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The human being, or Homo sapiens1, is an Earth-dwelling, carbon-based mammal and comes in a small variety of shapes and sizes. Its current population is estimated to be six billion (or six milliard under the British numerical system).
How to Spot a Human Being
There are several distinguishing features that make human beings different from other Earth life forms:
Bipedalism - Human beings walk on only two limbs, and are capable of sustaining this for extended periods of time depending on alcohol and caffeine levels.
Hands - Freed from the burden of locomotion, the front limbs include the opposable thumbs which allow humans to pick things up.
Tools - Humans create and use sophisticated tools to manipulate their environment, which has enabled them to construct sophisticated machines and buildings. Other animals have also been known to make and use tools. For example, chimpanzees have been observed removing the leaves from branches, then using those branches to draw termites from termite mounds. However, humans are the only animals so far who create tools and use them to create other tools.
Large brains - Admittedly, this is a bit subjective, but relative to their body mass, humans have more brain cells than many other kinds of animals.
Language - This is the big one. Language allows humans to communicate with each other, not just about the present (which things like mating calls and pheromones can do as well) but also about the past and the future.
How Human Beings Came to Be
There are several theories about how the human being came to be. These include:
Darwinism - In the mid-19th Century, Charles Darwin came up with the Theory of Evolution. This basically says that life originated in simple form and slowly evolved to the life we see today, including the human being. Most scientists follow this theory, as it explains unneeded features on animals such as wings on chickens. However, there are still many 'missing links' to substantiate this theory.
Creationism - Creation was the only popular theory until Darwin and is still believed by many, especially those who consider the human being to be above the rest of creation. It states God(s) created Man2 and then later created Woman out of Man. This theory is simple to explain as it cannot be proven because the only evidence possible to substantiate it is a reference to a couple without belly buttons.
Invadism - A theory drawn up by science fiction writers such as Douglas Adams. This theory can intermix with Darwinism and actually fills in the missing link between Homo erectus and Homo sapiens. According to The Hitchhiker's Guide to the Galaxy, for example, a spaceship containing frozen Homo sapiens lands on Earth and they eventually kill off the Homo erectus. This theory avoids mentioning or ignoring God and also does not put Man above or below other creatures.