It is widely agreed that the evolutionary lineage of the grey wolf can be traced back 2 million years to the Early Pleistocene species Canis etruscus, and its successor the Middle Pleistocene Canis mosbachensis. The grey wolf Canis lupus is a highly adaptable species that is able to exist in a range of environments and which possesses a wide distribution across the Holarctic. Studies of modern grey wolves have identified distinct sub-populations that live in close proximity to each other. This variation in sub-populations is closely linked to differences in habitat – precipitation, temperature, vegetation, and prey specialization – which affect cranio-dental plasticity.
The earliest specimens of the modern grey wolf date to around 400,000 years ago, or possibly earlier to 1 million years ago. Most modern wolves share most of their common ancestry within the last 25-23,000 years from earlier Siberian wolf populations. While some sources have suggested that this is the result of a population bottleneck, others suggest that this is a normal consequence of gene flow homogenising wolf genomes across their range.
Fossil record
The fossil record for ancient vertebrates is composed of rarely occurring fragments from which it is often impossible to obtain genetic material. Researchers are limited to morphologic analysis but it is difficult to estimate the intra-species and inter-species variations and relationships that existed between specimens across time and place. Some observations are debated by researchers who do not always agree, and hypotheses that are supported by some authors are challenged by others.
The Cretaceous–Paleogene extinction event occurred 66 million years ago and brought an end to the non-avian dinosaurs and the appearance of the first carnivorans. The name carnivoran is given to a member of the order Carnivora. Carnivorans possess a common arrangement of teeth called carnassials, in which the first lower molar and the last upper premolar possess blade-like enamel crowns that act similar to a pair of shears for cutting meat. This dental arrangement has been modified by adaptation over the past 60 million years for diets composed of meat, for crushing vegetation, or for the loss of the carnassial function altogether as in seals, sea lions, and walruses. Today, not all carnivorans are carnivores, such as the insect-eating aardwolf.
The carnivoran ancestors of the dog-like caniforms and the cat-like feliforms began their separate evolutionary paths just after the end of the dinosaurs. The first members of the dog family Canidae appeared 40 million years ago, of which only its subfamily the Caninae survives today in the form of the wolf-like and fox-like canines. The caniforms included the fox-like genus Leptocyon whose various species existed from 34 million YBP before branching 11.9 million YBP into Vulpes (foxes) and Canini (canines). The jackal-sized Eucyon existed in North America from 10 million YBP and by the Early Pliocene about 6–5 million YBP the coyote-like Eucyon davisi invaded Eurasia. In North America it gave rise to early Canis which first appeared in the Miocene (6 million YBP) in south-western US and Mexico. By 5 million YBP the larger Canis lepophagus appeared in the same region.
The canids that had immigrated from North America to Eurasia – Eucyon, Vulpes, and Nyctereutes – were small to medium-sized predators during the Late Miocene and Early Pliocene but they were not the top predators. The position of the canids would change with the arrival of Canis to become a dominant predator across the Holarctic. The wolf-sized C. chihliensis appeared in northern China in the Mid-Pliocene around 4–3 million YBP. The large wolf-sized Canis appeared in the Middle Pliocene about 3 million years ago in the Yushe Basin, Shanxi Province, China. By 2.5 million years ago its range included the Nihewan Basin in Yangyuan County, Hebei, China and Kuruksay, Tadzhikistan. This was followed by an explosion of Canis evolution across Eurasia in the Early Pleistocene around 1.8 million YBP in what is commonly referred to as the wolf event. It is associated with the formation of the mammoth steppe and continental glaciation. Canis spread to Europe in the forms of C. arnensis, C. etruscus, and C. falconeri. Other studies state that the oldest Canis remains that have been found in Europe were from France and dated to 3.1 million YBP, followed by Canis cf. etruscus (where cf. in Latin means confer, uncertain) from Italy dated to 2.2 million YBP.
The fossil record is incomplete but it is likely that wolves arose from a population of small, early canids. Morphological evidence and genetic evidence both suggest that wolves evolved during the Pliocene and Early Pleistocene eras from the same lineage that also led to the coyote, with fossil specimens indicating that the coyote and the wolf diverged from a common ancestor 1.5 million years ago. The ancestor of the jackal and the other extant members of the genus Canis had split from the lineage before this time.
After this separation from a common ancestor the species that were believed to be involved in the further evolution of the wolf and coyote – and the beliefs of some paleontologists – diverged. A number of researchers believed that the lines of C. priscolatrans, C. etruscus, C. rufus, C. lycaon, and C. lupus were components involved in some way that lead to the modern wolf and coyote.
Canis lepophagus Canis lepophagus lived in the early Pliocene in North America. Kurten proposed that the Blancan C. lepophagus derived from smaller Miocene Canis species in North America. It then became widespread across Eurasia where it was either identical to, or closely related with, C. arnensis of Europe.
Johnston describes C. lepophagus as having a more slender skull and skeleton than in the modern coyote. Robert M. Nowak found that the early populations had small, delicate and narrowly proportioned skulls that resemble small coyotes and appear to be ancestral to C. latrans. Johnson noted that some specimens found in Cita Canyon, Texas, had larger, broader skulls, and along with other fragments Nowak suggested that these were evolving into wolves.
