The Earth is estimated to have formed about 4.6 billion years ago. Earth is the only planet in which life forms have been found hitherto. Approximately 4.6 billion years ago the solar system was a cloud of dust and gas known as a solar nebula. Gravity collapsed the material in on itself as it began to spin, forming the sun in the centre of the nebula. The remaining materials began to clump up, small particles drew together by the force of gravity, into the larger particles. The solar winds swept away lighter elements such as hydrogen and helium, from the closer regions, leaving only heavy, rocky materials to create smaller terrestrial rocky planets like Earth.
Earth’s rocky core was formed first, with the heavy elements colliding and binding together. Dense materials sank to the centre, while the lighter materials formed the crust. The planets magnetic field was formed around this time. Gravity captured some of the gases that made up the planet’s early atmosphere.
The earliest life forms as known to us were microscopic organisms (microbes) that left signal of their presence in rocks 3.7 billion years ago. The signal consisted of a type of carbon molecule that is produced by living things. When cyanobacteria evolved at least 2.4 billion years ago, they set the stage for a remarkable transformation. They become Earth’s first organism to carry out photosynthesis, making food using water and energy taken from sun, and releasing oxygen as a result. This brought a dramatic rise in oxygen, making earth less hospitable for microbes who cannot tolerate oxygen. Microbes are just single cells with no organelles and no nuclei to package their DNA.
Something revolutionary happened as microbes living inside other microbes, functioning as organelles for them. Also for the first time DNA became packaged in nuclei. The new complex cells (“eukaryotic cells”) boasted specialized parts playing specialized roles that supported the whole cell. Later, eukaryotic cells engulfed photosynthetic bacteria and formed symbiotic relationship with them. The engulfed bacteria evolved into chloroplasts. 1.5 billion years ago the eukaryotes divide into three groups : the ancestors of modern plants, fungi and animals split into separate lineages, and evolve separately.
800 million years ago the early multicellular animals undergo their first splits. First they divide into essentially, the sponges and everything else – latter being known as Eumetazoa. 20 million years later, a small group called Placozoa breaks away from the rest of the Eumetazoa.
520 million years ago Conodonts appear and were given title of “earliest vertebrate” they probably looked like eels. 500 million years ago fossil evidence shows that animals were exploring the land at that time. The first animals to do so were probably euthycarcinoids- thought to be the missing link between insects and crustaceans. 489 million years ago the great Ordovician biodiversification event begins, leading to increase in biodiversity . Within each of the major group of animals and plants, many new varieties appear. Plants began to colonise the land. 200 million years ago dinosaurs started evolving. After some million years other mammals also started evolving and came into presence.