Answer to Question 1(1) An exoskeleton, a strong, lightweight, form-fitted external covering and support.
(2) Striated muscle, a quick, strong, lightweight form of muscle that makes rapid movement and flight possible.
(3) Articulation, the ability to bend appendages at specific points. The appendages of more
primitive phyla can usually bend anywhere, but arthropod appendages bend at a joint. There
are no ball-and-socket joints in arthropods; instead, each joint along an appendage moves
through a different plane to ensure a full range of motion.
Answer to Question 2The first primary producers, probably an early form of cyanobacteria, assembled their own
food from inorganic molecules and then broke the food down to release energy. With the
success and proliferation of these simple autotrophs, and with the abundance of oxygen they
eventually provided, the way was clear for the evolution of animals. The first animal-like
creatures were single-celled organisms. They began to prosper in the ocean during the
oxygen revolution, a time of radical change in Earths atmosphere. During the oxygen
revolution, between about 2 billion and 400 million years ago, the activity of photosynthetic
autotrophs changed the composition of the atmosphere from less than 1 free oxygen to its
present oxygen-rich mixture of more than 20. The growing abundance of free oxygen made
aerobic respiration practical, speeding the disassembly of food molecules that early animals
obtained by eating the autotrophs. Ozone derived from this oxygen blocked most of the suns
dangerous ultraviolet radiation from reaching Earths surface, permitting life to survive at the
surface of the ocean and, later, on land.
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