ORIGIN OF LIFE: –
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, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre- cellular life to protocells and metabolism. Many models fall into the “genes-first” category or the “metabolism-first” category, but a recent trend is the emergence of hybrid models that combine both categories.
There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses:
Life as we know it today synthesizes proteins, which are polymers of amino acids using instructions encoded by cellular genes—which are polymers of deoxyribonucleic acid (DNA). Protein synthesis also entails intermediary ribonucleic acid (RNA) polymers. One possibility is that genes came first and then proteins. Another possibility is that proteins came first and then genes. However, because genes are required to make proteins, and proteins are required to make genes, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because DNA and proteins function together so intimately, it’s unlikely that they arose independently. Therefore, many scientists consider the possibility, apparently first suggested by Francis Crick that the first life was based on the DNA- protein intermediary: RNA, In fact, RNA has the DNA-like properties of information storage and replication and the catalytic properties of some proteins. Crick and others actually favored the RNA-first hypothesis even before the catalytic properties of RNA had been demonstrated by Thomas Cech.
Recent experiments have demonstrated true Darwinian evolution of unique RNA enzymes (ribozymes) made up of two separate catalytic components that replicate each other in vitro. In describing this work from his laboratory, Gerald Joyce stated: “This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system.” Such experiments make the possibility of a primordial RNA World even more attractive to many scientists.
Recent findings by NASA, based on studies with meteorites found on Earth, suggests DNA and RNA components (adenine, guanine and related organic molecules) may also be formed extraterrestrially in outer space.