Prehistoric World

1.9 million

Homo ergaster
Part 1 of 3

The first truly human-like hominid species was Homo ergaster ('workman'). This species evolved either from Homo rudolfensis or Homo habilis during an accelerated period of global cooling and drying which cleared more and more tropical rainforest from Africa and regularly created a desert in the northern half of the continent.

Until recently, the general consensus was that habilis was the more likely ancestor, but finds from Kenya in 2007 revealed an overlap of about 500,000 years during which habilis and ergaster must have co-existed in the Turkana basin area, the region of East Africa in which the fossils were unearthed. Their co-existence seems to make it less likely that ergaster evolved from habilis.

If ergaster had evolved from [a group of] habilis and remained within the same location as other habilis groups then both must have been in direct competition for the same resources. Eventually, one would have out-competed the other. However, the fact that they stayed separate as individual species for a long time suggests that they had their own distinct ecological niches, therefore avoiding direct competition.

As if the choice were not already uncertain enough, an alternative theory formed in 2011 suggests that both rudolfensis and habilis are instead cousins and ergaster actually descends directly from Australopithecus sediba.

Homo ergaster typically possessed a thick, bony brow ridge across the eyes, large teeth sticking out from a vaguely apelike projecting mouth below a long, wide nose, and long limbs. Individuals seem to have grown to a height of around 185cm with a body shape which was perfectly adapted to an active life in the sun (just as with modern human populations living on equatorial grasslands today, such as the Masai in Kenya).

This body shape creates a large surface area over which the body can cool itself more easily, preventing Homo ergaster from overheating under the blazing sun.

This hominid was probably the first to regulate its temperature through sweating. For creatures which 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.

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.

In general, the use of Homo ergaster describes a species of hominid in Africa, but when examples of the same species leave Africa they are generally referred to as Homo erectus, although this is not a hard or fast rule - this example belongs to Turkana Boy, otherwise known as Nariokotome Boy, the most complete skeleton found to date and a perfect example of Homo ergaster of about 1.5 million years ago

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.

Climate variations in East Africa may also have influenced hominid development. Scientists have identified lake systems which formed and disappeared in East Africa between one and three million years ago, providing possible evidence that global climate changes were occurring.

There were three distinct periods during which extensive lakes covered the region and grew to depths of hundreds of metres. The growth of these lakes probably resulted from a moist local climate. The regional wet periods, which may have persisted for up to 100,000 years, occurred as much of Africa became increasingly dry.

The periods of wet weather in East Africa may reflect fluctuations of the Earth's climate as a whole. At the time at which the lakes grew - and this period was one of them - glaciers and the atmosphere were also going through major transformations.

This provides strong support for theories in which early human species evolved and spread out in response to a rapidly changing environment.

1.9 million

Paranthropus robustus

At the same time as Homo ergaster was emerging in Africa, Paranthropus boisei's southern African relative, Paranthropus robustus ('robust') also appeared.

P robustus was initially thought to have survived until 1.4 million years ago, a span of 500,000 years. This figure has since been revised down to 1.2 million years. It is uncertain if it evolved from the same ancestor as boisei or if, as is argued by some researchers, robustus is a case of parallel evolution in that it may have descended directly from Australopithecus africanus.

Evidence of what robustus was eating is less clear than for its northern 'relative'. They seem to have been consuming grass-eating insects, including termites. Archaeological finds show that robustus dug termites out of their mounds using sharpened animal bones. It could also have been eating the roots of plants such as papyrus.

The species name was chosen to describe the skull, jaw, and teeth, which were much more dense and thicker than what had been seen in previous species. There were also many more ridges and crests located on the skull. The front teeth of robustus were smaller, but the molars in the back were larger than with previous species. These dental characteristics support theories about the species' diet.

An advancement which appeared both with robustus and boisei was the presence of a sagittal crest, a ridge which ran from front to back on top of the skull in which muscles were attached. These muscles aided in moving the jaw so that chewing was possible. As more muscle was formed more powerful chewing was possible.

Most researchers agree that boisei and robustus are separate species rather than a widely-spread single, but variable, species. A small minority insist on refusing to accept the Paranthropus classification for them, referring to them instead as Australopithecus robustus.

1.8 million

Homo erectus
Part 1 of 4

Shortly after Homo ergaster appeared, humans began to leave Africa for what was long thought to be the first time and migrate to other continents. This has since been disproved with the discovery of Homo georgicus at Dmanisi in Georgia.

