Early pre-cambrian period the atmosphere consisted only of nitrogen and carbon dioxide without oxygen.
<span>A Prokaryotic cell does not contain a nucleus. It only contains one chromosome and is a single-celled organism. It was the only form of life on earth for millions of years. Examples of a Prokaryotic cell are the different types of bacteria present today.
</span><span>Autotrophs obtain energy by the process of photosynthesis. Any living organism need energy to survive and autotrophs are no different. Two types of autotrophs are present and they are Photoautotrophs and Chemoautotrophs. The Photoautotrophs rely on the energy produced during the process of photosynthesis and the Chemoautotrophs get their energy from most of the inorganic substances. Phtoautotrophs actually get their energy from the sunlight and then convert it into reusable form. In case of Chemoautotrophs ammonia is an important source of energy for them.</span>
The atmosphere has a much greater percentage of oxygen now that it did before. Cyanobacteria is the organism that is largely responsible for this change.
Cast fossils are the type people are most familiar with, as they make up the spectacular dinosaur skeletons that on view in museums. Cast fossils occur when minerals deposit into the mold left by the rotting organic material, resulting in a three-dimensional replica of the hard structures of the plant or animal
