Historical perspective

Based on the available evidence, scientists have reconstructed detailed information about the planet's past. Earth is estimated to have formed approximately 4.55 billion years ago out of the solar nebula, along with the Sun and other planets. The moon formed relatively soon afterwards.

Initially molten, the outer layer of the planet cooled, resulting in the solid crust. Outgas sing and volcanic activity produced the primordial atmosphere. Condensing water vapor, augmented by ice delivered by comets, produced the oceans. The highly energetic chemistry is believed to have produced a self-replicating molecule around 4 billion years ago.

Continents formed, then broke up and re-formed as the surface of Earth reshaped itself over the course of hundreds of millions of years, occasionally combining to make a super continent. Roughly 750 million years ago, the earliest known super continent Rodinia, began to break apart. The continents later recombined to form Pannotia which broke apart about 540 million years ago, then finally Pangaea, which broke apart about 180 million years ago.

Birds

Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviors including cooperative hunting, cooperative breeding, flocking and mobbing of predators. Birds are primarily socially monogamous, with engagement in extra-pair copulations being common in some species-other species have polygamous or polyandrous breeding systems. Eggs are regularly laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, and they provide other products. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120-130 species have become extinct as a result of human activity since 1600, and hundreds more before this. Currently around 1,200 species of birds are threatened with extinction by human activities and efforts are underway to protect them.