• First life forms arose on Earth more than 3.5 billion years ago  
  • Single-celled, microscopic organisms (living thing) appeared first & floated alone in seas  
  • Over 40 million species (types of organisms) exist with only about 2 million identified 
  • Many organisms are unidentified & new species are still discovered  
  • Biology is the study of all living things and how they interact with each other & their environment  
  • Over long periods of timespecies changed or evolved so that new species arose from earlier organisms & came to inhabit almost every part of the earth (bacteria living in thermal vents, parasites living inside another organism, etc.)  
  • Organisms must adapt to their specific environment to survive & reproduce