Francisco Javier Muñiz was born in San Isidro, Buenos Aires Province, Argentina on 21 December 1795.
Early military service and medical school
He studied at the Instituto Médico Militar beginning in 1814. The institute was founded by Dr. Cosme Argerich to train surgeons for the army. Muñiz graduated as a doctor in 1821 He transferred to the University of Buenos Aires and completed his surgical education in 1824. Muñiz obtained his doctorate in 1844; his dissertation was about the vaccination of indigenous peoples against smallpox. This work made him notable among European scientists. His dissertation followed the article, "A Case of Extensive Scabby Ulcerations, Cured by Vaccination" that he wrote and was published in the London Medical and Surgical Journal in 1833.
Muñiz was a paleontologist, particularly interested in variances among fossils. Working as a physician at Lujan was of particular paleontological interest because of a famous find in 1788 of the Megatherium, an immense ground sloth. He developed a collection that he intended to be used to create a natural history museum. The artifacts were sent to Juan Manuel de Rosas, the dictator of the Argentine Federation, whose support was required to establish a museum. Rosas, in an attempt to build alliances overseas, sent collected fossils to Jean Henri Dupotet, Rear Admiral of the French Navy. Dupotet then sent them to Paris. In France Muñiz collection ended up in the National Museum of Natural History where they were studied by Paul Gervais. Apuntes Toggraficos, published in 1847 by Muñiz, contained his topographical notes that discussed the study of fossils in the relative ages of sedimentary strata in areas south of the Buenos Aires Province by naturalists, including Charles Darwin. Darwin began corresponding with Muñiz after reading his work on ñata cattle, indicative of the reputation that he was gaining as a naturalist. Domingo Sarmiento, who researched Muñiz's papers, commented on his influence to Darwin's theory of the origin of species. He described a sabertooth, which he named Muni-felus Bonaerensis, in Gaceta Mercantil in 1845. He sent his description to Darwin on 30 August 1846 for his comments. Darwin encouraged him to send specimens to France, which were received in Paris, apparently sent by Rosas. It was determined to be a Smilodon bonaerensis. In their book, From Man to Ape: Darwinism in Argentina, Novoa and Levine identify Muniz as the first important naturalist from Argentina, who donated his later collection of fossils to the Museum of Buenos Aires.
Politics
He was a legislator, elected first as a representative and then a senator.