Johan (or Johann) Schreyer studied medicine at Jena, Germany, under J.T. Schenck and submitted a dissertation entitled Disputatio medica inauguralis de ephemera... (36p.) in 1658. A few years earlier he had already contributed an article or short thesis, "De palpitatione cordis et syncope", to a collection of theses published by W. Rolfinck under the title Epitome methodi cognoscendi... (Jena, 1655). In 1668 he joined the Dutch East India Company as a soldier and arrived in 'Table Bay on the Eendracht on 3 December 1669. Suffering from scurvy he was left behind when the ship sailed ten days later. His rank was that of "adelborst" - a soldier of superior education, not an officer in training. Early in 1670 he acted as surgeon to parties bartering for cattle in the interior and in March that year was promoted to under-surgeon. By 1671 he was listed as surgeon and on 11 April 1672 was promoted to upper-surgeon and put in charge of the colony's hospital. Meanwhile he had married the widow Jacomijntje Bakkers on 1 January 1672. He probably left the Cape in 1676.
Schreyer wrote a book entitled Neue ost-indianische Reisz-Beschreibung von anno 1669 bisz 1677... (New East Indian travel book, from the year 1669 to 1677; Saalfeld, 1679; 2nd ed. Leipzig, 1681). The first part comprised a description of the Cape of Good Hope, based on his own observations. The second part was a second-hand account of the Dutch posessions in the East and of little value, as there is no evidence that he visited those regions. A much later edition of the book, entitled Reise nach dem Kaplande und Beschreibung der Hottentotten, 1669-1677 (Journey to the Cape and description of the Hottentots, 1669-1677; The Hague, 1931) deals only with the Cape.
Schreyers's description of the Khoi is based almost entirely on his own observations and represents the earliest comprehensive, first-hand account of their culture. It is particularly valuable because it reflects their way of life before the changes brought about by European colonisation. The book also contains the earliest first-hand and reasonably detailed descriptions of the flora and fauna of the colony, with separate short chapters devoted to the birds, marine animals, land mammals, climate, plants, and vines of the Cape. In some respects the author was far ahead of his time, for example, he corrects several widely accepted errors pertaining to the ostrich, porcupine and rhinoceros. He took some boxes of bulbs with him to the Netherlands, but found that they did not flower as well there as at the Cape. His book, and particularly his description of the Khoi, was often quoted by later authors, including G. Meister*, and some of his statements can be traced in the work of P. Kolb*, F. Valentijn* and others.
A work by him published long after his death was entitled Erörterung und Erläuterung der Frage: ob es ein gewiss Zeichen, wenn eines todten Kindes Lunge im wasser untersincket, das solches in Mutter-Leibe gestorben sei?" (Discussion and explanation of the question: Whether it is a sure sign, if the lungs of a dead child sink in water, that the child died in the mother's body?; Zeitz, Germany, 1690; later editions 1725, 1745).