Hares and their relatives, the rabbits and pikas comprise the zoological Order Lagomorpha. Modern biochemical evidence suggests the lagomorphs have been a distinct lineage for around 90 million years. Since the dinosaurs became extinct around 65 million BP (years before present) it is possible that hare-like creatures were scurrying about the feet of the giant reptiles for 25 million years.
Until relatively recently lagomorphs were confused with rodents and even Darwin made the same mistake. Lagomorphs are unique in having a small pair of secondary incisor teeth in the upper jaw, just behind the main pair. These are never present in rodents and clearly distinguish the two animal groups.
The evolution of the lagomorphs was greatly influenced by the rise to dominance of grasses among the flowering plants during the Oligocene and Miocene periods between 35 and 5 million BP. Vast tracts of prairie land developed in Asia during those times and the lagomorphs evolved to take advantage of this abundant food supply.
The caecum, an offshoot of the gut and well developed in lagomorphs, acts as a fermentation chamber to break down the large amounts of cellulose which grasses contain. The caecum produces soft, lozenge shaped faeces which lagomorphs re-ingest to increase the efficiency of digestion and extract nutrients such as B vitamins. After a second passage of material through the gut the familiar currant-like pellets are produced. This behaviour is known as refection or caecotrophy.
The brown hare evolved in continental Europe, but probably did not radiate northwards before Britain was cut off from the mainland by the formation of the English Channel. If that was so, then the mountain hare is our only native hare species. The brown hare was possibly introduced by the Romans around 2,000 BP, or by an earlier civilisation.