Tedford disagreed with previous authors and found that its cranio-dental morphology lacked some characteristics that are shared by C. lupus and C. latrans, and therefore there was not a close relationship but it did suggest C. lepophagus was the ancestor of both wolves and coyotes.
Canis armbrusteri The North American wolves became larger, with tooth specimens indicating that C. priscolatrans diverged into the large wolf C. armbrusteri. during the Middle Pleistocene in North America. Robert A. Martin disagreed, and believed that C. armbrusteri was C. lupus. Nowak disagreed with Martin and proposed that C. armbrusteri was not related to C. lupus but C. priscolatrans, which then gave rise to A. dirus. Tedford proposed that the South American C. gezi and C. nehringi share dental and cranial similarities developed for hypercarnivory, suggesting C. armbrusteri was the common ancestor of C. gezi, C. nehringi and A. dirus.
Aenocyon dirus
In 1908 the paleontologist John Campbell Merriam began retrieving numerous fossilized bone fragments of a large wolf from the Rancho LaBrea tar pits. By 1912 he had found a skeleton sufficiently complete to be able to formally recognize these and the previously found specimens under the name C.dirus (Leidy 1858).
Canis dirus, or as it is now widely recognized, Aenocyon dirus, lived in the late Pleistocene to early Holocene in North and South America. In 1987, a new hypothesis proposed that a mammal population could give rise to a larger form called a hypermorph during times when food was abundant, but when food later became scarce the hypermorph would either adapt to a smaller form or go extinct. This hypothesis might explain the large body sizes found in many Late Pleistocene mammals compared to their modern counterparts. Both extinction and speciation – a new species splitting from an older one – could occur together during periods of climatic extremes. GloriaD.Goulet agreed with Martin and further proposed that this hypothesis might explain the sudden appearance of A.dirus in North America, and that because of the similarities in their skull shapes that C.lupus gave rise to the A.dirus hypermorph due to abundant game, a stable environment, and large competitors. Nowak, Kurten and Berta disagreed with Goulet and proposed that A. dirus was not derived from C. lupus. The three noted paleontologists Xiaoming Wang, R. H. Tedford and R. M. Nowak have all proposed that A. dirus had evolved from C. armbrusteri, with Nowak stating that there were specimens from Cumberland Cave, Maryland, that indicated C. armbrusteri diverging into A. dirus. The two taxa share a number of characteristics (synapomorphy), which suggests an origin of A. dirus in the late Irvingtonian in the open terrain in the midcontinent, and then later expanding eastward and displacing its ancestor C. armbrusteri.
However, in 2021, a study indicated the dire wolf to be a highly divergent lineage which last shared a most recent common ancestor with the wolf-like canines 5.7 million years ago. The morphological similarity between dire wolves and gray wolves was concluded to be due to convergent evolution. This finding indicates that the wolf and coyote lineages evolved in isolation from the dire wolf lineage. The study proposes an early origin of the dire wolf lineage in the Americas, and that this geographic isolation allowed them to develop a degree of reproductive isolation since their divergence 5.7 million years ago. Coyotes, dholes, gray wolves, and the extinct Xenocyon evolved in Eurasia and expanded into North America relatively recently during the Late Pleistocene, therefore there was no admixture with the dire wolf. As a result, the study found that the correct binomial name of the dire wolf is Aenocyon dirus, as proposed by Merriam in 1918. The long-term isolation of the dire wolf lineage implies that other American fossil taxa, including C. armbrusteri and C. edwardii, may also belong to the dire wolf's lineage.
Canis chihliensis
Wang and Tedford proposed that the genus Canis was the descendant of the coyote-like Eucyon davisi, and its remains first appeared in the Miocene (6 million YBP) in south-western USA and Mexico. By the Pliocene (5 million YBP), the larger Canis lepophagus appeared in the same region and by the Early Pleistocene (1 million YBP) Canis latrans (the coyote) was in existence. They proposed that the progression from Eucyon davisi to C lepophagus to the coyote was linear evolution. Additionally, C. edwardii, C. latrans and C. aureus form together a small clade and because C. edwardii appeared earliest spanning the mid-Blancan (late Pliocene) to the close of the Irvingtonian (late Pleistocene) it is proposed as the ancestor.
Nowak and Tedford also believed that it was possible for C. lupus to have been derived from a Miocene or Pliocene canid line that preceded and was separate from C. lepophagus. Based on morphology from China, the Pliocene wolf C. chihliensis may have been the ancestor for both C. armbrusteri and C. lupus before their migration into North America. C. chihliensis appears to be more primitive and smaller than C. lupus, and measurements of its skull and teeth are similar to C. lupus but those of its postcranial elements are smaller. C. armbrusteri appeared in North America in the Middle Pleistocene and is a wolf-like form larger than any Canis at that time. At the end of the most recent glacial retreat during the past 30,000 years, warming melted the glacial barriers across northern Canada allowing arctic mammals to extend their range into mid-latitude North America, including elk, caribou, bison, and the grey wolf.