ACROSS
3. period of Earth’s history from 4.6 billion to 541 million years ago the Hadean, Archean, and Protozoic Eons.
5. RNA that functions as an enzyme.
7. organism that lives inside another and does not harm it.
9. underwater opening where hot, mineral-rich water streams out.

DOWN
1. atmospheric region in the ozone that screens out incoming UV radiation.
2. theory that mitochondria and chloroplasts evolved from bacteria that entered and lived in a host cell.
4. hypothetical early interval when RNA served as the genetic material.
6. membranous sac that contains interacting organic molecules; hypothesized to have formed prior to the first life forms.
8. rocky structures composed of layers of bacterial cells and sediments.