What step number do you always start with when using a dichotomous key?

Which element is used in photosynthesis found in atmosphere and fixed by bacteria?

Which could be a pathway of food through an ecosystem

According to theories of how life began, how did early organic molecules begin to separate from the outside world? a: specialized enzymes were required b: chains of amino acids created a barrier c: formation of microspheres or vesicles d: rna catalyzed the formation of membranes