EARLIER than 900,000 years ago:

6 million years ago - Molecular clock estimates humans diverged from the apes. And: Sahara desert, Toumaï - walked upright.
http://www.pbs.org/wgbh/nova/beta/evolution/becoming-human-part-1.html

Brains stayed small. For almost 4,000,000 years, there was a profusion of upright walkers with chimp-sized brains: Orrorin tugenensis, Ardipithecus ramidus, Australopithecus africanus, Kenyanthropus platyops. Bipeds, big snouts, chimp-sized brains. Small-brained bipedal apes. Extremely successful, flourished. Brain size stayed the same.

AFRICA:
3.3 million years ago, Africa Rift Valley, northeastern Ethiopia: Australopithecus afarensis, nicknamed "Selam": small, chimp-like, but walked on two legs, and slower-developing brain so greater length of childhood, and less deep Lunate sulcus in brain, so smarter. Discovered by Zeray Alemseged (ca 2001). Same species as "Lucy", discovered in the 1974 by Don Johanson.

About 2.5 - 2.7 million years ago, an ice age sent global temperatures plummeting as much as 20F.
http://www.mc.maricopa.edu/dept/d10/asb/anthro2003/origins/manbegan.html

The current ice age, the Quaternary glaciation, started about 2.58 million years ago. During the late Pliocene the spread of ice sheets in the Northern Hemisphere began. http://en.wikipedia.org/wiki/Ice_age [*]

2.5 - 1.5 million years ago - Oldowan Tradition, defined by Louis and Mary Leakey at Olduvai Gorge - stone tools made from cobbles by hard-hammer percussion, including large scrapers, choppers, hammerstones, and a range of smaller tools from stone flakes such as awls and smaller scrapers. Oldowan includes the earliest stone tools found to date (with the possible exception of KBS), and is usually associated with our ancestor Homo habilis. http://archaeology.about.com/od/oterms/g/oldowan.htm

2 million years ago, Africa: Homo habilis. Skulls different, toolmaking. Still about the same size—three to four feet tall. But expansion in brain size. Sloping, elevated forehead. Projecting snout of an ape gone.

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http://www.sciam.com/article.cfm?id=is-the-out-of-africa-theory-out&sc=WR_20070814
Three major waves of migration from Africa to Europe: the first occurring about two million to 1.5 million years ago during the late Pliocene and early Pleistocene epochs.

1.7 million year old skull found in Dmanisi in the south of the Republic of Georgia, and also that a mixture of human-like creatures lived together in Eurasia, as this skull is smaller and more primitive than two others found at the same site two years earlier. Another "hobbit". (Discovered 2002.)

1.2 million year old jawbone, including some teeth, found at Sima del Elefante, in Atapuerca, northern Spain, near stone tools and animal bones bearing cut marks means that the oldest known human inhabitants of the continent of Europe. (Discovered March 2008.)
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900,000 - first evidence of controlled rafting by Homo erectus on the island of Flores, 11.4 miles from Indonesia (Archeology Magazine 5/98, in News Briefs section, "First Mariners"). Tool stone was not to be had on this island, so it had to be brought from the mainland. http://www.ancientsites.com/aw/Post/238372

http://www.archaeology.org/9805/newsbriefs/mariners.html
Volume 51 Number 3, May/June 1998: New dates from two sites on the Indonesian island of Flores prove that Homo erectus was able to navigate open waters between 800,000 and 900,000 years ago. Previously, modern humans who colonized Australia were credited with the earliest sea crossings, 40,000 to 60,000 years ago.
Michael J. Morwood of Australia's University of New England and his colleagues presented their conclusions, based on excavations at Tangi Talo and Mata Menge, in the journal Nature. Tangi Talo yielded the remains of pygmy stegodon (a type of elephant), giant tortoise, and Komodo dragons, but no tools. Mata Menge, however, produced a small number of stone tools, including some made of nonlocal chert, as well as remains of large stegodon, crocodile, giant rat, freshwater molluscs, and plants.
Morwood dated the sites using a technique that analyzes individual zircon crystals from volcanic deposits. A sample from Tangi Talo, taken near a pygmy stegodon tusk and giant tortoise shell fragments, yielded a date of about 900,000 years ago. At Mata Menge, a sample from just beneath the artifact-bearing level dated to about 880,000 years ago, while another, taken above in situ artifacts, gave a date of about 800,000. The sites' early dates and the identification of the stone tools seem secure, according to Carl Swisher of the Berkeley Geochronology Lab.
Tools this early in Southeast Asia can only have been made by Homo erectus. Unlike Java, which was periodically connected to mainland Asia and accessible to early humans on foot, Flores could be reached only by crossing an 11.4-mile-wide strait, even at times of lowest sea level. The Mata Menge artifacts prove that H. erectus was able to make the crossing.
The new dates also support the suggestion made by Dutch paleontologist Paul Sondaar more than a decade ago that the extinction of pygmy stegodon on Flores ca. 900,000 years ago was the result of human predation. Also: http://www.austmus.gov.au/archive.cfm?id=437 (Homo floresiensis) [This far back the continents may have been in different relations, i.e. Flores may not have been an island.]

