Homo habilis – an educated guess (Wikimedia Commons)
4.3 million YA (Years Ago) In what today is Ethiopia, creatures labeled Ardipithecus ramidus lived, represented today by the nickname created by scientists: "Ardi". Her species was either directly ancestral to humans or closely related to a species ancestral to humans. She was 1.2 meters (4 feet) tall. She walked on two feet – not knuckle-walking as gorillas and chimps do, but did not have arched feet like us, indicating that she could not walk or run for long distances. She had opposable great toes and she had a pelvis that allowed her to negotiate tree branches well.
3.2 million YA In what today is Ethiopia, members of the biological family Hominidae lived, represented today by the nickname "Lucy." The angle of her knee joint indicates that she walked upright. She was 1.1 meters (3 feet 8 inches) tall. Walking upright improves the ability to run after game and to run from danger.
2.5 million YA Rocks are split into flakes and used as tools.
2.5 to 1.6 million YA A species called Homo habilis lives in what today is Tanzania. It is shorter and has disproportionately long arms compared to modern humans and is using stone tools.
1.8 to 1.3 million YA A species called Homo erectus has come into being and spreads as far as India, China and Java. (There are still disagreements about the Homo erectus classification.) Homo Erectus is to be described as the first human species to walk fully upright.
1.77 million YA Hominids (humans) in what today is the Dmanisi Republic of Georgia have a gum disease that scientists will think must have been caused by the use of toothpicks.
1 million YA (or shortly thereafter) Creatures using stone tools exist in Eastern England.
200,000 YA Give or take thousands of years, Homo sapiens have come into being in Africa. They create what will be a fossil record of their species. They are to remain very rare in Africa for much more than 100,000 years. They will be described as having a greater part of their brain devoted to language and speech than Homo erectus.
130,000 YA The Eemian interglacial period begins. Greater warmth in the next 5,000 years will allow forests to reach above the Arctic Circle. By now another creature belonging to the homo genus (biological grouping), Neanderthals, exist in Europe. They are a species apart from Homo erectus and Homo sapiens.
110,000 YA Give or take thousands of years, the Eemian interglacial period ends and another ice age begins, but humans and the Neanderthal will endure.
75,000 YA Give or take thousands of years, people in Africa have begun to expand from the east or the south, to the west and to the north. Genetic evidence suggests that they will replace other peoples, except for the Khoisan and pygmy peoples. In density of population they will remain rare.
73,000-68,000 YA The Toba Catastrophe Theory holds that on the island of Sumatra a super-volcanic eruption created a volcanic winter that extended to Africa and reduced the world's human population there to between 1,000 and 10,000 breeding couples. A mini ice age followed, lasting around 1,000 years. Where the eruption occurred a lake developed – Lake Toba.
60,000-55,000 YA The planet warms a bit. Ice retreats a little. Changes in climate will eventually begin to alternate between warmer and colder conditions, often in sudden jumps. Much of what would be Indonesian islands are a part of the Asian mainland. New Guinea, Australia and Tasmania are one continent, known today as Sahul.
50,000 YA Humans running from drought have left Africa, taking a coastal route to India.
50,000 YA Mating between Neanderthals and people called Denisovans introduces genes that will help modern humans cope with viruses. The interbreeding will embody as much as 4 percent of the human genome.
48,000 YA In Asia, Neanderthals are becoming extinct.
43,000 YA Humans are in an area around 500 kilometers south of what is today Moscow, their presence to be surmised in CE 2007 by archaeologists who have uncovered artifacts at what today is called the Kostenki Site.
42,000 YA By now, humans have crossed a body of water from Sunda in Southeast Asia to the continent of Sahul, including what today are called New Guinea, Australia and Tasmania.
40,000 YA Near what today is Beijing, human bones dating to around this year have been found. At least one person to whom these bones belong wore shoes. According to Erik Trinkaus of Washington University in Missouri, evidence also exists of some shoe or sandal wearing among Neanderthals.
38,000 YA Neanderthals in Europe have numbered no more than 10,000. On average, they are about as tall as contemporaneous humans, with around the same size skulls, suggesting similar brain size. Their bones are a little heavier and they tend to have stronger arms and hands. Like humans they use stone tools. DNA studies will indicate that because Neanderthal and human genes are so nearly identical some interbreeding MAY have occurred between the two species.
30,000 YA Homo Erectus becomes extinct. This species will be described as having used the same basic hand axe for more than a million years. Homo Sapiens, meanwhile, have been using the spear.
26,000 YA In Europe, Neanderthals are becoming extinct. It will be theorized that, in obtaining their food, Neanderthals have not adapted as well as humans to climate changes. In warmer weather, Neanderthal body structure requires more calories. Climate changes have produced new species of plants and animals, and it will be theorized that Neanderthal ambush hunting techniques have failed as grasslands have replaced trees. It will be theorized also that humans have been pushing Neanderthals from territories and theorized also that interbreeding has been absorbing the Neanderthals. Scientists will describe Neanderthals as highly intelligent, that in weapons making they were the first to use "dry distillation." Genetic analyses will reveal modern European individuals as 1 to 4 percent Neanderthal genetically. (PBS Nova: http://www.pbs.org/wgbh/nova/evolution/decoding-neanderthals.html)
20,000 BCE (Before the Common Era) By now humans are in southern Greece.
20,000 BCE A single Siberian population group moves across the Bering Strait to North America, according to genetic evidence. They will remain in Alaska for thousands of years, blocked from moving south by glacial ice.
18,000 BCE People in what today is Hunan province, in central China near the Yangzi River, are making pottery.
14,500 BCE An ice-free corridor in Canada allows migration from Alaska southward.
14,000 BCE A melting ice sheet begins a rise in sea levels and warming in Europe. Rising waters separate have separated New Guinea, Australia and Tasmania.
13,000 BCE Rice is being grown in Korea.
12,000 BCE The epoch described by geologists as the Pleistocene has ended. The epoch spanned nearly 1.8 million years. The last continental glacier is in retreat, and for archaeologists the Paleolithic age – a cultural period – ends.
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