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What is Life?

What is life? If this strikes you as an odd question, consider it for a moment. (It's almost as hard as describing the origin of life). Of course we all have an intuitive sense of what life is, but how would you actually define it? Given the incredible diversity of living things, from the mold on your old tuna sandwich to monkeys in the rainforest, how could one word encompass all that "life" represents?

Even biologists have a difficult time defining the very concept they study. However, given our knowledge of the life forms present on Earth, it is recognized that all living things do share a set of characteristics in common:

1) Living things take in and use energy
2) Living things eliminate waste products
3) Living things grow and develop
4) Living things respond to their environment
5) Living things reproduce
6) Living things show variation based on heredity
7) Over time, living things evolve (change slowly) in response to their environment.

Therefore, in order for something to be considered to be "alive" or to "have life" as we know it, it must possess these characteristics.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA