The Origin of Life
Somewhere, and at some time on this earth, these gases combined together. They took energy from heat and light to form amino acids, the essential chemicals for all forms of life. As earth cooled over millions of years these compounds accumulated within oceans, lakes and ponds to evolve into the simplest forms of life that could reproduce. This is understood as the origin of life. Early life was anaerobic, fermentative and chemoautotrophic. There was no free oxygen in the atmosphere. It was like this for almost 2 billion years.

Photosynthesis
Nature simultaneously was unfolding a new drama. The first photosynthetic bacteria had started appearing in the sea some 3.5 billion years ago. Oxygen started evolving as a by-product of photosynthesis. It took marine photosynthesisers another billion years to enrich the oceans and the atmosphere with enough oxygen and to create the ozone layer. The oxygen poisoned the then predominant life forms, which were anaerobic. Today their descendants survive in obscure, oxygen deficient environments.

Life in the Oceans
Till about 450 millions years ago, life was confined to the oceans. Terrestrial life had not evolved. Marine ecosystems were well developed. There were shallow water and deep-water organisms, floating and swimming and the bottom dwelling. Most of the animal phyla known today were present – except the chordates. The most abundant fossils from the period which was known as the Cambrian were Brachiopods and Arthropods, which were again very different from present day forms. The Trilobites, of which more than a 1000 species have been described from the Cambrian rocks alone formed a diverse group which eventually disappeared about 425 million years ago. The brachiopods and the coelenterates and sponges exist still, though in different forms.

The Great Migration
Early organisms in the oceans critically modified the physical environment by giving earth an oxygen rich environment. This enabled living things to leave the sea and colonise lands. The first forest ecosystems started fringing the shallow seas about 300 million years ago. They were mostly relatives of the present day horsetails, club mosses and ferns. Some were gymnosperms. Angiosperms have appeared just about 75 million years ago.

Our distant ancestors, the amphibians also left the oceans to colonise the lands about 300 million years ago and through a series of evolutionary steps, culminated in the immense biodiversity we witness today. A diversity that has come about with precision and perfection, each species being unique in its own way