Answer to Question 1
The first land plants were believed to have become established between 500 and 425 million years ago during
the Ordovician period. This is also the period of time when invertebrates, such as arthropods and worms,
appeared on land. These first terrestrial invertebrates fed on decaying plant material.
Answer to Question 2
The oldest direct traces of life on earth date back 3.4 to 3.5 billion years. In rocks of that age in Australia
and Southern Africa, geologists have found stromatolites, layered structures created through the activity
of primitive algae and bacteria. Other Australian rocks of similar age provide even more direct evidence
of ancient life. Sections of these rocks, known as cherts, show fossilized remains of blue-green algae.
Rocks also reveal even more distant, indirect traces of life. Living things use particular isotopes (atomic
forms) of the element carbon preferentially. The mix of carbon isotopes detected in rocks from Greenland
more than 3.8 billion years ago shows evidence of life on earth.
