Earth is impacted upon by other objects in outer space, including the Sun and the Moon. At present, Earth orbits the Sun once for every roughly 365.26 times it rotates about its axis. This length of time is a sidereal year, which is equal to 365.26 solar days.[12] The Earth's axis of rotation is tilted 23.4° away from the perpendicular to its orbital plane,[13] producing seasonal variations on the planet's surface with a period of one tropical year (365.24 solar days). Earth's only known natural satellite, the Moon, which began orbiting it about 4.53 billion years ago, provides ocean tides, stabilizes the axial tilt and gradually slows the planet's rotation. A cometary bombardment during the early history of the planet played a role in the formation of the oceans.[14] Later, asteroid impacts caused significant changes to the surface environment.[edit]HumanityFurther information: Human and World populationHumans are bipedal primates belonging to the mammalian species Homo sapiens. Compared to other animals, humans have a relatively highly developed brain capable of abstract reasoning, language, and introspection. This mental capability, combined with an erect body carriage that frees their upper limbs for manipulating objects, has allowed humans to make far greater use of tools than any other species. DNA evidence indicates that modern humans originated in Africa about 200,000 years ago.[15] Humans now inhabit every continent and low Earth orbit, with a total population of over 6.73 billion humans as of January 2009.[16]Like most primates, humans are social by nature. However, humans are particularly adept at utilizing systems of communication for self-expression, the exchange of ideas, and organization. Humans create complex social structures composed of many cooperating and competing groups, from families to nations. Social interactions between humans have established an extremely wide variety of traditions, rituals, ethics, values, social norms, and laws which form the basis of human society. Humans have a marked appreciation for beauty and aesthetics which, combined with the human desire for self-expression, has led to cultural innovations such as art, literature and music.Humans are noted for their desire to understand and influence the world around them, seeking to explain and manipulate natural phenomena through science, philosophy, mythology and religion. This natural curiosity has led to the development of advanced tools and skills.