principle, produce similar structures—although it isn’t obvious that such processes were at work in the Barberton seaway. The Barberton spheres, thus, fall only a few steps ahead of Warrawoona filaments. They could be fossil cyanobacteria or some other type of microorganism. Theycould record a primordial microbe that is long extinct. Or they could be carbonaceous spheres formed by physical processes on the Barberton seafloor. We simply don’t know. More recently, Maud Walsh of Louisiana State University has made a careful study of the organic matter in Barberton cherts, finding bedding textures most easily explained as mats and thin filaments that may be microfossils.
Figure 4.6. (a) Carbonaceous microstructure, possibly preserving a microbe during cell division, in 3.4-billion-year-old rocks from South Africa. Sphere is 4 microns in diameter. (b) Filamentous microfossils in 3.2-billion-year-old rocks from northwestern Australia. Each filament is about 2 microns across. (Photo (b) courtesy of Birger Rasmussen)
What kind of planet can we piece together from these fragments? Geologically, it appears to have been a world of familiar processes but not-so-familiar patterns. Continents began to form at least 4.2 billion years ago, and chemical details of volcanic rocks from Barberton, Warrawoona, and other old terrains suggest that a large volume of continental crust had formed by the time they were deposited. Little of these early continents remains, however, implying that on the early Earth, continents were recycled back into the mantle more easily than they are today. Three and a half billion years ago, plate tectonics had already begun to pattern our planetary surface, but Earth’s upper mantle appears to have been hotter, the basaltic crust beneath the oceans thicker, and, perhaps, the continents smaller and less stable. Then, as now, continental crust probably formed at plate margins, where descending slabs of oceanic crust cause overlying rocks to melt. On the other hand, early continent formation may have received a significant boost from a source that is no longer important—partial melting of basalts buried beneath thick piles of lava spilled onto the seafloor.
The rock record that survives from the early Earth is not simply the fragment of a geologically modern planet buffeted by time. Something about the character and mix of processes that form and destroy continents was different, and though many insightful scientists have hazarded opinions, we don’t fully understand what it was.
We have a bit more confidence that when the Warrawoona seaway formed, Earth was a biological planet. Moreover, the evidence of carbon isotopes suggests that the great ecological liberation of photosynthesis may already have begun. Whether or not contemporary microorganisms included the oxygen-producing cyanobacteria is uncertain, but the presence of any type of photosynthetic organism in the Warrawoona ocean speaks volumes, because it allows us to place a calibration point on the Tree of Life introduced in chapter 2 . In the new view of microbialevolution symbolized by the tree, photosynthetic organisms are relative latecomers that diversified long after the origin of life and the divergence of biology’s principal domains. If Warrawoona organic matter was made by photosynthesis, then a great deal of evolution must already have taken place.
Microorganisms appear to have cycled carbon, sulfur, and nitrogen through early Archean ecosystems, just as they do today. There is no record of eukaryotes or archaeans in these oldest rocks, but then there aren’t many fossils, period, and it would be hazardous to interpret the absence of evidence as evidence for absence. The branching pattern of the Tree of Life tells us that if photosynthetic bacteria lived in the Warrawoona sea, then at least some Archaea were almost certainly present.
We have one more constraint on early Archean biology. Consistent with our environmental reading of the
Figure 4.6. (a) Carbonaceous microstructure, possibly preserving a microbe during cell division, in 3.4-billion-year-old rocks from South Africa. Sphere is 4 microns in diameter. (b) Filamentous microfossils in 3.2-billion-year-old rocks from northwestern Australia. Each filament is about 2 microns across. (Photo (b) courtesy of Birger Rasmussen)
What kind of planet can we piece together from these fragments? Geologically, it appears to have been a world of familiar processes but not-so-familiar patterns. Continents began to form at least 4.2 billion years ago, and chemical details of volcanic rocks from Barberton, Warrawoona, and other old terrains suggest that a large volume of continental crust had formed by the time they were deposited. Little of these early continents remains, however, implying that on the early Earth, continents were recycled back into the mantle more easily than they are today. Three and a half billion years ago, plate tectonics had already begun to pattern our planetary surface, but Earth’s upper mantle appears to have been hotter, the basaltic crust beneath the oceans thicker, and, perhaps, the continents smaller and less stable. Then, as now, continental crust probably formed at plate margins, where descending slabs of oceanic crust cause overlying rocks to melt. On the other hand, early continent formation may have received a significant boost from a source that is no longer important—partial melting of basalts buried beneath thick piles of lava spilled onto the seafloor.
The rock record that survives from the early Earth is not simply the fragment of a geologically modern planet buffeted by time. Something about the character and mix of processes that form and destroy continents was different, and though many insightful scientists have hazarded opinions, we don’t fully understand what it was.
We have a bit more confidence that when the Warrawoona seaway formed, Earth was a biological planet. Moreover, the evidence of carbon isotopes suggests that the great ecological liberation of photosynthesis may already have begun. Whether or not contemporary microorganisms included the oxygen-producing cyanobacteria is uncertain, but the presence of any type of photosynthetic organism in the Warrawoona ocean speaks volumes, because it allows us to place a calibration point on the Tree of Life introduced in chapter 2 . In the new view of microbialevolution symbolized by the tree, photosynthetic organisms are relative latecomers that diversified long after the origin of life and the divergence of biology’s principal domains. If Warrawoona organic matter was made by photosynthesis, then a great deal of evolution must already have taken place.
Microorganisms appear to have cycled carbon, sulfur, and nitrogen through early Archean ecosystems, just as they do today. There is no record of eukaryotes or archaeans in these oldest rocks, but then there aren’t many fossils, period, and it would be hazardous to interpret the absence of evidence as evidence for absence. The branching pattern of the Tree of Life tells us that if photosynthetic bacteria lived in the Warrawoona sea, then at least some Archaea were almost certainly present.
We have one more constraint on early Archean biology. Consistent with our environmental reading of the
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