Another uniqueness of the Earth is the temperatures suitable for life. If the solar system were billions of years old, the formation of the solar system, the Sun would have had to be nearly one-third less luminous than it is now. 1
Early in Earth's history, the Sun's output would have been only 70 percent as intense as it is during the modern epoch. In the environmental conditions existing at that time, this solar output would have been insufficient to maintain a liquid ocean. Astronomers Carl Sagan and George Mullen pointed out in 1972 that this is contrary to the geological and paleontological evidence. 4 According to the Standard Solar Model, stars similar to the Sun should gradually brighten over their main sequence lifetime due to contraction of the stellar core caused by fusion. 2 However, with the supposed solar luminosity 4 billion years ago and with greenhouse gas concentrations the same as are current for the modern Earth, any liquid water exposed to the surface would freeze.
if the sun were 25% less bright than it is today, the earth would simply be too cold to support life. In fact, it would be too cold to have liquid water present in any reasonable amount. This is a serious problem, however, because there is ample evidence that there has been significant amounts of liquid water on the earth during the earliest parts of its history. 4 So if the nuclear reactions in the sun play by the same rules as those in the lab, there shouldn’t have been liquid water on the earth billions of years ago. Nevertheless, there was. 7
It is postulated that certain greenhouse gases must have been present at higher concentrations to prevent the Earth from becoming a frozen planet. Carbon dioxide levels could not have been sufficiently high to compensate for the lower solar luminosity. The presence of other greenhouse gases, for example, ammonia or methane, is also problematical, as some propose that the Earth possessed an oxidative atmosphere 4 billion years ago. Moreover, ammonia is highly sensitive to solar UV radiation, and ammonia at levels required to influence the Earth’s temperature would have prevented photosynthetic organisms from fixing nitrogen (i.e., protein, DNA, and RNA synthesis would have been prevented). Fossil evidence supposedly suggests that photosynthetic organisms have been present on the Earth for at least 3.5 billion years. Methane also has an identical problem to ammonia; it is sensitive to solar UV radiation in an oxidative atmosphere. The problem is still unsolved, but some unique conditions have existed to prevent the Earth from becoming a planet frozen in solid ice in the early stages or a sweltering inferno at present.
By devouring 300 million tons of methane each year, archaea organisms may help keep this greenhouse gas in check 3
Buried in the ocean floor are more than 10 trillion tons of methane-twice the amount of all known coal, oil, and other fossil fuels. Methane (CK) is also 25 times more potent, molecule for molecule, as a greenhouse gas than carbon dioxide is. That means that the ocean's hidden methane reservoirs could play havoc with the world's climate if they were to escape to the atmosphere. Yet most of the methane that does rise toward the surface of the ocean floor vanishes before it even reaches the water. A team of researchers provided the clinching evidence for where all that methane goes: It is devoured by vast hordes of mud-dwelling microbes that microbiologists once said couldn't exist. These methane-eating microbes-once thought to be impossible now look to be profoundly important to the planet's carbon cycle.
For instance, without CO2 and other greenhouse gases, Earth would be a frozen ball of rock. With too many greenhouse gases, however, Earth would be like hothouse Venus. Just right means balancing between the two extremes, which helps to keep the planet’s temperature relatively stable.
It has been estimated that these bacteria-like organisms consume 300 million tons of methane every year to prevent the Earth from turning into a furnace. If they had not been established at some point in the history of Earth, we probably would not have been here. On early Earth, the microbes might have been even more significant. Atmospheric scientists pointed out that methane levels in the atmosphere may have been 1000 times greater than they are today, initially formed by volcanoes and then by methane-producing microbes. Initially, methane may have been beneficial, producing a greenhouse effect that prevented the planet from freezing. However, if the rise in methane levels had gone unchecked, Earth might have become too hot for life to exist, as Venus. We may have the evolution of methane-eating archaea to thank for saving us from that grim fate. "If they hadn't been established at some point in Earth's history," says Hinrichs, "we probably wouldn't be here."
