AP BIOLOGY

Detailed explanation-1: -The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old.

Detailed explanation-2: -"This means life could have begun as little as 300 million years after Earth formed.

Detailed explanation-3: -The geologic record shows evidence for early life in two ways: 1) carbon isotopes, and 2) fossil stromatolites.

Detailed explanation-4: -Researchers at UCLA and the University of Wisconsin–Madison have confirmed that microscopic fossils discovered in a nearly 3.5 billion-year-old piece of rock in Western Australia are the oldest fossils ever found and indeed the earliest direct evidence of life on Earth.