Scientists studying early life on Earth found evidence of simple organisms dating back about 3.5 billion years. Which type of organisms were most likely the first forms of life?

A geologist discovers marine fossils in a dry, landlocked area. What does this most strongly suggest?

Why are rock layers important in understanding Earth’s history?

Why were ancient zircon crystals important in estimating Earth’s age?

A science teacher compares Earth’s 4.6-billion-year history to a single calendar year. Most of that year is dominated by which major biological phase?