they are caused by mutation. the best example of it are human blood groups.
Carbo dioxide, water and sun's energy comes fron the outside.... energy and Oxygen molecule are produced
Answer:
Early land animals had both lungs AND gills, and their bodies consisted of few hard parts. Meaning they lacked things like an exoskeleton for instance. Animals today have either lungs OR gills and many have protecting hard parts of their bodies.
Explanation:
