It is widely known that languages change over the years, and therefore is not easy to estimate how many languages are currently spoken in the world: the figure commonly known is about 5,000. Most languages are known to belong to language families which can be subdivided into smaller units, referred to as branches. There are at least fifteen language families which are the ancestors of modern languages throughout the world. Most languages of Europe except for Basque, Hungarian, or Finnish and the languages of Iran, Afghanistan, Pakistan, and India belong to the Indo-European language family. Four of the five official languages of the United Nations are Indo-European (English, French, Russian, and Spanish are Indo-European). It has twelve branches: Romance, Germanic, Slavic, Baltic, Celtic, Iranian, Indic, Tocharian, Anatolian, Armenian, Greek and Albanian. Germanic group includes English, German, Yiddish, Norwegian, Swedish, Danish, Dutch and a few others. Germanic group shares its similarities in vocabulary and phonology and some are mutually intelligible. With about 350 million speakers, English is spoken in many countries such as the British Isles, the United States, Canada, Australia and New Zealand, being the second most spoken language in the world after Chinese. Romance Group includes French, Spanish, Italian, Portuguese, and Romanian among which Spanish is the third most spoken language in the world. These languages are also closely related to each other. There are various theories about how Indo-European languages came to spread out to Europe and Western Asia, but the most dominant one presumes that the Indo-European language family originated from the Black Sea during the Neolithic period about 7000BC, spreading west to Europe between 3500BC and 2500BC, south to the Mediterranean, north to Scandinavia, and east to India. Scattered throughout Asia and Europe are a few smaller language families among which the Altaic Family is named after the Alti Mountains, in Central Asia where nomadic people lived in the plains. Of the three branches of Altaic, Turkic ranges from Anatolia to the Volga basin and central Asia; Mongolian from China and Mongolia to the lower Volga and Afghanistan; Manchu-Tungus from the northern coast of northeastern Siberia to the Amur and to the Yenisei. Korean and Japanese languages are assumed to be remotely related to the Altaic language family, but it is still controversial. Some scholars consider the Altaic languages are related to the Uralic languages and group them as a larger Ural-Altaic language family, but there has been too little evidence to support this. Included in the Sino-Tibetan family are about 300 East Asian languages, which are divided into a Sinitic Group and a Tibeto-Burman group. The Sinitic group includes a dozen languages among which Mandarin, spoken by about 700 million people in northern China, is the official language of the People's Republic of China. The Tibeto-Burman group includes many rarely spoken languages, among which are Burmese and Tibetan. The Sino-Tibetan language family has the largest number of speakers, estimated to have 1 billion speakers. In addition to the three above mentioned language families, there are other language families such as the Austronesian family, the Afroasiatic family, the Uralic family, the Malayo-Polynesian family and the Caucasian family. A very large majority of the people in the world speaks languages which belong to only one family. There are, however, also languages that cannot be classified into any family known as language isolates.