A chemical equation is a way of describing how
A.
a mixture of substances are separated into two or more pure substances that do not react with each other.
B.
pure substances are combined into a mixture that has the same properties as the original substances.
C.
pure substances change to produce one or more pure substances with different properties from the original substances.
D.
a mixture of substances change to produce one or more mixtures with the same properties as the original substances.

several tissues working together form an organ, and several organs together make up an organisms.

the group of cell that carries out special jobs are tissue.

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the oldest known insects are believed to live around 400 millions ago as suggested by the fossil records.   some plants having woody stems also evolved during this time.

after the evolution of insects the first four legged species or the tetrapods evolved in shallow freshwater habitats from the intermediate species approximately 397 million years ago. the tetrapods went on conquering the land giving rise to amphibians, reptiles, mammals and birds.

the major split in tetrapods first occurred 340 million years ago branching off from amphibians.

363 million years ago when the carboniferous period began the insects started roaming on land and taking flights, following which the four limbed tetrapods adapted in the terrestrial habitat.