Earliest known life forms
The earliest known life forms on Earth are putative fossilized microorganisms found in hydrothermal vent precipitates. The earliest time that life forms first appeared 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 are microfossils of microorganisms permineralized in 3.465-billion-year-old Australian Apex chert rocks.
Earth remains the only place in the universe known to harbor life. The Earth's biosphere extends down to at least 19 km (12 mi) below the surface, and up to at least 76 km (47 mi) into the atmosphere, and includes soil, hydrothermal vents, and rock. Further, the biosphere has been found to extend at least 914.4 m (3,000 ft; 0.5682 mi) below the ice of Antarctica, and includes the deepest parts of the ocean, down to rocks kilometers below the sea floor. In July 2020, marine biologists reported that aerobic microorganisms (mainly), in "quasi-suspended animation", were found in organically-poor sediments, up to 101.5 million years old, 76.2 m (250 ft) below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean"), and could be the longest-living life forms ever found. Under certain test conditions, life forms have been observed to survive in the vacuum of outer space. More recently, in August 2020, bacteria were found to survive for three years in outer space, according to studies conducted on the International Space Station. The total mass of the biosphere has been estimated to be as much as 4 trillion tons of carbon. According to one researcher, "You can find microbes everywhere – [they are] extremely adaptable to conditions, and survive wherever they are."
Of all species of life forms that ever lived on Earth, over five billion, more than 99%, are estimated to be extinct. Some estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent remain undescribed. However, a May 2016 scientific report estimates 1 trillion species currently on Earth, with only one-thousandth of one percent described. Additionally, there are an estimated 10 nonillion (10 to the 31st power) individual viruses (including the related virions) on Earth, the most numerous type of biological entity, and which some biologists consider to be life forms. Moreover, there are more individual viruses than all the estimated stars in the universe; which, in turn, are considered to be more numerous than all the grains of beach sand on planet Earth. About 200 virus types are known to cause diseases in humans. Other possible virus-like forms, some pathogenic, less likely to be considered living, much smaller than viruses and possibly much more primitive, include viroids, virusoids, prions and nanobes.
Earliest life forms
The age of the Earth is about 4.54 billion years; the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago. Some computer models suggest life began as early as 4.5 billion years ago.
A December 2017 report stated that 3.465-billion-year-old Australian Apex chert rocks once contained microorganisms, the earliest direct evidence of life on Earth. A 2013 publication announced the discovery of microbial mat fossils in 3.48 billion-year-old sandstone in Western Australia. Evidence of biogenic graphite, and possibly stromatolites, were discovered in 3.7 billion-year-old metasedimentary rocks in southwestern Greenland, and described in 2014 in the journal Nature. Potential "remains of life" were found in 4.1 billion-year-old rocks in Western Australia and described in a 2015 study.
By comparing the genomes of modern organisms, it is possible to postulate the existence of a last universal common ancestor (LUCA), for which no specific fossil evidence exists. A 2018 study from the University of Bristol, applying a molecular clock model, concluded that the LUCA may have lived 4.477 to 4.519 billion years ago, within the Hadean eon. In March 2017, fossilized microorganisms (microfossils) were announced to have been discovered in hydrothermal vent precipitates from an ancient sea-bed in the Nuvvuagittuq Belt of Quebec, Canada. These may be as old as 4.28 billion years, the oldest evidence of life on Earth, suggesting "an almost instantaneous emergence of life" after ocean formation 4.41 billion years ago. Some researchers even speculate that life may have started nearly 4.5 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe".
As for life on land, in 2019 scientists reported the discovery of a fossilized fungus, named Ourasphaira giraldae, in the Canadian Arctic, that may have grown on land a billion years ago, well before plants are thought to have been living on land. In July 2018, scientists reported that the earliest life on land may have been bacteria 3.22 billion years ago. In May 2017, evidence of microbial life on land may have been found in 3.48 billion-year-old geyserite in the Pilbara Craton of Western Australia.
Extant ancient Korarchaeota
The Korarchaeota are a group of microorganisms that appear to have diverged early in the evolution of the archaea. Korarchaeota have been detected in several geographically isolated terrestrial and marine thermal environments. One species, Korarchaeum cryptofilum has been studied in order to gain insight into the early evolution of the archaea. Based on phylogenetic analysis this organism has a deep-branching archaeal lineage. Korarchaeum cryptofilum has an ultrathin filamentous morphology. Its predicted gene functions indicate the K. cryptofilum relies on a simple mode of peptide fermentation for carbon and energy and is unable to synthesize de novo purines, Coenzyme A and other cofactors. In view of the known composition of archaeal genomes, K. cryptofilum may have retained a set of cellular characteristics that represents the ancestral archaeal form.
Earliest known life forms (click "show" on right for images)
- LUCA is not thought to be the first life on Earth, but rather the only type of organism of its time to still have living descendants.
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