Which of the following is true? species on earth today are but a fraction of all species that ever lived. the number of species existing at one time has decreased throughout history. extinctions of past species has happened gradually and on a small scale. most organisms present early in earth's prehistory were more complex than modern organisms. "bacteria represent a newer form of life, not present during the early prehistory of earth."

Hormones are chemical molecules produced by endocrine glands. one such endocrine gland is the thyroid gland, which synthesizes the thyroid hormone, which in turn affects the heart muscles. which two statements describe the probable reason for the function of the hormone? the cells in the heart have specific receptors that allow for the intake of hormones. the heart and the endocrine glands have the exact same types of cells. all cells make the same types of hormones. thyroid hormones show their effect on the heart by means of specialized cells. thyroid cells and cardiac cells have different dna.