Life on Earth

Dating of radioactive material from the Earth, meteorites, and the Moon, indicates that the Earth is about 4.5 billion years old. The most ancient fossils date life back about 3.5 billion years, with recent reports of isotopic dating of possibly once organic material pushes that date to 3.8 billion years. Considering that the surface of the Earth solidified and cooled about 4 billion years ago, it is remarkable that life emerged on a such short geological time-scale.

It is clear that life on Earth, from bacteria to humans, has a common origin, as attested by the paleontological record, analysis of and the commonality in the machinery of the cell. Furthermore, while molecules might have a symmetry such that they come right and left handed, or one the mirror image of the other, molecules (amino-acids) produced by living matter always comes with a left-handed symmetry, again underscoring the common origin of all life forms.

The big question of interest to scientists looking for life on other planets is: how did the first living cell form? There are three scenarios actively studied:

- life began in warm tidal pools, where water provided the medium to move molecules from place to place and the warm temperature speeded up chemical reactions. Against this scenario one might consider the fact that back then there was much more ultra-violet radiation reaching the Earth, since there was no ozone for shielding (free oxygen in the atmosphere is the product of reactions of living matter chiefly through photosynthesis; this occurred about 2.5 billion years ago). Considering that at the beginning the Earth was subjected to intense bombardment of asteroids, comets, etc. survival of emerging life forms could have been difficult. Warm tidal pools might not have been common on Earth, since the Sun, at that time, emitted 30% less energy and some scientists speculate that the Earth was covered in large part by ice sheets. To counter this argument, other scientists think that the Earth could have been covered by a blanket of greenhouse gases (such as carbon dioxide, methane, etc.) that would have raised the temperature of the surface of the Earth, as it happened to Venus in a much more dramatic way. There is no definite answer on which of the scenarios described above actually took place in the early history of the Earth;

- life began in hot springs or in vents on the ocean floors (hydrothermal vents). Simple bacteria have been found to live in conditions that, until recently, were considered off limit for any life-form, such as the extreme pressure and temperature (450 C) of vents in the ocean floors. In this scenario, life could have survived the harsh conditions at the surface of the Earth that we mentioned above;

- life began after shocks imparted by one or more extraterrestrial bodies impacting the Earth and producing chemicals that eventually assembled in primitive forms of RNA. At present there is no known test that we can do to find out how life actually emerged; however, the different scenarios give scientists a roadmap of possible places where to look for life on other planets.