The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in the Barberton fossil record in rocks as old as 3.5 billion years. [1] The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks.
Studying the earliest forms of life on Earth can provide valuable information to help understand how life can evolve on other planets. It has long been hypothesized that life may have existed on Mars due to the similarity of environmental and tectonic conditions during the Archean time. [2] By knowing the environments in which early life evolved on Earth, and the rock types that preserve them, scientists can have a better understanding of where to look for life on Mars.
Fossil life of 3.5 billion years of age is also found in the Pilbara craton of western Australia. [3] This evidence, along with Barberton fossils, show that cellular life must have existed by this point in the evolution of Earth. There is work that potentially demonstrates life at 3.8 billion years ago, in what is now western Greenland, [4] [5] but it is highly debated. Cellular life existed 3.5 billion years ago and thus it evolved prior to this time. Because the Earth is 4.5 billion years old, [6] there is a window of about one billion years for cellular life to evolve on a lifeless earth.
The Barberton Greenstone Belt is located on the Kaapvaal craton, which covers much of the southeastern part of Africa, and was formed by the emplacement of granitoid batholiths. [7] The Kaapvaal craton was once part of a supercontinent geologists term Vaalbara that also included the Pilbara craton of western Australia. [7] Though the exact timing is still debated, it is likely that Vaalbara existed from approximately 3.6 to 2.2 billion years ago, [8] and then split into two different continents.
Preserved life in Archean rocks has been altered over its 3.5 billion year history and, thus, can be difficult to distinguish. The cell wall structure can be preserved, but the original composition changes over time and becomes mineralised. There are six established criteria to determine the plausibility of a given microstructure being a microfossil: [9] [10]
Cells are preserved in the rock record because their cell walls are made of proteins which convert to the organic material kerogen as the cell breaks down after death. Kerogen is insoluble in mineral acids, bases, and organic solvents. [12] Over time, it is mineralised into graphite or graphite-like carbon, or degrades into oil and gas hydrocarbons. [13]
There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8 μm, filamentous microfossils have diameters typically less than 5 μm and have a length that can range from tens of μm to 100 μm, and spindle-like microfossils can be as long as 50 μm. [1] [14]
Stable isotope fractionation is a useful way of characterising organic carbon and inorganic carbon. These numbers are reported as δ13C values, where C is for the chemical element carbon. Isotope analysis of inorganic carbon typically yields δ13C values heavier than −10 per mil, with numbers usually falling between −5 and 5 per mil. Organic carbon, however, has δ13C values that range from −20 per mil for photoautotrophic bacteria [15] to −60 per mil for microbial communities that recycle methane. [16] The large range in values for organic carbon has to do with the cellular metabolism. For instance, an organism that uses photosynthesis (a phototroph) will have a different isotope δ13C value than an organism that relies on chemical substances for energy (an autotroph).
The oldest microfossils from the Barberton Greenstone belt are found in the Onverwacht Group, specifically, in both the Kromberg and Hooggenoeg Formations. [1] Both of these formations are predominantly igneous rock; the sedimentary rock has been metamorphosed. However, it is still possible to find microfossils in chert, a type of evaporite that forms in sedimentary environments. From the evidence in these rocks, it is likely that early life existed in the form of microbial mats and stromatolites. Evidence for this hypothesis is preserved in both chert and lithified stromatolites. [1]
Stromatolites represent large colonies of microorganisms, and are found both in the fossil record and rarely in modern hypersaline environments. A typical stromatolite consists of alternating layers of sediment and microbes. The microbes are photosynthetic; thus stromatolites represent shallow water environments in the fossil record due to their necessity to exist in the photic zone of water bodies. Stromatolites typically consist of filamentous microfossils. [17] The oldest stromatolites have been dated to approximately 3.5 billion years old. [18] Stromatolites in Barberton have been dated to about 3.3 billion years.
