The unicellular organism, Cyanobacteria or ’blue-green algae’, was the first form of life on earth around two thousand million years ago.
The geologic time scale is divided into two eons – Precambrian and Phanerozoic. The Precambrian eon is divided into the Hadean, Archean and Proterozoic eras, while the Phanerozoic eon is divided into the Palaeozoic, Mesozoic and Cenozoic eras.
Different life forms evolved in the three eras of the Phanerozoic eon. The Palaeozoic era spans from around five hundred forty million years ago to about two hundred forty eight million years ago. It is divided into six periods which are the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. In the Cambrian period, plant life consisted of primitive algae and seaweeds while animal life consisted of invertebrates such as sponges, starfish and sea urchins. In the Ordovician and Silurian periods, true plants with a stem, leaves and roots developed. The Ordovician period also marked the evolution of the first vertebrate. In the Devonian period plants grew and covered a large part of the land. About three hundred million years ago, in the Carboniferous period, Pteridophytes dominated the earth, there were lung fish, lobe fin fish which later evolved as amphibians. The last period of the Paleozoic era was the Permian period in which amphibians evolved into reptiles. Reptiles became the first animals to successfully lay eggs on land and are known to be first terrestrial animals.
Mesozoic era, which extends from two hundred forty five million years ago to sixty five million years ago. This era includes the Triassic, Jurassic and Cretaceous periods. In the Mesozoic era, the land reptiles of the Mesozoic era were the famous dinosaurs with terrifying, large teeth. They were a diverse group of reptiles which remained the most important terrestrial vertebrates for more than one hundred and sixty million years. Dinosaurs suddenly disappeared sixty five million years ago. Birds have descended from Ornithischian dinosaurs during the Jurassic period of the Mesozoic era. The flora in the Mesozoic era was dominated by flowering plants and non-flowering plants such as ferns, pines, conifers and cycads. Cenozoic era, which began sixty five million years ago and continues till today. This era is also known as the Age of Mammals, which are highly evolved vertebrates. The most successful evolution story was that of human beings as they developed language skills and self-consciousness as well.