They do not consider humans to be life-forms because we are carbon-based- They do not consider themselves to be life-forms, as they are actually biological weapons created by a being known only as the "Creator" (創造主 "Souzoushu"), a silicon-based life-form, and so only consider silicon-based entities to be life-forms (the superior states that it is impossible for carbon-based life-forms to occur naturally). We find out more about the BETA from Takeru's conversation with the Superior (上位存在, "Jouisonzai", also referred to as "Target A"/あ号標的, "A-gou hyouteki") at the end of Alternative. All that mankind knows of them during Unlimited and at the start of Alternative is that they are (obviously) hostile, are carbon-based lifeforms (this was discovered when specimens were examined in Alternative 2), and don't consider humans to be living beings (this was found out through Soviet psychics' readings in Alternative 3). Their very existence is shrouded in mystery. They cannot fly, and so shoot down anything that does, rendering aircraft useless and making evacuation a lot more difficult. They vary greatly in size, the smallest identified strains (soldier-class) being around three metres tall, and the largest identified strains (fort-class) at over sixty metres tall. When they first arrived in Japan in 1998, 36 million people were dead within a week. By 2001, the world population is down to 1 billion people. Muv-Luv Unlimited and Alternative take place in alternate worlds where the BETA invaded Earth in 1973 after arriving on the Moon in 1967 and being spotted on Mars in 1958. The Beings of the Extra Terrestrial origin which is Adversary of human race or BETA are a fictional extraterrestrial lifeform in the Muv-Luv computer games.