Source : howstuffworks
The earth is estimated to be about 4.5 billion years old, and for much of this history it has been home to life in one way or another.
In fact, some scientists believe that life came about at a time when our planet’s environment was stable enough to sustain it.
The earliest evidence of life on earth comes from fossilized mats of cyanobacteria, the stromatolites in Greenland, which are around 3.7 billion years old. No matter how old their origins are, these bacteria (which still exist today) are already biologically complex – they have cell walls that protect their protein-producing DNA, so scientists think life must have started much earlier. In fact, there are signs of life in even more primitive rocks: the 4.1 billion year old zircons of Western Australia contain high amounts of a form of carbon that is typically used in biological processes.
But while scientists have a rough idea of when it first appeared on Earth, scientists are still a long way from answering how it came into being.
“Many theories have been proposed about the origin of life, but because they are difficult to prove or disprove, there is no fully accepted theory,” said Diana Northup, a cave biologist at the University of New Mexico.
It would only fill one of the greatest gaps in scientists’ understanding of nature, but it would also have important implications for the likelihood of finding life elsewhere in the universe.
Many Ideas
Today there are several competing theories about how life on earth came about. Life began on Earth, claiming instead that it came from a distant world or from the heart of a fallen comet or asteroid. Some even say that life could have originated here more than once.
“There can be multiple origins,” said David Deamer. , Biochemist from the University of California, Santa Cruz. “We usually do ‘origins’ in the plural just to indicate that we are not necessarily claiming that there is a single origin, just one origin that has not been criticized for huge [asteroid] impacts.
Most scientists agree that life was a period when RNA was the primary molecule and life moved through its nascent stages. According to this “RNA World” hypothesis, RNA was the crucial molecule for primitive life and the second level when DNA and proteins are developed to do their jobs much more efficiently than RNA.
“Many of the brightest and most talented people in my field have accepted that the world of RNA is not just possible, it is likely,” said Deamer.
RNA is very similar to DNA and today fulfills numerous important functions in each of our cells, including as a transition molecule between DNA and protein synthesis and as an on and off switch for some genes.
But the RNA world hypothesis doesn’t explain how RNA itself came about. RNA is a complex molecule made up of repeating units of thousands of smaller molecules called nucleotides that come together in very specific and patterned ways. While there are scientists who think that RNA could have formed spontaneously on early Earth, others say the likelihood that something like this are astronomical.
“The appearance of such a molecule is incredibly unlikely, given the way chemistry works. It would be a possibility in the universe, “said Robert Shapiro, a chemist at New York University.” To take that [point of view] one has to believe that we have been incredibly lucky.
The anthropic principle
But “astronomical” is a relative term. In his book The God Delusion, biologist Richard Dawkins considers another possibility, inspired by work in astronomy and physics.
Assuming Dawkins says that the universe contains a billion trillion planets (a conservative estimate he says) then the odds that life will arise in one of them are not really remarkable, just one of many, and each universe contains a billion trillion Planets, so it is almost certain that life will arise on at least one of them.
As Dawkins writes: “There can be universes whose skies have no stars, but also have no inhabitants who notice the lack.” Shapiro does not believe the need to conjure up multiple life-laden universes or comets to collide with ancient Earth, but instead believes that life began with molecules smaller and less complex than RNA that performed simple chemical reactions which eventually leads to a self-sustaining system that involves the formation of more complex molecules.
“If you go to a simpler theory, the odds are no longer astronomical,” Shapiro said.
Trying to recreate an event that happened billions of years ago is a daunting task, but many scientists believe that it is still possible, like the genesis of life itself.
“The solution to a puzzle of this magnitude is completely unpredictable,” said Freeman Dyson, professor emeritus of physics at Princeton University in New Jersey.”It could happen next week or may take 1000 years.
Original article on Science Atom.
