In warm pools?
Darwin suggested life began in “a warm little pond”. Earth’s early atmosphere would have been rich in methane, ammonia, and other gases on which lightning and radiation might have acted to create complex organics including amino acids. These would have rained down into pools on the surface. As concentrations grew, proteins and nucleic acids would have formed, eventually producing the first life-forms.
In deep-sea vents?
Some scientists believe life began near deep-sea hydrothermal vents. The energy liberated there would have fuelled the chemical reactions needed to construct complex proteins. This idea finds support in DNA sequences of modern organisms, which suggest the most recent common ancestor of all life was probably an aquatic micro-organism that lived in extremely high temperatures – a thermophile.
Life requires water but too much can be harmful. Some scientists think that drier Mars would have been the best bet as a birthplace for life. Meteorite impacts would have blasted Martian rocks into space and all the way to Earth, carrying life with them. The hardy ones would have colonised Earth. In short, we might all be Martians.
On unknown planetary systems?
Panspermia, the idea that life comes from space, was developed by Swedish scientist Svante Arrhenius who argued that bacterial spores from another planetary system, propelled through space by light pressure, seeded life here. British astronomers Fred Hoyle and Chandra Wickramasinghe later argued that comets could carry bacterial life across space and protect it from radiation damage en route.
A gift from aliens?
Scientists such as Francis Crick and the late Carl Sagan have suggested microbes might have been sent to Earth by an advanced civilisation was facing catastrophic annihilation or hoping to adapt planets for later colonisation. This concept is known as directed panspermia, and we are the end product.