When considering the entire period of time during which life on Earth has been evolving, we must look at: D. billions of years.
Evolution can be defined as a process of gradual change in the characteristics of various species of living organism over several generations.
Basically, these characteristics refer to gene expressions that are typically transferred from parent organisms to their offspring or successive generations.
The earliest life forms known to humans existed about 3.7 billion years ago and they are microscopic organisms (microbes) whose fossils were found on rocks.
In conclusion, when you are considering the entire period of time during which life on Earth has been evolving, you must look at billions of years.
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