Out of Africa: When and to Where?

When did humans first expand out of Africa, and where did they go? Only a few years ago, the general patterning seemed to indicate that the exodus was just before 1 mya, that the human type involved was Homo erectus and that the destination was Asia, not Europe. The earliest well-dated sites with definitive traces of human activity in Europe all appeared to cluster in the Middle Pleistocene after about 730,000 or even 500,000 years ago. New dates for both Asia and Europe as well as new finds suggest that this scenario, like the others mentioned in this article, may be far too simplistic.

The most widely accepted early dates in Asia are for Ubeidiya, a well-known site in Israel where Oldowan artifacts appear to go back to cat 1.4 mya based on faunal comparisons with Africa. New chronometric dates for the eastern part of the continent have been even more surprising. Carl Swisher and Garniss Curtis of the Berkeley Geochronology Center have published several dates older than 1.0 mya for the Modjokerto child, an early Homo erectus find from Java. These cluster around 1.8 mya. Some who disagree with these dates have argued that while there is indeed a volcanic ash near the site of the find that is of this age, it is far from clear how that relates to the age of the find, which was made by a local farmer in the 1930s. Swisher and Curtis have responded that the ash that lines the skull is a close match chemically to the dated ash; others have either disputed their conclusions or pointed out that both the ash and the skull could have washed into the site together. In the latter case the skull could be much younger than the ash. The continuing accumulation of new dates for other sites in Java such as Sangiran, however, appear to confirm the presence of Homo erectus in Java between 1.4 and 1.8 mya.

An even more controversial site, Longgupo, in South China, was recently described by Huang, Ciochon and others in both Nature and Natural History. This site contains a small jaw fragment of what the authors argue is early Homo, either habilis or ergaster, the first such fossil outside Africa. The find was associated with early Asian mammals (Late Pliocene to early Pleistocene in age) including a giant ape (Gigantopithecus). Also found were two very minimally fashioned objects of stone that the authors argue are tools. The possible attribution to habilis is based on the size and forward position of the cusps of the second premolar together with its double root. Others point out that these characteristics are not unknown from Homo ergaster or early erectus, or even some early Asian apes.

In addition, the dating of Longgupo is based on paleomagnetism, which measures the direction and strength of the earth's magnetic field in samples of earth taken from around the bones. The earth's magnetic field periodically dissolves, reorganizes and changes direction; 800,000 years ago, for example, a compass needle would have pointed south rather than north. These reversals are encoded in newly forming sediments, as the atoms align themselves with the prevailing magnetic field at the time. The ancient magnetic signal is locked in to the sediment and can be measured in the lab. Precise dating of reversals in volcanic sediments using the potassium argon technique has led to a sequence of ages for 'normal' (north-oriented) and 'reversed' (couth-oriented) periods. In non-volcanic sediments, such as those at Longgupo, researchers must try to guess which 'normal' or 'reversed' interval they are looking at, based on the entire sequence. The important levels at Longgupo are normal', below a layer that is 'reversed' and several meters below a date of 1.02 mya, based on the decay of uranium isotopes in a sample of fossil tooth enamel and dentine. The researchers argue that the closest 'normal' period before 1.02 mya is the one at 1.78 to 1.96 mya. If the uranium series age is closer to 0.78 - 0.84 mya, which the authors admit is possible, then the earth around the 'human' bones could date to 0.9 to 1.0 mya, also a normal period, and much closer to the age of other old Chinese hominids.

What about Europe? The oldest European, and the only clear Homo erectus fossil from that continent recently turned up in the Republic of Georgia, in the Caucasus Mountains that separate Europe from the Near East. The fossil jaw, which looked very much like one from Kenya, was located above a basalt flow dating to 1.8 mya. in a normally polarized horizon. One additional problem is that the find was not in some undisturbed cave but in the wall of a medieval storage cellar in the town of Dmanisi. A recent expedition suggested that the fossil came from a series of burrows or dens, excavated by prehistoric mammals. Although the earth into which the dens were excavated is of normal polarity, the earth that fills the dens is reversed. This means that the fossil is younger than 1.8 mya (when polarity was normal) but must be older than 0.78 mya (polarity has been normal from that time to the present). The most likely estimate at the moment is cat 1.4 mya, around the same age as Ubeidiya.

A final European site in the news is much further into Europe than Dmanisi: the site of Atapuerca in northern Spain, where literally hundreds of human bones have been recovered from narrow fissures in the rock. Most relate to Middle Pleistocene times, but in the oldest site, the dating may suggest an age of 800-900,000 years ago. It is especially interesting that these are not classic examples of Homo erectus, but already suggest some specializations in the direction of Neanderthals, such as tooth row with a space behind the last tooth, deep pulp cavities in the teeth, semicircular brow ridges, and some enlargement of the middle face. How did all those human bones end up in this area? Excavation and analysis of this site are ongoing, and perhaps further publication will soon enlighten us.

Ex Africa Semper Aliquid Novi

(Ancient Greek proverb, "Always something new Out of

Africa," cited by Pliny the Elder and Charles Darwin)

Just as we thought that the general picture of human evolution was becoming clear, new finds have suggested that our picture was too simplistic. The tree is more bushy, the causes more complex, and the migrations multiple and in several directions. These are very exciting times in palaeoanthropology, and we look forward with great anticipation to the next few years of research and analysis.

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