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The panspermia hypothesis therefore answers questions of where, not how, life came to be; it only postulates that life may have originated in a locale outside the Earth.Nonetheless, Earth remains the only place in the Universe known to harbour life, and possibly as early as the Eoarchean Era, after a geological crust started to solidify following the earlier molten Hadean Eon.
The current Japanese probe Akatsuki is designed to give answers to the question why did Venus, which probably originated with conditions similar to those on early Earth, within the Circumstellar Habitable Zone or CHZ, take such a different evolutionary pathway?
The answer to this question, not only will help us find what proportion of exoplanets currently being discovered are likely to harbour life, but is also likely to help us discover how life evolved on Earth.
On current evidence it is believed that there may be 40 billion planets orbiting within the CHZ within the Milky Way Galaxy that could harbour life.
The question of the origin of life on Earth is likely to impact our thinking on the prevalence of life in our galaxy.
The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors.
Precambrian stromatolites in the Siyeh Formation, Glacier National Park.
In 2002, a paper in the scientific journal Nature suggested that these 3.5 Ga (billion years) old geological formations contain fossilized cyanobacteria microbes.
This suggests they are evidence of one of the earliest known life forms on Earth. Abiogenesis is studied through a combination of laboratory experiments and extrapolation from the characteristics of modern organisms, and aims to determine how pre-life chemical reactions gave rise to life on Earth.
The classic Miller–Urey experiment and similar research demonstrated that most amino acids, the basic chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth.
Various external sources of energy that may have triggered these reactions have been proposed, including lightning and radiation.
Other approaches ("metabolism-first" hypotheses) focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
The panspermia hypothesis alternatively suggests that microscopic life was distributed to the early Earth by meteoroids, asteroids and other small Solar System bodies and that life may exist throughout the Universe.