Homo ergaster evolved during an accelerated period of global cooling and drying that cleared more and more tropical rainforest from Africa and created a desert in the northern half of the continent.
About a boy
One of the best sources of information about Homo ergaster is a skeleton discovered in 1984 by Alan Walker and Kamoya Kimeu at Nariokotome in West Turkana, Kenya. The remains were found to be those of a teenage boy between the ages of 11 and 13 when he died. Around 1.5 million years ago, the boy's body sank into the marsh where he died and became fossilised. His teeth show signs of an abscess where his milk teeth fell out, indicating that he may have died from septicaemia (blood poisoning).
Nariokotome Boy, as he has been dubbed, was already developing a thick, bony ridge across his eyes. A pair of buck teeth stuck out from a large, projecting mouth below a long, wide nose.
He was about 160 centimetres (5 feet 3 inches) tall and would have stood at 185 centimetres (6 feet 1 inch) had he reached adulthood.
This was clearly a strapping lad, with a body shape that was perfectly adapted to an active life in the sun. Human populations living on equatorial grasslands today, such as the Masai in Kenya, have the same tall, linear physique.
This body shape creates a large surface area over which the body can cool itself more easily, preventing Nariokotome Boy from overheating under the blazing Sun.
This hominid was probably the first to regulate its temperature through sweating. For creatures that must remain active at midday in a sunny, dry habitat, sweating is the most effective mechanism for maintaining safe body and brain temperatures.
Homo ergaster's body was probably smooth and largely hairless, since heat loss through sweating occurs most efficiently through naked skin. Its skin was almost certainly dark, to protect it from the Sun's harmful rays.
The outdoors type
Homo ergaster travelled long distances on foot, as it worked hard to scavenge enough meat to feed its growing body and brain.
In order to increase the energy efficiency of muscles involved in upright walking, ergaster developed a narrower pelvis. But its snake hips came at a price.
Firstly, the narrowing of the pelvis caused the lower part of the ribcage to narrow. In order to prevent constriction of the lungs, the upper part of ergaster's rib cage expanded, giving its chest a human barrel shape. Secondly, and more importantly, the narrowing of the pelvis constricted the female birth canal. This single anatomical change seems to have had profound consequences for human relationships.
Reasons to be faithful
A tight pelvis could have caused problems during birth. As brains increased in size, mothers had to push increasingly big-brained infants through an already tight pelvis. The solution was a trade off. While chimpanzees are born with their brains almost fully mature, humans are born with a comparatively immature brain. This makes human babies helpless and vulnerable during their first year of life as their brains make vital neural connections.
As a result, human mothers need to be well nourished to keep up with the demands of their babies, making them increasingly reliant on the support of their male partner and other members of their social group. Many experts regard this shift as the beginning of the nuclear family.
Less differences between the sexes in Homo ergaster may reflect a distinctively human pattern of sharing and cooperation between males and females.
Homo ergaster probably communicated using gestures combined with a limited range of sounds. The vertebral canal of Nariokotome Boy does not seem developed enough to have given him the control over his breathing needed for complex speech.
The small cheek teeth of Nariokotome Boy suggest that ergaster relied more on stone tools for processing food. To begin with, ergasterused primitive 'Oldowan stone tools,' which are little more than chipped rocks with sharp edges. But by around 1.6 million years ago, ergaster developed symmetrical, heart-shaped handaxes known as 'Acheulean bifaces', which gave the hominid greater control over the butchering of meat for food.
Out of Africa
Shortly after Homo ergaster appeared 1.9 million years ago, humans began to leave Africa for the first time and migrate to other continents. Early humans reached Dmanisi in ex-Soviet Georgia around 1.8 million years ago. Here, they encountered cool, seasonal grasslands where African animals such as ostriches and giraffes mingled with Eurasian species such as wolves and the sabre-toothed cat Megantereon.
Humans quickly spread east as far as the Indonesian island of Java. The hominids that inhabited subtropical Asia at this time belong to the species Homo erectus. This early human learned to survive in the bamboo forests that covered this region of Asia. The paucity of stone tools from Southeast Asian hominid sites suggests that erectus may have created a technology based on bamboo, a strong and versatile material.
"They may have used bamboo to make spears for hunting and poles to knock animals down from the tall trees", says Professor Russell Ciochon of the University of Iowa.
Homo erectus shared these bamboo forests with pigs, a type of elephant called Stegodon and the biggest primate that has ever lived - the giant vegetarian ape Gigantopithecus. It's possible that Gigantopithecus may even have been hunted by early humans in Asia. "They probably wouldn't have taken on the big adults, but they may have targeted juveniles. If we look at people who live in forests today, they also eat apes", says Ciochon.
Dates for the arrival of Homo erectus in subtropical Asia are highly controversial. While erectus was clearly established throughout the region by 1.8 million years ago, some sites suggest an even earlier date for its arrival. A hominid jaw and stone tools unearthed at Longuppo Cave, China, may date to as early as 1.9 million years ago.
Similar dates have been established for hominid sites at Mojokerto and Sangiran in Java. This newfound wanderlust may have been dictated by an increasing reliance on meat for food. Carnivores generally need much larger home ranges than similar-sized herbivores because carnivores have fewer total calories available to them per unit area of their territory.