<span>undergo a dramatic decline in size, possibly to a stable level at or below 1,800 individuals.</span>
Definite article is used before a noun (person place or thing) to indicate that the identity of the noun is known to the reader.
indefinite article is used before a noun that is general or when its identity is not known.
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<em>Merit-Ptah
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<em>Ancient history. The involvement of women in the field of medicine has been recorded in several early civilizations. An ancient Egyptian physician, Merit-Ptah ( c. 2700 BC), described in an inscription as "chief physician", is the earliest known female scientist named in the history of science.</em>
