Currently, Earth remains the only place in the universe known to harbor life. The Earth's biosphere extends down to at least below the surface, and up to at least into the atmosphere, and includes soil, hydrothermal vents, and rock. Further, the biosphere has been found to extend 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, in "quasi-suspended animation", were found in organically-poor sediments, up to 101.5 million years old, 250 feet below the seafloor in the South Pacific Gyre , 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. 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 – 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 individual viruses 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 and prions.
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. There is evidence that suggests 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 biogenicgraphite, 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. suggests that life on Earth may have come frombiological matter carried by space dust or meteorites. A 2016 genetic study concluded that the last universal common ancestor may have lived in deep-sea hydrothermal vents 3.5 to 3.8 billion years ago. A 2018 study from the University of Bristol, applying a molecular clock model, places the LUCA shortly after 4.5 billion years ago, within the Hadean. In March 2017, fossilized microorganisms 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 were 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. In January 2018, a study found that 4.5 billion-year-old meteorites found on Earth contained liquid water along with prebioticcomplex organic substances that may be ingredients for life.
