According to the series mythology, the Colonists are actually Earth's original inhabitants, forced to abandon the planet during the last ice age (although the exact details concerning their departure are unknown). They highly resemble the well-known "grey aliens" in their mature form. In their immature stage, they are more yellowish colored, tall, and very aggressive. This form is sometimes referred to as the "reptilian" form, due to the fangs, claw, and scale like texture of the skin. They are a symbiotic species, as their blood contains an intelligent virus/life force (similar in texture to crude oil), which can incubate within other life forms, including humans. Once incubated, a human host, within 96 hours (Fight the Future), gestates the immature alien form, and in the process, kills