As life has evolved on Earth, it has been organised into biological communities of increasing complexity. The earliest evidence of true terrestrial ecosystems comes from the Ordovician Period, about 460 million years ago. During the Paleozoic, terrestrial animal life progressed from the earliest arthropods and tetrapod vertebrates through many of the modern invertebrate orders, amphibians and reptiles. The extinction event at the end of the Permian allowed new groups of plants and animals to dominate, including gymnosperms and dinosaurs. The mass extinction at the end of the Cretaceous likewise allowed angiosperms and mammals to come to dominance in the Cenozoic. From an ecological perspective, perhaps the most significant evolutionary event during the Quaternary Period was the rise of our own species, Homo sapiens, as we have reshaped the biotic world during the time since the last glaciation. The fossil evidence clearly shows that biological communities are temporary associations of species that shift in response to environmental disturbances.

Key Concepts:

• The history of life on Earth has been punctuated by a series of mass extinctions that have ended the prominence of more primitive groups and given new groups the opportunity to flourish.
  
• Biological communities are temporary associations of species that shift in response to environmental disturbances.
  
• The important time scales of ecological change are centuries and millennia: intervals too long to be observed by one generation of scientists.
  
• Biodiversity adds to the stability of ecosystems.