Microfossils found in chert extend the Barberton microfossil record back to 3.5 billion years. All three types of microfossil morphologies are found in cherts. Chert can have a variety of colours, but microfossils are typically found in black cherts, as the dark color can indicate organic material. [1]
Scientists have established the approximate age that life first appears in the fossil record, but that doesn't necessarily equal the time that life first evolved on Earth. Though fossils have not been found in older rocks, evidence for life can be found in other ways, such as extended carbon isotope data and Raman Spectroscopy. There is also ongoing work within the scientific community to solve the problem of how cellular life evolved in a hostile early earth.
The Proterozoic is a geological eon spanning the time interval from 2500 to 538.8 million years ago. It is the most recent part of the Precambrian "supereon". It is also the longest eon of the Earth's geologic time scale, and it is subdivided into three geologic eras : the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic.
The Archean Eon is the second of four geologic eons of Earth's history, representing the time from 4,000 to 2,500 million years ago. In this time, the Earth's crust had cooled enough for continents to form and for the earliest known life to start. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen. The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
Marble Bar is a town and rock formation in the Pilbara region of north-western Western Australia. Its extremely hot climate, with a mean maximum temperature second only to Wyndham, Western Australia has resulted in the town being well known for its hot weather.
A microfossil is a fossil that is generally between 0.001 mm and 1 mm in size, the visual study of which requires the use of light or electron microscopy. A fossil which can be studied with the naked eye or low-powered magnification, such as a hand lens, is referred to as a macrofossil.
The Mesoarchean is a geologic era in the Archean Eon, spanning 3,200 to 2,800 million years ago, which contains the first evidence of modern-style plate subduction and expansion of microbial life. The era is defined chronometrically and is not referenced to a specific level in a rock section on Earth.
The Paleoarchean, also spelled Palaeoarchaean, is a geologic era within the Archaean Eon. The name derives from Greek "Palaios" ancient. It spans the period of time 3,600 to 3,200 million years ago. The era is defined chronometrically and is not referenced to a specific level of a rock section on Earth. The earliest confirmed evidence of life comes from this era, and Vaalbara, one of Earth's earliest supercontinents, may have formed during this era.
The Gunflint chert is a sequence of banded iron formation rocks that are exposed in the Gunflint Range of northern Minnesota and northwestern Ontario along the north shore of Lake Superior. The Gunflint Chert is of paleontological significance, as it contains evidence of microbial life from the Paleoproterozoic. The Gunflint Chert is composed of biogenic stromatolites. At the time of its discovery in the 1950s, it was the earliest form of life discovered and described in scientific literature, as well as the earliest evidence for photosynthesis. The black layers in the sequence contain microfossils that are 1.9 to 2.3 billion years in age. Stromatolite colonies of cyanobacteria that have converted to jasper are found in Ontario. The banded ironstone formation consists of alternating strata of iron oxide-rich layers interbedded with silica-rich zones. The iron oxides are typically hematite or magnetite with ilmenite, while the silicates are predominantly cryptocrystalline quartz as chert or jasper, along with some minor silicate minerals.
The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, and the Oxygen Crisis, was a time interval when the Earth's atmosphere and the shallow ocean first experienced a rise in the amount of oxygen. This occurred approximately 2.4–2.0 Ga (billion years) ago, during the Paleoproterozoic era. Geological, isotopic, and chemical evidence suggests that biologically-produced molecular oxygen (dioxygen, O2) started to accumulate in Earth's atmosphere and changed it from a weakly reducing atmosphere practically free of oxygen into an oxidizing atmosphere containing abundant oxygen.
The Isua Greenstone Belt is an Archean greenstone belt in southwestern Greenland, aged between 3.7 and 3.8 billion years. The belt contains variably metamorphosed mafic volcanic and sedimentary rocks, and is the largest exposure of Eoarchaean supracrustal rocks on Earth. Due to its age and low metamorphic grade relative to many Eoarchaean rocks, the Isua Greenstone Belt has become a focus for investigations on the emergence of life and the style of tectonics that operated on the early Earth.