Researchers have discovered a possible new species of bacteria that survives by producing and 'breathing' its own oxygen. The finding suggests that some microbes could have thrived without oxygen-producing plants on the early Earth — and on other planets — by using their own oxygen to garner energy from methane (CH4). The oxygen-producing bacterium, provisionally named Methylomirabilis oxyfera, grows in a layer of methane-rich but oxygen-poor mud at the bottom of rivers and lakes. 4
This bacterium is a member of the deep-branching ‘NC10’ phylum, thus it is evolutionary unrelated to the previously known methanotrophs 8 W e found that “Ca. Methylomirabilis oxyfera” cells possess an unusual polygonal cell shape. To our knowledge, the presence of a star-like cell shape was reported only once in the literature.
Jetten and his colleagues have described a fourth pathway, in which microbes extract energy from methane through a chemical process linked to denitrification, which releases nitrogen and oxygen from nitrogen oxides. The two known groups of methane-consuming bacteria live in either the absence of oxygen (anaerobic methanotrophs) or exploit oxygen from the atmosphere (aerobic methanotrophs). But M. oxyfera can survive in methane-rich areas that are inhospitable to many other bacteria. It does this with the help of an enzyme, perhaps a nitric oxide dismutase, that combines two molecules of nitric oxide to form nitrogen and oxygen. The oxygen is then used to metabolize methane to produce water and carbon dioxide.
"It's a very unusual form of metabolism in that it's not directly utilizing oxygen from photosynthesis," says David Valentine, a geomicrobiologist at the University of California in Santa Barbara, who was not involved with the study. "It's an anaerobic form of metabolism at heart that then produces oxygen and becomes an aerobic form of metabolism."
Which came first?
The order of evolution of metabolic pathways on the early Earth is still hotly debated. Numerous enzymes exist that use oxygen but seem to pre-date the actual presence of oxygen on Earth from photosynthesis, says Ettwig. One possible explanation for this is that these enzymes did not originally use oxygen, but rather nitric oxide, which would require a similar metabolic pathway. "We add another possibility to this debate — that some microorganisms could have produced their own oxygen," says Ettwig. Oremland adds that the study introduces numerous questions of evolutionary significance that can only be answered through further studies. "We need to figure out who came first — aerobic methanotrophs that we've studied for so long and know so much about, or these guys?"
So there is clearly a further essential factor to keep the climate of our planet in balance. That is methane-eating Archean Organisms on the ocean floor. Question: Had these Organisms not have to be on earth right from the start, otherwise, life could never have taken off based on an inhospitable climate on earth? The fact is, however, that multiple factors would have to be taken into account, and the data we have today at hand simply does not suffice to draw a clear picture. No wonder, do the authors of Natura Magazine admit: Despite all of these proposed warming mechanisms, there are still reasons to think that the faint young Sun problem is not yet solved. 4 And Alicia Newton writes in Nature Geoscience: Challenges for each hypothesis remain and are likely to remain for some time. The faint early Sun paradox persists, but not for lack of research efforts. 5
The surface temperature of the early Earth is another controversial point in determining the likely environmental conditions present. 6 At the lower luminosities of billions of years ago, less radiation would have reached the surface of the Earth, resulting in a surface temperature below the freezing point of water. Considering this, it is plausible that the Earth was completely icebound, in a similar manner to the later “Snowball Earth” periods that have been suggested. However, given the geological evidence of liquid water on the Earth at this time, this is at least an incomplete picture of the early Earth. This is the so-called faint young sun paradox, based on the conflicting data between decreased solar luminosity and evidence of liquid water. This paradox can be reconciled by invoking an atmospheric composition with sufficient greenhouse gases to trap enough heat on the Earth to compensate for the lower influx of light.
1. Jeffrey M. Lichtman - Solar Planetary Systems, page 158
4. Faint young Sun redux, Nature magazine, NATURE|Vol 464|1 April 2010
5. Newton, A., Warming the early Earth, Nature Geoscience 3:458, 2010