Answer:
Earth is estimated to be about 4.5 billion years old, and for much of that history it has been home to life in one weird form or another.
Indeed, some scientists think life appeared the moment our planet's environment was stable enough to support it.
The earliest evidence for life on Earth comes from fossilized mats of cyanobacteria called stromatolites in Australia that are about 3.4 billion years old. Ancient as their origins are, these bacteria (which are still around today) are already biologically complex - they have cell walls protecting their protein-producing DNA, so scientists think life must have begun much earlier, perhaps as early as 3.8 billion years ago.
But despite knowing approximately when life first appeared on Earth, scientists are still far from answering how it appeared.
"Many theories of the origin of life have been proposed, but since it's hard to prove or disprove them, no fully accepted theory exists," said Diana Northup, a cave biologist at the University of New Mexico.
The answer to this question would not only fill one of the largest gaps in scientists' understanding of nature, but also would have important implications for the likelihood of finding life elsewhere in the universe.
Lots of ideas
Data: The rise and fall of Earth's species
Today, there are several competing theories for how life arose on Earth. Some question whether life began on Earth at all, asserting instead that it came from a distant world or the heart of a fallen comet
or asteroid. Some even say life might have arisen here more than once.
"There may have been several origins," said David Deamer, a biochemist at the University of California, Santa Cruz. "We usually make 'origins' plural just to indicate that we don't necessarily claim there was just a single origin, but just an origin that didn't happen to get blasted by giant [asteroid] impacts."
Most scientists agree that life went through a period when RNA was the head-honcho molecule, guiding life through its nascent stages. According to this "RNA World" hypothesis, RNA was the crux molecule for primitive life and only took a backseat when DNA and proteins - which perform their jobs much more efficiently than RNA - developed.
"A lot of the most clever and most talented people in my field have accepted that the RNA World was not just possible, but probable," Deamer said.
by-Galib alfayed