All the different life forms that we see on earth now evolved only gradually. Radiometric dating of geological (earth) strata and detailed study of the fossils found in them help us in reading important chapters in the history of life on earth since its origin 4.5 billion years ago.
Geologists have given names to different periods of the history. The microscopic, unicellular prokaryotes were the exclusive life forms on earth for nearly three billion years. Dinosaurs ruled the earth for nearly 150 million years and became extinct 65 million years ago.
If you could have traveled 200 million years back in earth’s history, you would have found neither birds nor flowering plants anywhere on earth. When did we humans arrive on this planet? Just 2 million years back. If you consider the 24 hours geological clock with the origin of life set at midnight, we can say that humans have come on this planet just less than a minute ago.
When human evolution began, forests had dwindled because of glaciation (ice age). Much of the land surface was, however, still covered by forests. The common ancestors of apes and humans had to come down from trees where they lived and walk on the ground using all four limbs.
Recent molecular studies have shown that from common ancestors, evolution of apes (Chimpanzee, gorilla, gibbon and orangutan) and that of humans, diverged about 6 million years ago.
The trends of human evolution are towards:
Fossil history reveals that human evolution began approximately 1.5 to 2 million years ago. Australopithecus is deemed to be the first human like ancestor. Fossils of an australopithecine named "Lucy" has been found in African rock deposits. Thereafter, fossils of Homo erectus which walked on two legs, were unearthed from many parts of the world.
Next to evolve was Neanderthal man and Cromagnon man. They were both Homo sapiens. Modern man, Homo sapiens sapiens meaning the wise one evolved about 50,000 years ago.