when first cellular life form on earth appeared?
According to NCERT it is given 4000-3500 mya and 2000 mya which is ambiguous.
Which is correct?
3.8 billion years ago?
This is our current “best guess” for the beginning of life on Earth. It is distinctly possible that this date will change as more evidence comes to light. The first life may have developed in undersea alkaline vents, and was probably based on RNA rather than DNA.
At some point far back in time, a common ancestor gave rise to two main groups of life: bacteria and archaea.
3.5 billion years ago
The oldest fossils of single-celled organisms date from this time.
3.46 billion years ago
Some single-celled organisms may be feeding on methane by this time.
3.4 billion years ago
Rock formations in Western Australia, that some researchers claim are fossilised microbes, date from this period.
3 billion years ago
Viruses are present by this time, but they may be as old as life itself.
2.4 billion years ago
The “great oxidation event”. Supposedly, the poisonous waste produced by photosynthetic cyanobacteria – oxygen – starts to build up in the atmosphere. Dissolved oxygen makes the iron in the oceans “rust” and sink to the seafloor, forming striking banded iron formations.
Recently, though, some researchers have challenged this idea.
2.3 billion years ago
Earth freezes over in what may have been the first “snowball Earth”, possibly as a result of a lack of volcanic activity. When the ice eventually melts, it indirectly leads to more oxygen being released into the atmosphere.
2.15 billion years ago
First undisputed fossil evidence of cyanobacteria, and of photosynthesis: the ability to take in sunlight and carbon dioxide, and obtain energy, releasing oxygen as a by-product.
2 billion years ago?
Eukaryotic cells – cells with internal “organs” (known as organelles) – come into being. One key organelle is the nucleus: the control centre of the cell, in which the genes are stored in the form of DNA.