BBC Nature – History of life on Earth

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  • Geological timeline | Geological time periods | Big Five mass extinction events |
  • Mass extinction theories | Ancient Earth habitats
  • Geological timeline

History of life on Earth

Pangea supercontinent from space, as it may have looked 300 million years ago.Pangea supercontinent from space, as it may have looked 300 million years ago.

The Earth is a little over 4.5 billion years old, its oldest materials being 4.3 billion-year-old zircon crystals. Its earliest times were geologically violent, and it suffered constant bombardment from meteorites. When this ended, the Earth cooled and its surface solidified to a crust – the first solid rocks. There were no continents as yet, just a global ocean peppered with small islands. Erosion, sedimentation and volcanic activity – possibly assisted by more meteor impacts – eventually created small proto-continents which grew until they reached roughly their current size 2.5 billion years ago. The continents have since repeatedly collided and been torn apart, so maps of Earth in the distant past are quite different to today’s.

The history of life on Earth began about 3.8 billion years ago, initially with single-celled prokaryotic cells, such as bacteria. Multicellular life evolved over a billion years later and it’s only in the last 570 million years that the kind of life forms we are familiar with began to evolve, starting with arthropods, followed by fish 530 million years ago (Ma), land plants 475Ma and forests 385Ma. Mammals didn’t evolve until 200Ma and our own species, Homo sapiens, only 200,000 years ago. So humans have been around for a mere 0.004% of the Earth’s history.

Jump to:Geological timeline | Geological time periods |Big Five mass extinction events |
Mass extinction theories | Ancient Earth habitats

Geological timeline

During its dramatic 4.5 billion year history, Earth has gone through a series of major geological and biological changes. The timescale below highlights a number of notable prehistoric events and the geological periods in which they occurred. As things didn’t get interesting from a biological perspective until around 570 million years ago, we’ve included a couple of zoomed in timelines to show the detail of more recent evolutionary history. Show text only timeline

Geological time periods

Geologists have organised the history of the Earth into a timescale on which large chunks of time are called periods and smaller ones called epochs. Each period is separated by a major geological or palaeontological event, such as the mass extinction of the dinosaurs which occurred at the boundary between the Cretaceous period and the Paleocene epoch.

Big Five mass extinction events

Although the Cretaceous-Tertiary (or K-T) extinction event is the most well-known because it wiped out the dinosaurs, a series of other mass extinction events has occurred throughout the history of the Earth, some even more devastating than K-T. Mass extinctions are periods in Earth’s history when abnormally large numbers of species die out simultaneously or within a limited time frame. The most severe occurred at the end of the Permian period when 96% of all species perished. This along with K-T are two of the Big Five mass extinctions, each of which wiped out at least half of all species. Many smaller scale mass extinctions have occurred, indeed the disappearance of many animals and plants at the hands of man in prehistoric, historic and modern times will eventually show up in the fossil record as mass extinctions. Discover more about Earth’s major extinction events below.

Mass extinction theories

Asteroid impacts, climate change, volcanoes – there have been many theories about the causes of mass extinctions. In some cases, such as the Cretaceous mass extinction event, more than one such factor was involved in the global catastrophe.

Ancient Earth habitats

If you were able to travel back far in time, you’d find Earth to be a very different place – at times a giant hot molten ball of rock, at others a frozen planet completely covered in snow and ice. During its long history, Earth has been covered by habitats and experienced climates that no longer exist. Discover more about these and about the dramatic story of ancient Earth.

via BBC Nature – History of life on Earth.

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