Nasar Raja mula digambarkan oleh Carl Linnaeus pada tahun 1785 dalam Systema Naturae edisi ke-10 dengan nama Vultur papa.[2] Ia kemudiannya dimasukan kedalam jenis Sarcoramphus pada 1805 oleh André Marie Constant Duméril. Nama umum Sarcoramphus berasal dari bahasa Latin, sarco-/σαρκο- yang bermaksud daging dan ramphos/ραμφος yang bermaksud paruh bengkok burung pemakan daging.[3] Burung ini juga telah dimasukan kedalam kumpulan oleh Constantin Wilhelm Osama Lambert Gloger pada 1841, tetapi klasifikasi ini tidak digunakan dalam kesusasteraan moden kerana Sarcoramphus mendapat keutamaan sebagai nama yang awal digunakan.[4] Nasar Raya mempunyai kaitan dengan burung Andean Condor, Vultur gryphus.[5] Sesetengah penulis memasukan spesies ini kedalam subfamili yang berpisah dari burung Nasar Dunia Baru lain, meskipun sesetengah penilis menyedari subdivision adalah tidak penting.[5]

Terdapat dua tiori mengapa Nasar Raja mendapat nama "Raja". Pertama, nama "raja" diberikan kerana ia berjaya mengalahkan dan mengusir burung nasar yang lebih kecil dab membiarkannya menunggu sehingga ia makan sehingga kenyang.[6] Teori alternatif melaporkan bahawa nama raja berasal dari lagenda kuno iaitu burung nasar adalah raja yang menjadi utusan antara manusia dan tuhan.[7] Burung ini juga dikenali sebagai "Gagak Putih" oleh orang Spanyol di Paraguay.[8]

Penempatan sistematik Nasar Raja dan enam spesis Burung Nasar Dunia Baru masih belum jelas.[9] Walaupun mempunyai mirip dan ekologi yang sama, Nasar Dunia Lama dan Baru adalah evolusi keturunan yang berlainan di kawasan yang berlainan. Bagaimanapun, perbezaan ini diperdebatkan dengan beberapa sumber yang terdahulu yang menyatakan Nasar Dunia baru lebih berkait dengan bangau.[10] Sumber terbaharu menunjukan posisi mereka kembali kepada kumpulan Falconiformes bersama dengan Nasar Dunia Lama[11] atau meletakkan mereka di kumpulan mereka sendiri, Cathartiformes.[12] Jawatankuasa Pengelasan Amerika Selatan telah mengeluarkan Nasar Dunia Baru dari kumpulan bangau (Ciconiiformes) dan meletakan dalam status tidak menentu (Incertae sedis), tetapi menyatakan bahawa perpindahan ke Falconiformes atau Cathartiformes adalah mungkin.[9]

Fossil record and evolution

The genus Sarcoramphus, which today contains only the King Vulture, had a wider distribution in the past. The Kern Vulture, Sarcoramphus kernense, lived in southwestern North America during the mid-Pliocene (Piacenzian), some 3.5-2.5 million years ago). It was a little-known component of the Blancan/Delmontian faunal stages. The only material is a broken distal humerus fossil, found at Pozo Creek, Kern County, California. As per Loye H. Miller's original description, "[c]ompared with [S. papa] the type conforms in general form and curvature except for its greater size and robustness."[13] The large span in time between the existence of the two species suggests that the Kern Vulture might be distinct, but as the fossil is somewhat damaged and rather non-diagnostic, even assignment to this genus is not completely certain.[14]

During the Late Pleistocene, another species probably assignable to the genus, Sarcoramphus fisheri, occurred in Peru. This would have been the chronospecies ancestral to the present-day King Vulture, or possibly a sister species. Few avian species of today evolved after that time, and of those that did most are small, short-lived Passeriformes.

A supposed King Vulture relative from Quaternary cave deposits on Cuba turned out to be bones of the eagle-sized hawk Buteogallus borrasi (formerly in Titanohierax).[15]

Little can be said of the evolutionary history of the genus, mainly because remains of other Neogene New World vultures are usually younger or even more fragmentary. The teratorns held sway over the ecological niche of the extant group especially in North America. The Kern Vulture seems to slightly precede the main bout of the Great American Interchange, and it is notable that the living diversity of New World vultures seems to have originated out of Central America.[13] The Kern Vulture would then seem to represent a northwards divergence possibly sister to the S. fisheri - S. papa lineage. Be that as it may, the fossil record, though scant, supports the theory that the ancestral King Vultures and South American Condors separated at least some 5 mya.

Bartram's "Painted Vulture"

A "Painted Vulture" ("Sarcoramphus sacra" or "S. papa sacra") is described in William Bartram's notes of his travels in Florida during the 1770s. The notes were considerably altered and expanded in the printed edition, possibly by an editor who had seen images of the King Vulture and believed the bird briefly and equivocally described by Bartram to be a King Vulture and therefore fleshed out the details as he saw fit.

Several researchers have attempted to prove the former existence of a King Vulture relative in Florida at this late date, suggesting that the population was in the process of extinction and finally disappeared during a cold spell.[16] However, Bartram saw his "Painted Vulture" in the area from which the Northern Crested Caracara was later described. This bird was common and conspicuous in Bartram's days, but it is conspicuously missing from Bartram's notes if the "Painted Vulture" is accepted as a Sarcoramphus.

While the original notes are somewhat ambiguous regarding the appearance of the bird, the description does fit that of a caracara. The birds' behavior, as recorded by Bartram, is in complete agreement with the caracara's. For example, Bartram observed the birds following wildfires to scavenge for burned insects and box turtles. Such behavior is typical of caracaras, but the larger and shorter-legged King Vultures are not well adapted for walking.[16]