The Earth has a human population of 7.8 billion, with an overall population density of 50 people per km2 (129 per sq. mile), excluding Antarctica. Nearly two-thirds of the world's population lives in Asia, with more than 2.7 billion in the countries of China and India combined. The world's literacy rate has increased dramatically in the last 40 years, from 66.7% in 1979 to 86.3% today. Lower literacy levels are mostly attributable to poverty. Lower literacy rates are mostly found in South Asia and Sub-Saharan Africa. The world's largest ethnic group is Han Chinese, with Mandarin being the world's most spoken language in terms of native speakers.