The Silurian appeared in 1970 TV series called Doctor Who. The Silurians, also known as Earth Reptiles, Eocenes, Psionosauropodomorpha, Homo Reptilicus, Homo reptilia and Reptilia sapiens, were a species native to Earth that pre-dated humanity. Technologically advanced, they lived alongside their aquatic cousins, the Sea Devils. Unlike other species, Silurians showed an important intraspecific variation, with vast differentiation between breeds, such as the number of eyes, the formation of their pupils and the color of their skin.
The Silurians were an extremely varied species, with different subspecies and appearances; there were at least ten or eleven Silurian variations. Whilst many of them appeared similar, there were subtle differences which were attributed to caste. There were also clans or families with differentiated physical characteristics, some suited for environments of extreme cold or high plateaus. In general, they were humanoid reptilians with scaly crests on their head.
One group of Silurians was found at Wenley Moor; these had long fingers and three webbed toes on each foot, and their mouths were small and sucker-like. The most noticeable feature of these Silurians were their three eyes. Their main eyes were bright yellow with cat-like pupils, while their third eye was red and high on the forehead, surrounded by a fluted bone structure. They also had rectangular external ears. They were active creatures who spoke quickly in deep voices. A tribe fitting this description also lived under the Galápagos Islands. The cadaver of another such Silurian was identified by Vastra as being of the "Scholar" caste.
Another group was found at Sea Base 4. They looked similar to those of Wenley Moor, but had stockier builds, brown skin, suckers on the tips of their fingers, smaller mouths, ornate, curving spikes around their heads and the third eye on top. The third eye would glow along with their speech, but was not used in attack. They were also slower and spoke in a higher-pitched voice. Identical Silurians were seen elsewhere as part of the aforementioned Scholar caste.
Horlak, a leader of a tribe of scholars which included Silurians matching the aforementioned groups, appeared much like them but was distinct in that his cranial ridges were further pronounced.
A tribe seen on prehistoric Earth were distinguished in that they had only two eyes, no external ears, five digits on each hand, crests on their heads and down their neck, and faces with bulging black eyes and beak-like mouths. A further subspecies, found in Wales, were more radically different with more human-shaped faces. Of this subspecies, Silurian Hunters wore masks resembling the faces of their beaked counterparts. They had long tongues which they could flick to inject a venom, which was mutagenic to humans. They moved and spoke much like humans. This tribe also appeared to have extended into the London area as well, with the colony Madame Vastra belonged to having been uncovered during an expansion to the London Underground. Vastra was identified by her Scholar brethren as "Warrior". These Silurians would become darker green when blushing.
The Silurians were also distantly related to the Sea Devils. They were genetically divergent enough to create problems for the offspring of matings with Silurians. Offspring suffered from biological problems including sterility and a shorter lifespan, due to cellular degeneration.
In at least one alternate reality, there was a human-Silurian hybrid. This demonstrated that mating between the two was possible.