While there are some gaps in the fossil record, deer are believed to have evolved from antlerless, tusked ancestors that resembled modern day pigs, known as the gelocids, that lived during the early Cenozoic. Ling Huang and colleagues at the Kunming Institute of Zoology in China have used chromosome painting studies to show that six chromosomal fissions appear to have transformed the ancestral pecoran (pig-like) karyotype of 58 chromosomes (2n) into the ancestral cervid (deer) karyotype of 70.
The late Eocene, around 46 million years ago (mya), saw widespread extinction, and ruminants similar in appearance to today's duikers (small African antelopes) and chevrotains (small Asian and African “mouse-deer”) arose during the Oligocene. In the same period, we see the development large canines for use in combat. In his appraisal of cervid evolution, published in 1966, Valerius Geist notes that the early genera of cervids were antlerless but possessed large upper canines. Most present-day genera, by contrast, have medium to large antlers and have either lost or reduced to insignificance their upper canines.
These first artiodactyls were small, short-legged creatures that probably fed on leaves and succulent plant material. By the end of the Eocene, the three suborders we see today are recognised: the Suina (pigs); the Tylopoda (camels); and the Ruminantia (the goats and cows). We're lacking data on the ancestor of modern ruminants, but it seems probable that they evolved from a rabbit-sized mammal that inhabited middle Eocene forests, some 50 mya, called Diacodexis. Regardless of their precursor, genetic analysis by Clément Gilbert and colleagues, published in 2006, suggests an Asian origin for true deer, with the foundations of the Cervidae running back to some point during the Late Miocene (around 10 mya).
Water deer have 70 chromosomes, as do roe (Capreolus), grey brocket (Mazama gouazoubira) and the white-tailed and mule deer (Odocoileus), while other cervids either have the same or commonly fewer (Cervus have 60-68, for example). In their 1988 publication on deer genetics, Z. Wang and R. Du suggest that chromosome evolution in the Cervidae has been towards fewer chromosomes, suggesting that the Hydropotes/Capreolus/Odocoileus/Mazama karyotype represents the ancestral state and these species appeared early in the Cervidae lineage.
The Capreolinae subfamily, according to the data presented by Gilbert's team, emerged in Central Asia at the Miocene/Pliocene boundary (~5 mya) as Procapreolus and diversified here before migrating into what is now North America*. Similarly, in their 2004 analysis of Old World deer phylogeny, Christine Pitra and colleagues pointed out that while the Asian genus Procapreolus is considered ancestral to Capreolus, it remains unclear (and in their view, unlikely) whether Procapreolus latifrons, a species inhabiting the Late Miocene-Early Pliocene forests of Asia some 5.5 million years ago that appears to be the ancestor of modern day roe, is also ancestral to Hydropotes. Indeed, in a 1998 paper to the Proceedings of the Royal Society, a team led by Ettore Randi at the Istituto Nazionale per la Fauna Selvatica in Italy calculated that, based on cytochrome b nucleotide sequences, water deer separated from Capreolus during the Late Miocene, about 11 mya, well before Procapreolus latifrons appeared on the scene. During the 1960s and '70s, several authors suggested that Dremotherium from the Late Oligocene of Europe and Asia (~28 mya) as the ancestor of both musk and water deer. The difference in soft tissue anatomy between the species and molecular data suggesting that the Cervidae and Bovidae/Moschidae diverged around that time (i.e. about 23 mya) cast some doubt on Dremotherium as an ancestor, however.
Yeong-Seok Jo, John Baccus and John Koprowski note, in their 2018 Mammals of Korea, that the water deer was present in Korea during the first half of the Quaternary, suggesting they've been in Asia for approaching three million years. Wei Dong, in her chapter in the 1993 Deer of China compendium, lists the oldest fossil of Hydropotes from the Middle Pleistocene of northern China (800,000-120,000 ya). I think this is from the Sinanthropus Site at Chouk'outien, although she doesn't specify. In line with Dong, however, in his 1972 paper to the Journal of Mammalogy, Peter Brokx wrote that the water deer was “not known from before the Pleistocene”. Brokx cited George Simpson's 1945 work on the principles of mammalian classification for this, although Simpson lists the water deer only as “Recent”, which going by his scheme is Holocene.
I'm currently not aware of any fossil evidence of water deer from Korea, but would be interested to hear from any readers who are.
A curious feature of this species, and one that sets it apart from all other members of the deer family, is that the males lack antlers. Bony structures grown from specialised plates on the skull called pedicles, antlers are more than just ornaments evincing fitness; they're an integral component in securing mating opportunities in a family where males must fight one another for access to females (see Antler Development). Consequently, their absence here raises an interesting evolutionary question. Does the Chinese water deer represent the ancestral phenotype (appearance) of deer, or is antler loss a derived autapomorphy? In other words, did the first deer to walk the earth look like water deer and antlers evolved later on in one lineage, or did they sport antlers that got lost somewhere along the water deer's ancestral line?
In their deer classification scheme, published in 1987, zoologists Paul Groves and Peter Grubb considered water deer to be the sister group to all other cervids, which is to say that they viewed both the presence of well-developed canines and the absence of antlers as primitive traits. In 1998, however, renowned deer evolutionary biologist Valerius Geist argued that the morphology and life history traits of water deer don't appear to match what we know about the primitive tropical deer from which we know modern cervids are descended. As such, Geist championed the idea first proposed by Russian zoologist Konstantin Flerov in 1950 - he regarded the lack of antlers as a secondary trait in Hydropotes. In the same year, in a paper to the Proceedings of the Royal Society of London, Université Montpellier geneticist Ettore Randi and his colleagues arrived at the same conclusion, writing:
“Contrasting with current taxonomy, Hydropotes is not the sister group of all the antlered deers, but it is nested within the [roe deer subfamily]. Therefore, Hydropotes lost the antlers secondarily.”
More recently, in 2003, Ming Li and her team published a cytochrome b dataset in Acta Theriologica that supported the emergence of water deer from an antlered ancestral line and later, in his 2005 revision of the scheme, Grubbs updated his stance, placing the water deer as a sister subfamily to the Capreolinae (roe deer); an arrangement that, while not widely adopted, more closely reflects the current thinking that antler loss is actually a secondary characteristic in this species. Presumably, the hypertrophication (enlargement) of the canines was a compensatory response to the loss of antlers. Indeed, we do tend to see a reduction in canine length as antler size and complexity increases.