Homo ergaster migrants were forced to hunt for new foodstocks by progressively cooler global temperatures at the start of the Pleistocene era. After groups of ergaster left Africa they are referred to as Homo erectus ('upright man') - an older term which formerly included a large range of specimen types. H erectus survived mainly in South Asia, while H ergaster remained in Africa as a direct ancestor of modern man. [1]

Homo erectus' migration took place during a rather brief period called the Olduvai subchron (1.98 to 1.79 million years ago). The East African Rift and extreme South East Asia were endpoints on a grand east-west geotectonic pathway called the Tethys corridor, a feature which was extremely unstable.

Homo erectus and companion mammals took advantage of open linear landscapes to migrate north from the rift to the Caucasus, and then both ways across the Tethys corridor - west towards Gibraltar and east to the Himalayan foreslope.

Homo erectus reached Dmanisi, which is eighty kilometres south-west of the Georgian capital, Tbilisi, at around 1.8 million years ago. Here they encountered cool, seasonal grasslands where African animals such as ostriches, rhinoceros and giraffes mingled with Eurasian species such as wolves and the sabre-toothed cat megantereon. This is where they also encountered the diminutive Homo georgicus.

They were also in Romania as early as 1.95 million years, showing that they ventured outside of Africa as soon as the ecological window made it possible (see 'external links' in the sidebar).

Homo erectus quickly spread further east to the emergent Sunda continental shelf off East Asia's present south coast, before rising sea levels cut the shelf into a series of islands of which the modern Indonesian island of Java is the southernmost.

Populations existed throughout subtropical Asia, but extended no further north than the Himalayas or southern China. Instead, erectus learned to survive in the bamboo forests which covered this region of Asia. The paucity of stone tools from South East 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,' saids 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 which has ever lived.

This was the giant vegetarian ape, Gigantopithecus - a descendant of the earlier Ramapithecus or Sivapithecus. It is 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,' said Ciochon.

Dates for the arrival of Homo erectus in subtropical Asia are 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 in China may date to as early as 1.9 million years ago - 100,000 years earlier than expected.

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.

Shortly after settling in its new Asian homelands, from about 1.6 million years ago Homo erectus began to diverge from Homo ergaster populations. This divergence may be related to the first full scale ice age, which occurred around 1.5 million years ago, following half a million years of continued cooling of the world climate.

1.8 million?

Homo naledi
Part 1 of 2

Homo naledi made worldwide headlines in 2015 when researchers announced the discovery of an unusually large collection of odd-looking Homo fossils in the bowels of South Africa's Rising Star cave system.

The cave - the Dinaledi Chamber - was extremely hard to reach, being accessed via narrow passageways and down a vertical chute which only the more slender experts could manage. The level of difficulty in reaching the site left archaeologists puzzling over how the bones got there in the first place.

They recovered 1,550 fossils from a minimum of fifteen individuals of all age groups. Some experts have asserted that naledi intentionally dropped dead comrades into the underground chamber, but that's far from proven yet.

One of the biggest mysteries surrounding naledi has been its age. A solid date for the fossils is essential for deciphering their place in Homo evolution.

The archaeological team assigned the bones to H naledi based on an unexpected mix of humanlike features and traits which are typical of Australopithecus species from more than three million years ago. Fossil analyses have challenged the speculative claim that naledi actually represents a variant of Homo erectus, a species which is known to have existed by 1.8 million years ago.

In fact, naledi possessed a shoulder which was unlike those of other Homo species. Its collarbone and upper arm bone resemble corresponding Australopithecus bones. The shoulder blades must have been positioned low and behind the chest, an arrangement which would have been more conducive to climbing trees than running long distances.

H naledi's hand was built both for climbing and gripping stone implements. It had a human-like wrist and thumb which were combined with Australopithecus-like curved fingers.

The lower jaw of Homo naledi (on the left) is not as strongly built as those of many hominins, while the hand (right) has a very human shape but the curved finger bones and strong thumb are archaic features (text partially from the Natural History Museum, London)

Its curved toes and flaring pelvis also recall Australopithecus. Still, a preliminary lower-body reconstruction - incorporating fossil evidence of humanlike legs, knees, and feet - suggests that naledi walked almost as well as modern humans.

Dating remained unconfirmed at the start of 2017, but a window between 2 million years and 1.5 million years seemed most likely based on similarities in the skull and other body parts to other species in this timescale.

One late-2016 estimate which has been claimed as reasonably reliable (although possibly using a flawed process of analysis) suggests a date as recent at 912,000 years, which would place it in Africa at the same time as Homo ergaster. A young date for naledi perhaps should not be unexpected. At least some naledi bones appear not to have fossilised, which would be consistent with a more recent age.

A study released in April 2017 suggested a date for the remains of 300,000-200,000 years ago, making naledi contemporary with Neanderthals.

Further finds would provide better dating for this species, with that also being in the same Neanderthal-contemporary period (see next part for this group).