Biotic material or biological derived material is any material that originates from living organisms. Most such materials contain carbon and are capable of decay.
The early Earth is loosely defined as Earth in its first one billion years, or gigayear (Ga, 109y). The “early Earth” encompasses approximately the first gigayear in the evolution of our planet, from its initial formation in the young Solar System at about 4.55 Ga to sometime in the Archean eon at about 3.5 Ga. On the geologic time scale, this comprises all of the Hadean eon (starting with the formation of the Earth about 4.6 billion years ago), as well as the Eoarchean (starting 4 billion years ago) and part of the Paleoarchean (starting 3.6 billion years ago) eras of the Archean eon.
Vaalbara was an Archean supercontinent consisting of the Kaapvaal Craton and the Pilbara Craton. E. S. Cheney derived the name from the last four letters of each craton's name. The two cratons consist of crust dating from 2.7 to 3.6 Gya, which would make Vaalbara one of Earth's earliest supercontinents.
The Kaapvaal Craton, along with the Pilbara Craton of Western Australia, are the only remaining areas of pristine 3.6–2.5 Ga crust on Earth. Similarities of rock records from both these cratons, especially of the overlying late Archean sequences, suggest that they were once part of the Vaalbara supercontinent.
The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia.
The Barberton Greenstone Belt is situated on the eastern edge of Kaapvaal Craton in South Africa. It is known for its gold mineralisation and for its komatiites, an unusual type of ultramafic volcanic rock named after the Komati River that flows through the belt. Some of the oldest exposed rocks on Earth are located in the Barberton Greenstone Belt of the Eswatini–Barberton areas and these contain some of the oldest traces of life on Earth. Only the rocks found in the Isua Greenstone Belt of Western Greenland are older. The Makhonjwa Mountains make up 40% of the belt.
The Warrawoona Group is a geological unit in Western Australia containing putative fossils of cyanobacteria cells. Dated 3.465 Ga, these microstructures, found in Archean chert, are considered to be the oldest known geological record of life on Earth.
This timeline of natural history summarizes significant geological and biological events from the formation of the Earth to the arrival of modern humans. Times are listed in millions of years, or megaanni (Ma).
James William Schopf is an American paleobiologist and professor of earth sciences at the University of California Los Angeles. He is also Director of the Center for the Study of Evolution and the Origin of Life, and a member of the Department of Earth and Space Sciences, the Institute of Geophysics and Planetary Physics, and the Molecular Biology Institute at UCLA. He is most well known for his study of Precambrian prokaryotic life in Australia's Apex chert. Schopf has published extensively in the peer reviewed literature about the origins of life on Earth. He is the first to discover Precambrian microfossils in stromatolitic sediments of Australia (1965), South Africa (1966), Russia (1977), India (1978), and China (1984). He served as NASA's principal investigator of lunar samples during 1969–1974.
The Fig Tree Formation, also called Fig Tree Group, is a stromatolite-containing geological formation in South Africa. The rock contains fossils of microscopic life forms of about 3.26 billion years old. Identified organisms include the bacterium Eobacterium isolatus and the algae-like Archaeosphaeroides barbertonensis. The fossils in the Fig Tree Formation are considered some of the oldest known organisms on Earth, and provide evidence that life may have existed much earlier than previously thought. The formation is composed of shales, turbiditic greywackes, volcaniclastic sandstones, chert, turbiditic siltstone, conglomerate, breccias, mudstones, and iron-rich shales.
The earliest known life forms on Earth are putative fossilized microorganisms found in hydrothermal vent precipitates, considered to be about 3.42 billion years old. The earliest time for the origin of life on Earth is at least 3.77 billion years ago, possibly as early as 4.28 billion years, or even 4.41 billion years—not long after the oceans formed 4.5 billion years ago, and after the formation of the Earth 4.54 billion years ago. The earliest direct evidence of life on Earth is from microfossils of microorganisms permineralized in 3.465-billion-year-old Australian Apex chert rocks.