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:
Answer:
Explanation:
Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from oxygen molecules or nutrients into adenosine triphosphate, and then release waste products.
While scientists have made enormous discoveries, there is always more to learn. Knowledge is built on and reviewed. Sometimes new information reveals an error in understanding from the past. The study of how the world works is always ongoing.
Less expensive source of energy is available.
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