Life - Origin

Origin

Evidence suggests that life on Earth has existed for about 3.7 billion years, with the oldest traces of life found in fossils dating back 3.4 billion years. All known life forms share fundamental molecular mechanisms, reflecting their common descent; based on these observations, hypotheses on the origin of life attempt to find a mechanism explaining the formation of a universal common ancestor, from simple organic molecules via pre-cellular life to protocells and metabolism. Models have been divided into "genes-first" and "metabolism-first" categories, but a recent trend is the emergence of hybrid models that combine both categories.

There is no current scientific consensus as to how life originated. However, most accepted scientific models build on the following observations:

• The Miller-Urey experiment, and the work of Sidney Fox, show that conditions on the primitive Earth favored chemical reactions that synthesize amino acids and other organic compounds from inorganic precursors.
• Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.

Living organisms synthesize proteins, which are polymers of amino acids using instructions encoded by deoxyribonucleic acid (DNA). Protein synthesis entails intermediary ribonucleic acid (RNA) polymers. One possibility for how life began is that genes originated first, followed by proteins; the alternative being that proteins came first and then genes.

However, since genes and proteins are both required to produce the other, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because of this, it is unlikely that genes and proteins arose independently.

Therefore, a possibility, first suggested by Francis Crick, is that the first life was based on RNA, which has the DNA-like properties of information storage and the catalytic properties of some proteins. This is called the RNA world hypothesis, and it is supported by the observation that many of the most critical components of cells (those that evolve the slowest) are composed mostly or entirely of RNA. Also, many critical cofactors (ATP, Acetyl-CoA, NADH, etc.) are either nucleotides or substances clearly related to them. The catalytic properties of RNA had not yet been demonstrated when the hypothesis was first proposed, but they were confirmed by Thomas Cech in 1986.

One issue with the RNA world hypothesis is that synthesis of RNA from simple inorganic precursors is more difficult than for other organic molecules. One reason for this is that RNA precursors are very stable and react with each other very slowly under ambient conditions, and it has also been proposed that living organisms consisted of other molecules before RNA. However, the successful synthesis of certain RNA molecules under the conditions that existed prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction. This study makes the RNA world hypothesis more plausible.

In 2009, experiments demonstrated Darwinian evolution of a two-component system of RNA enzymes (ribozymes) in vitro. The work was performed in the laboratory of Gerald Joyce, who stated, "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system."

NASA findings in 2011, based on studies with meteorites found on Earth, suggest DNA and RNA components (adenine, guanine and related organic molecules) may be formed extraterrestrially in outer space.