LIFE ON EARTH
It is believed that all organisms on Earth are descended from a common ancestor or ancestral gene pool. Traits shared between all living organisms is projected many times as evidence for common descent. In these days of Darwin, visible observations of morphologic similarities, like all birds have wings, even those which do not fly is shown as evidence of shared traits. Today, debates are also going on whether or not all organisms are descended from a common ancestor. The universality of genetic coding is also an indication of common ancestry. For instance, every living cell makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins, eventhough exceptions to these basic twenty amino acids are also found. But now exceptions to this universal genetic code were found in mitochondria, the tiny energy factories inside cells and hence the common ancestoral hypothesis stands challenged.
The develoment of oxygenic photosynthesis around 3 billion years ago and the subsequent development of an oxygen-rich, non-reducing atmosphere can be traced back through the formation of banded iron deposits, and later red beds of iron oxides. This is reported as a necessary prerequisite for the development of aerobic cellular respiration, that have emerged around 2 billion years ago.
In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the appearance of the first animals, the Cambrian explosion led to the creation of all the major body plans, or phyla, of modern animals. This event is now believed to have been triggered by the development of the Hox genes. Again, about 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems which we see now. Geological evidences indicates that the Earth is approximately 4.6 billion years old.