age (Rozanov, 2002),

particularly in epicontinental basins. Moreover, not only bacteria but also glycocalyx is well fossilized, and its perfectly preserved specimens a few nanometers thick can be found (Fig. 5). This phenomenon has been confirmed by many laboratory experiments in Europe, Japan, America, and Russia. Note that the abundance of fossilized microorgan isms in sedimentary rocks is accounted for by specific conditions of epicontinental basins. These shallow PALEONTOLOGICAL JOURNAL

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843 C1

Recent

O2

3 µm Fig. 5. Living and fossil (Cambrian and Ordovician) cyanobacteria; perfect preservation of filaments and films is particularly interesting; the films are 10–20µm thick.

water basins with extensive photic zone are saturated by bacteria. Therefore, epicontinental sedimentation has nothing in common with the oceanic one and requires comparative analysis. The presence of cyanobacteria in Archean rocks was repeatedly reported (Schopf, 1983; Westall and Walsh, 2002; etc.). This has long been rejected by many colleagues, particularly the Oxfordian team headed by Brasier (Brasier et al., 2002). For me, the data on Archean cyanobacteria is rather reliable because there are no serious arguments against them. Therefore, the records from the Baltic Shield seem to be very important. Definite cyanobacterial mats were found in rocks aged about 3.0 Ga (Fig. 6). Another evidence for the abundance of cyanobacteria at 3.5 Ga in the Earth is stromatolites in the Archean strata and their regular distribution in the Archean through Pro terozoic (Fig. 7). The problem of land colonization by bacteria (microorganisms) is of great importance. In the litera ture, there are some indications to the existence of paleosoils. I think we should speak rather of weather PALEONTOLOGICAL JOURNAL

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ing crusts and an active role of microorganisms in their formation. All the studied weathering crusts of the Baltic Shield contain abundant microorganisms, which are evidence that the Archean land was colonized by microorganisms and, probably, by fungi (Fig. 8). Dif ferences in the pH conditions in the crusts and basins are appropriate to note (Fig. 9). The appearance of eukaryotes in the geological record is a very contradictory problem arousing heated debates. I see no reasons to distrust the data of Timofeev (1982). The related material stored in the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences and that collected again in the sections of the same localities were restud ied. The restudies have shown that spherical forms up to 100–150 µm across and tubes up to 30–40 µm across are probably remains of eukaryotic organisms (Fig. 10). Undoubted eukaryotic organisms were described by Belova and Akhmedov (2006) from deposits aged 2–2.2 Ga (Fig. 11) and phosphorites aged 2.0 Ga of the Pechenga Group of the Pechenga River Basin

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ROZANOV

10 µm

10 µm

10 µm Fig. 6. Deposits composed entirely of cyanobacterial filaments, which form mats, from the Lopian of Karelia (after Astafieva, 2006).

(Fig. 12). A form from these phosphorites is referred with confidence to prasinophytes and named Pechen gia (Rozanov and Astafieva, 2008). Belova and Akhmedov described peculiar organic remains, some of which may be candides of fungi or pro tists. The occurrence of Leiosphaeridia with crumpling folds is worthy of special attention. For some reasons, the genus Leiosphaeridia is some times regarded as a heterogeneous group, comprising envelopes of eukaryotes and prokaryotic colonies (e.g., Sergeev et al., 2007). However, the groping of smooth

and folded structures in Leiosphaeridia seems incorrect. Leiosphaeridia comprises envelopes with crumpling folds, and their specific matter excludes referring them to prokaryotic structures. Therefore, only forms with crum pling folds should be named Leiosphaeridia. The absence of these folds makes this identification incorrect. The appearance of multicellular eukaryotic organ isms and particularly coelomates in the paleontologi cal record indicates a high, almost Recent, content of atmospheric oxygen. Fossilized metazoans are identi fied by their remains, fossil traces, particularly, move PALEONTOLOGICAL JOURNAL

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LIFE CONDITIONS ON THE EARLY EARTH AFTER 4.0 Ga Archean

EAr

PAr

Proterozoic

MAr NAr

PR12

PR1b

3600 3200 2800 2300 2500±50 AR

PR1c

PR1d R11

R21

R21

845

Number of genera 80 70 60 50 40 30 20 10 R32 V C 0 R22 R31

2000 1800 1850±50 1350±20 1000±50 PR2 PR1

650±50

Fig. 7. Distribution of stromatolites in the Precambrian (based on the data of Raaben, Semikhatov, Hoffman, and Sumina).

10 µm

3 µm

10 µm

3 µm

Fig. 8. Microorganisms characteristic of weathering crusts from Paanajärvi, 2.4 Ga (after Rozanov et al., 2008).

ment traces, and fossil biomarkers. Among the known biomarkers, there is a spongy form (Reitner, 2005), which was found in deposits dated 1.8 Ga. Ichnofossils were repeatedly recorded in many studies. Boring and burying traces described by Kaufmann and Steidt mann (1981) from North America and dated as 1.6 Ga PALEONTOLOGICAL JOURNAL

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as well as creeping traces discovered by B.B. Shishkin in deposits aged 1.4 Ga in the northwestern Siberian Platform are most interesting (Rozanov, 2006). The finding of Udokania is the most interesting. It is long time ago when Udokania was described by

846

ROZANOV pH = 4–6

crust of weathering

pH > 6

Fig. 9. Relationships between weathering crusts and basin deposits, including carbonates, geomorphological features, and pH of the environment.