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One million to 500,000 years ago - Second major migration into Europe, PRIMARILY FROM ASIA. http://www.sciam.com/article.cfm?id=is-the-out-of-africa-theory-out&sc=WR_20070814

All the ancestors of contemporary Europeans apparently did not migrate out of Africa as previously believed. According to a new analysis of more than 5,000 teeth from long-perished members of the genus Homo and the closely related Australopithecus, many early settlers hailed from Asia. . . .The new findings, published this week in Proceedings of the National Academy of Sciences USA, casts doubt on the second migration out of Africa. (August 2007) http://www.sciam.com/article.cfm?id=is-the-out-of-africa-theory-out&sc=WR_20070814

700,000 years old - Eight dwellings in Qatar indicate that an early human species crossed the Red Sea to leave their origins in Africa. Artefacts include axes and knives. http://www.thepeninsulaqatar.com/ [see full cite]

700,000-200,000 BP - Beijing Man. Zhou Kou Dian Site, Fang Shan County, Beijing - http://www.chinaculture.org/cnstatic/doc/exhibition/20e.doc

Humanity's ancestors may have departed Africa . . . eons earlier than scientists have assumed. . . . Chinese scientists have contended that the skull of a modern-looking human, found in their country a decade ago, is at least 200,000 years old. . . .U.S. and Indonesian researchers said they had redated fossil skull fragments found at two sites on the island of Java. Instead of being a million years old, as earlier analysis suggested, the fossils appear to date back nearly 2 million years. They are from the species known as Homo erectus - the first primate to look anything like modern humans and the first to use fire and create sophisticated stone tools. If the evidence from Java holds up, it means that protohumans left their African homeland hundreds of thousands of years earlier than anyone had believed, long before the invention of the advanced stone tools that, according to current textbooks, made the exodus possible. (1994) http://www.mc.maricopa.edu/dept/d10/asb/anthro2003/origins/manbegan.html

400,000-350,000 - Temperatures descend from high to Glacial low - http://commons.wikimedia.org/wiki/File:Ice_Age_Temperature.png

The first proto-Neanderthal traits appeared in Europe as early as 600,000–350,000 years ago. http://en.wikipedia.org/wiki/Neanderthal
400,000 to 340,000 - Homo habilis split into Neanderthals and "modern humans" (Cro-Magnon, Homo sapiens). http://dsc.discovery.com/news/2008/03/17/human-neanderthal-split.html

340,000-330,000 - Return to high temperatures.
330,000-300,000 - Temperatures descend from high to Glacial low - http://commons.wikimedia.org/wiki/File:Ice_Age_Temperature.png

300,000 years ago - Many ‘modern’ traits like the use of grind stones or big game hunting began to accumulate in Africa. (Neanderthal Man Was An Innovator) http://www.sciencedaily.com/releases/2007/06/070619164133.htm (2007)

300,000-275,000 - Return to high temperatures.
275,000-250,000 - Temperatures descend from high to Glacial low - http://commons.wikimedia.org/wiki/File:Ice_Age_Temperature.png

250,000-240,000 - Return to high temperatures.
240,000-220,000 - Temperatures descend from high to Glacial low - http://commons.wikimedia.org/wiki/File:Ice_Age_Temperature.png

Balkan caves, gorges were pre-Neanderthal haven: A fragment of a human jaw found in Serbia and believed to be 250,000 to 130,000 years old is helping anthropologists piece together the story of prehistoric human migration from Africa to Europe. "This is the earliest evidence we have of humans in the area". Discovered in a small cave in the Sicevo gorge in south Serbia. "It is a pre-Neanderthal jaw." The jaw might belong to homo erectus, the first type of human to walk upright, who appeared in Africa 1.8 million years ago and was the precursor of both modern man, or homo sapiens, and the separate species of Neanderthal man.  The jaw was found at a depth of four meters, below a Neanderthal village in a linked cave, one of the richest archaeological sites in the region. The remains of a hearth, primitive stone and bone tools and animals indicated an 80,000 year old home base. "What we found there was enough to reconstruct the way of living, changes in culture, climate, vegetation and animal life during a longer period of some 50,000 years." http://www.reuters.com/article/newsOne/idUSL2768278020080627

220,000-215,000 - Return to high temperatures.
215,000-185,000 - Long warm period.
185,000-140,000 - Temperatures descend from high to Glacial low - http://commons.wikimedia.org/wiki/File:Ice_Age_Temperature.png

200,000--150,000 - African origin of early modern humans - http://www.sciencedaily.com/releases/2007/08/070828155004.htm

140,000-125,000 - Return to high temperatures.

In Africa, early modern humans appeared at least as long ago as 160,000 years BP at sites such as Bouri in Ethiopia, and perhaps as long ago as 195,000 years ago, if the dating of Omo Kibish, also in Ethiopia, is correct. http://archaeology.about.com/od/earlymansites/a/cro_magnon.htm

164,000 years ago humans were eating shellfish, making complex tools and using red ocher pigment—all modern human behaviors. The shellfish remains—of mussels, periwinkles, barnacles and other mollusks.
http://www.smithsonianmag.com/history-archaeology/human-migration.html

158,000 - "oldest fossilized remains of modern humans yet found" -- portions of skulls. Village of Herto, Ethiopia (Africa). http://www.npr.org/templates/story/story.php?storyId=1295624 (2003)
http://www.talkorigins.org/faqs/homs/herto.html (2003)