Homo ergaster was named in 1994, when researchers recognized that it did not display very well the set of features exhibited by the former Asian Homo erectus. Homo ergaster sites are found in Koobi Fora, Nariokotome (West Turkana), and Olduvai Gorge. The specimens include KNM-ER 3733, KNM-ER 3883, KNM-WT 15000 (Nariokotome boy), OH 9. It existed between 1.9 to 1.2 million years ago. Homo ergaster had a higher skull dome, thinner cranial bones, slim brow ridges, a light skull, and generally, a lighter build than Homo erectus. It had a swift gait, with long, well-muscled limbs, and narrow hips. The body shape and proportions were very similar to those of modern humans living in tropical climates.
There is no sexual dimorphism between the two genders in Homo ergaster, which means that they both participated in, pretty much, the same economic activities. This also means that males rarely fought for access to females anymore. The enlarged brain gave H. ergaster a greater intelligence than his forbears, which undoubtedly aided him in adapting to new habitats. The molars and premolars are also more diminished in size--which is the mark of an omnivorous creature, whose diet includes a proportional amount of meat.
It is believed that the stone tools were not used mainly for predatory means, but for the break-up of tough foods before ingesting. The stone tools in use during this period is the Acheulean tool kit, named after a Homo erectus site named St. Acheul, in France. Bi-facial chipping is one of the defining characteristics of this tool industry, the by-product of which are the famous hand-axes found throughout the world. Scientists estimate the year at which Homo ergaster arrives at a particular region of the world by tracking the date at which bi-facial tools appear in the fossil record at that particular site.
Then, there is the question of why Homo ergaster traveled out of Africa. As you remember, around 2 million years ago, there was an ice age, and the spread of savanna habitats throughout Africa. This would have caused many neighboring bands of Homo ergasters to compete with each other for resources. There were two choices: (1) specialize to create a new niche for oneself or (2) migrate elsewhere to relieve the pressure. While the paranthropines chose #1, Homo ergaster chose to migrate elsewhere. If each year, Homo ergaster moved on an average of 200 meters, then in just 5000 years, they would be out of Africa and into the Middle East. Thus, it could easily have traveled all over the world in the 700,000 years of its existence.
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