10 µm

Fig. 10. Lopian fossils (Timofeev, 1982; reexamined in PIN in 2006).

A.M. Leites. R.F. Hecker, V.V. Menner, and B.S. Sokolov initially identified it as a true fossil of coelenterate or polychaete organization. Subse quently, Salop (1982) interpreted it as scapolite crys tals. A restudy undertaken by Sayutina and Vil’mova (1990) has shown that they are undoubtedly of biolog ical and, moreover, metazoan nature. However, this disagrees with the models for the biosphere evolution supported by the majority of Western scientists and, then, by Russian researchers (Sergeev et al., 2007), because Udokania is dated about 2 Ga. The paradox is that the data on Udokania are per sistently ignored by distinguished experts on the Pre cambrian. Moreover, nobody tried to reexamine numerous specimens stored in the Paleontological Institute of the Russian Academy of Sciences (PIN)

and some other institutions. At the beginning of the third millennium, it has become apparent that special ists who studied and restudied the stone material have never doubted about the metazoan nature of Udoka nia. Those who have not examined the material silently supported the abiogenic point of view. I repeatedly drew attention of my colleagues to the fact that silence about the Udokania problem should not continue any longer. Finally, the situation began to change. The colleagues from Moscow (Sergeev et al., 2007, p. 32) indicated that “there are divergent inter pretations of the nature of Udokania and associated structures. Some researchers regard it as multicellular animals of Ediacaran appearance, some interpret it as abiogenic forms, while others refer it to biogenic struc tures without specifying their nature (Sayutina and PALEONTOLOGICAL JOURNAL

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1

2

847

3

4

6

8 10 µm 7 5

9

10

Fig. 11. Remains of eukaryotic organisms from Karelian deposits dated 2–2.2 Ga (after Belova and Akhmedov, 2006).

30 µm

10 µm

Fig. 12. The prasinophyte Pechengia from Proterozoic phosphorites dated 2.0 Ga (after Rozanov and Astafieva, 2008). PALEONTOLOGICAL JOURNAL

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ROZANOV (a)

(b)

1 cm

1 cm

Fig. 13. Udokania: (a) a form of Udokania, (b) fine longitudinal section through the tubes.

RNA World Protobionta

Earth history documented geologically

Earth’s surface T°C 100

Metazoa Metaphyta Fungi Eukaryota

Protoeukaryota Archaea Cyanobacteria Bacteria

Prokaryota

H2O

100% O2 (Recent)

O2

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10% O2

70 60

1.0 O2

50 40 T°

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20 10 0

biomarkers

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15°C

2 1.8

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Archean

1

0.54

0

Phanerozoic

Cryptozoic Last intense meteorite bombardment

First glac

Onset of wide diversity of Eukaryota

Eukaryota (2.7 Ga) Metazoa (Spongia) (1.8 Ga)

Fig. 14. Integrated picture of some geobiological events in the Precambrian.

Vil’mova, 1990; Rozanov, 2003, 2004; Sinitsa et al., 2003; and references in these studies). A complex analysis of these disciform (marked out by me, A. Rozanov) forms made by A.A. Terleev, A.A. Postni

kov, B.B. Kochnev, K.E. Nagovitsin, D.V. Grazhdankin, and A.M. Stanevich (2006) revealed that they are impressions of complex colonies of unicellular microor ganisms, i.e., bacteria, fungi, and eukaryotic algae.” A PALEONTOLOGICAL JOURNAL

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reader can easily seize a strangeness of the situation after comparing a photograph of Udokania (Fig. 13) and the words “disciform structures” and “animals of Ediacaran appearance” in the publication cited. Colleagues from Novosibirsk, who are referred to by Moscow researchers, analyzed stone material that was collected by them alone and did not restudied the type collections. Thus, Metazoa aged 2.0 Ga is a fact which is impossible to ignore. This agrees with the data on biomarkers (see Figs. 1 and 14). The history of organic evolution on the Earth dis plays two main states of every group of organisms, i.e., the appearance and wide expansion. They are not coincident in time and there may be intervals of hun dreds of million years between them. The first appearance is indicative of corresponding content of atmospheric oxygen, whereas wide expan sion, as in the case of Metazoa, shows stabilization of salt content and water volume in the World Ocean or certain other similar events. CONCLUSIONS The main conclusions are summarized in Fig. 14. There was little or no water on the Earth’s surface between 4.6 and 4.0 Ga. A significant volume of water appeared about 4.0 Ga, after the last intense meteorite bombardment. At that time, sedimentation began in shallowwater basins and life flourished on the planet surface. The appearance of green algae and probably even fungi, along with bacteria, including cyanobacte ria, should not be excluded. The land surface was inhabited by microorganisms as early as the Archean. Cyanobacteria, eukaryotes, Metaphyta, and Metazoa appeared much earlier than was previously thought. The level of their organization provides information concerning the oxygen content in the atmosphere. The water volume commensurable with the Recent state was formed about 1.3 Ga. Since then, wide expansion of different organisms began. The average subaerial temperature differed from the modern one by at most 15–25°C. The RNA World could have existed only earlier than 4.0 Ga or probably before the formation of the Earth. It is highly improbable that the Earth was the place of the origin of life. ACKNOWLEDGMENTS This study was supported by the Presidium of the Russian Academy of Sciences, Program “Origin of the Biosphere and Evolution of Geobiological Systems” (Subprogram II), Russian Foundation for Basic Research (project no. 080400484), and the Russian PALEONTOLOGICAL JOURNAL

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