Homo habilis are thought to have lived in larger groups than their ancestors, with some groups consisting of perhaps as many as eighty individuals, and this was a driver for increased intelligence. Scientists have identified a strong association between the size of the neocortex and the size of social groups within a species. The more individuals you have to interact with, the more brain power you need to manage the complex social interactions you are continuously faced with. Our neocortex size predicts we can cope with a social group of 150 individuals, which is about the average size of our social networks. This number is termed Dunbar’s number, after Robin Dunbar, the British anthropologist who first estimated it. If Homo habilis did live in groups that were eighty strong, early homo species were already starting to become quite smart. Nonetheless, Homo habilis, although more human than the Australopithecines, still differed from us in several ways. Males were significantly larger than the females, and they cooperated to protect the group from predators and to scavenge meat from them. Although meat formed a greater proportion of the diet than in Australopithecines, examination of fossilized teeth shows that fruit and vegetation were key constituents of most meals.
Some descriptions of the lifestyle of Homo habilis draw comparisons with modern-day baboons and savannah chimpanzees. In all these species, males are considerably larger than females, with one or a small number of males monopolizing matings. Species where a small number of males conduct most of the matings are known as polygynous, and in such species males do not contribute to rearing offspring. It is usually assumed that Homo habilis was polygynous, although this is highly speculative as patterns of reproductive success across individuals cannot be fossilized. Perhaps females worked together to raise offspring, but some evidence suggests that Homo habilis offspring did not have a long childhood, being less dependent upon their parents than we are, and being independent at a much earlier age.
Regardless of the species’ mating behaviour, Homo habilis was successful. Like the Australopithecines before, they, or species similar to them, appear to have spread to many parts of Africa, colonizing savannah habitats as the forests retreated. And just like their ancestors they too adapted to each environment they inhabited. Some of the skulls that have been found have an anatomical feature known as Broca’s area (the motor speech area) that is essential for speech. Some scientists have interpreted this as evidence that some populations of Homo habilis might have developed rudimentary speech, although conclusively proving this is likely to be a challenge.
It seems unlikely that Homo habilis had all the attributes needed to develop nation states and complex organizations, but it did have some of them. Individuals could live in large social groups and cooperate with one another in a more advanced manner than their ancestors could. They may also have had a primitive form of speech, and they could produce simple tools, and such behaviour may suggest the first murmurings of abstract thought. Nonetheless, tool use was rudimentary, and there is no evidence that Homo habilis produced art, used fire or buried their dead. Their descendants did start to do these things, and next we turn to a hugely successful ancestor of ours, Homo erectus, or upright man.
If you were to see an individual of the species Homo erectus walking in the distance, you would be excused for thinking she was a human. Anatomically they were similar to us. They were larger than Homo habilis, ranging in height from 145 to 185 centimetres, and weighed up to 70 kilograms. In addition to their large size, they also had bigger brains than their ancestors, though these were smaller than ours. The average human brain volume is 1,350 cubic centimetres, while that of Homo erectus was a little under a thousand. This next step on the journey to you and me first appeared in the fossil record 2 million years ago, 400,000 years before the last Homo habilis died.
Homo erectus differed from Homo habilis in that it was an apex predator, meaning that, like us, it was top of the food chain. Homo species had moved on from scavenging meat from other carnivores to also killing their own. Groups of the species hunted animals as large as wild cattle and elephants, as well as smaller prey, but they still supplemented their meat diet with some fruit and vegetation. The Australopithecines and Homo habilis had been successful in colonizing many environments in what is now sub-Saharan Africa (the Sahara desert did not form until 11,000 years ago), but no fossils of these species have been discovered outside of Africa. In contrast, Homo erectus fossils have been found throughout Asia and Europe.
Exactly how human Homo erectus was is hard to know. There was variation in anatomy across populations, with numerous other Homo species evolving from Homo erectus. A bit like the Australopithecines before them, different populations adapted to the different environments in which they found themselves. There are compelling arguments that Homo erectus colonized the island of Flores, before evolving into the diminutive Homo floresiensis, the hobbit human. Flores has always been an island, so its colonization required Homo erectus to cross the ocean. Whether this happened by chance, with a small number of individuals surviving the journey by clinging to logs or mats of vegetation, or by design in purpose-built dugout canoes or other forms of boat, is the subject of debate. The answer is important, because if Homo erectus was a seafaring species, it suggests an ability to communicate complex ideas and issue instructions of when to paddle in order to maintain a particular course. Unfortunately, the likelihood of a seafaring craft surviving for hundreds of thousands of years in the waters around Flores is extremely slim, so we may never know whether Homo erectus journeyed to the island by chance or by design.
Tool use was a ubiquitous feature across Homo erectus populations. The tools they produced were more complex than those of Homo habilis, with sharp edges chiselled on multiple sides, and stone knapping used to form sharp points. Hand axes of a variety of sizes were commonplace, with the tools used to cut up meat, wood and vegetation. The production of these tools was an important technological advance on the journey from the first primates to us.
Homo erectus roamed the Earth for nearly 2 million years, with the species disappearing from the fossil record about 110,000 years ago on the island of Java. Over that time, they evolved. In some populations they developed early forms of art by engraving shells, in others they tamed fire, and in some they built shelters. Given these abilities, it is not beyond the realm of possibility that individuals in some populations of Homo erectus built seafaring craft. During the course of their 2 million years of existence, the Homo erectus brain would have evolved, and the neocortex likely grew in size, giving them more human cognitive abilities than their predecessors. Other human traits also evolved in Homo erectus, including monogamy.