J. William Schopf

Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life

Eleven taxa (including eight heretofore undescribed species) of cellularly preserved filamentous microbes, among the oldest fossils known, have been discovered in a bedded chert unit of the Early Archean Apex Basalt of northwestern Western Australia. This prokaryotic assemblage establishes that trichomic cyanobacterium-like microorganisms were extant and morphologically diverse at least as early as ∼3465 million years ago and suggests that oxygen-producing photoautotrophy may have already evolved by this early stage in biotic history.

Laser-Raman imagery of Earth's earliest fossils

Unlike the familiar Phanerozoic history of life, evolution during the earlier and much longer Precambrian segment of geological time centred on prokaryotic microbes. Because such microorganisms are minute, are preserved incompletely in geological materials, and have simple morphologies that can be mimicked by nonbiological mineral microstructures, discriminating between true microbial fossils and microscopic pseudofossil ‘lookalikes’ can be difficult. Thus, valid identification of fossil microbes, which is essential to understanding the prokaryote-dominated, Precambrian 85% of lifes history, can require more than traditional palaeontology that is focused on morphology. By combining optically discernible morphology with analyses of chemical composition, laser–Raman spectroscopic imagery of individual microscopic fossils provides a means by which to address this need. Here we apply this technique to exceptionally ancient fossil microbe-like objects, including the oldest such specimens reported from the geological record, and show that the results obtained substantiate the biological origin of the earliest cellular fossils known.

PRECAMBRIAN MICRO‐ORGANISMS AND EVOLUTIONARY EVENTS PRIOR TO THE ORIGIN OF VASCULAR PLANTS

The nature of the Precambrian biota-its antiquity. composition and evolutionand the well-known faunal discontinuity near the beginning of the Palaeozoic. have long been recognized as particularly puzzling problems in palaeontology . The evolutionary continuum well-documented in Phanerozoic sediments and the diversity and complexity of the early Palaeozoic biota augur well for a substantial period of Precambrian evolutionary development . Until recently. however. evidence of this development remained largely undeciphered ; the nature of Precambrian life was a fertile subject for speculation. essentially unfettered by the poorly known fossil record . The past few years have witnessed a renewed interest in these classic problems and a marked proliferation of available data; this increased activity has resulted in the

Morphological Biosignatures and the Search for Life on Mars

This report provides a rationale for the advances in instrumentation and understanding needed to assess claims of ancient and extraterrestrial life made on the basis of morphological biosignatures. Morphological biosignatures consist of bona fide microbial fossils as well as microbially influenced sedimentary structures. To be recognized as evidence of life, microbial fossils must contain chemical and structural attributes uniquely indicative of microbial cells or cellular or extracellular processes. When combined with various research strategies, high-resolution instruments can reveal such attributes and elucidate how morphological fossils form and become altered, thereby improving the ability to recognize them in the geological record on Earth or other planets. Also, before fossilized microbially influenced sedimentary structures can provide evidence of life, criteria to distinguish their biogenic from non-biogenic attributes must be established. This topic can be advanced by developing process-based models. A database of images and spectroscopic data that distinguish the suite of bona fide morphological biosignatures from their abiotic mimics will avoid detection of false-positives for life. The use of high-resolution imaging and spectroscopic instruments, in conjunction with an improved knowledge base of the attributes that demonstrate life, will maximize our ability to recognize and assess the biogenicity of extraterrestrial and ancient terrestrial life.

The Fossil Record: Tracing the Roots of the Cyanobacterial Lineage

Since the mid-1960s, following a century of unrewarded search, impressive progress has been made toward deciphering the Precambrian fossil record, evidence of life extant during the earliest seven-eighths of geologic time. Hundreds of fossiliferous units have been discovered containing thousands of microbial fossils—dominantly but not exclusively cyanobacterial — and the documented antiquity of life has been extended to an age roughly three-quarters that of the Earth. Mutually reinforcing lines of evidence, paleontological, geological, and isotopic geochemical, indicate that stromatoliticmicrobial ecosystems,evidently including cyanobacteria and other members of the bacterial domain, were extant ~3500 Ma ago; methanogenic archaeans by ~2800 Ma ago; and Gram-negative sulfate-reducing bacteria at least as early as ~2700 Ma ago.The discrepancy between these dates and those suggested for emergence of these groups by a recently proposed amino acid-based “molecular clock” is too great and too consistent to be ignored. The challengeis to unify the molecular data with the increasingly well-established paleobiologicrecord.

Carbon isotopic composition of individual Precambrian microfossils.

Ion microprobe measurements of carbon isotope ratios were made in 30 specimens representing six fossil genera of microorganisms petrified in stromatolitic chert from the approximately 850 Ma Bitter Springs Formation, Australia, and the approximately 2100 Ma Gunflint Formation, Canada. The delta 13C(PDB) values from individual microfossils of the Bitter Springs Formation ranged from -21.3 +/- 1.7% to -31.9 +/- 1.2% and the delta 13C(PDB) values from microfossils of the Gunflint Formation ranged from -32.4 +/- 0.7% to -45.4 +/- 1.2%. With the exception of two highly 13C-depleted Gunflint microfossils, the results generally yield values consistent with carbon fixation via either the Calvin cycle or the acetyl-CoA pathway. However, the isotopic results are not consistent with the degree of fractionation expected from either the 3-hydroxypropionate cycle or the reductive tricarboxylic acid cycle, suggesting that the microfossils studied did not use either of these pathways for carbon fixation. The morphologies of the microfossils suggest an affinity to the cyanobacteria, and our carbon isotopic data are consistent with this assignment.

Alga-Like Fossils from the Early Precambrian of South Africa

Micropaleontological studies of carbonaceouis chert from the Fig Tree Series of South Africa (> 3.1 x 109 years old) revealed the presence of spheroidal microfossils, here designated Archaeosphaeroides barbertonensis, interpreted as probably representing the remnants of unicellular alga-like organisms. The presumed photosynthetic nature of these primitive microorganisms seems corroborated by organic geochemical and carbon isotopic studies of the Fig Tree organic matter, and is consistent with the geologically and mineralogically indicated Early Precambrian environment. These alga-like spheroids, together with a bacterium-like organism previously described from the Fig Tree chert, are the oldest fossil orgisms now known.

Carbon isotopic fractionation by Archaeans and other thermophilic prokaryotes

This study of carbon isotopic fractionation in a wide array of 21 phylogenetically diverse microbial species provides an opportunity to correlate carbon isotopic fractionations with a biochemical pathway. These carbon isotopic fractionation experiments included two members of the Aquificales and two members of the Thermoproteales using the reductive TCA cycle, three members of the Sulfolobales using the 3-hydroxypropionate cycle, as well as three Archaeoglobales and seven methanogens using the acetyl-CoA pathway. In these experiments, microorganisms using the reductive tricarboxylic acid cycle (with e values between 2.0 and 5.5‰) and the 3-hydroxypropionate cycle (with e values between 0.2 and 3.6‰) demonstrated significantly less carbon isotopic fractionation than methanogens using the acetyl-CoA pathway. The results reported here for the acetyl-CoA pathway-utilizing microbes, however, vary over a remarkably wide range with e values of 2.7 to 8.0‰ for the Archaeoglobales and e values of 4.8 to 26.7‰ for the methanogens. The magnitude of carbon isotopic fractionation observed in species of Methanococcus were related to the particular growth status that had been attained by the various cultures, with increasing isotopic fractionation as growth proceeded.

How Old Are the Eukaryotes

Evidence from Precambrian sediments appears to indicate that nucleated (eukaryotic) organisms had become well established and relatively diverse about 850 � 100 million years ago and that eukaryotes were probably extant, and may have first appeared, as early as 1400 � 100 million years ago.

Microorganisms from the Late Precambrian of Central Australia

An assemblage of structurally and organically well preserved microorganisms, interpreted as both green and blue-green algae, has been found in chert facies of the Bitter Springs limestone from the upper Precambrian of central Australia. This appears to be the earliest known occurrence of green algae in the fossil record. These organisms are among the oldest known multicellular and unicellular fossils exhibiting distinct histological preservation.