Biologists can infer what the breeding system of an extinct mammal was by looking at the difference in sizes of adult males and females. In polygynous species, such as red deer, where a dominant male outcompetes other males before mating with many females and then plays no role in rearing offspring, males tend to be much larger than females. In contrast, in more monogamous species, where males and females form longer-term pair bonds and raise offspring together, males and females are of more similar size. In Homo erectus, males and females were similar heights, suggesting that the species was monogamous and hinting at both parents providing some parental care. If that was the case Homo erectus had evolved a different breeding behaviour from its Homo habilis ancestors. However, in some populations it appears that the Homo erectus infant brain did not develop much beyond birth, revealing that, as with Homo habilis, the young may have required less parental care than those of modern-day humans. Nonetheless, if they were monogamous, both parents would have likely provided parental care as such behaviour is seen in most other monogamous mammal species alive today, even if the length of childhood was relatively short.
Homo erectus lived in groups, but we know little about how big these were. Analysis of nearly a hundred fossilized footprints from a site in Kenya reveals that a group of twenty individuals walked through the site 1.5 million years ago. There are four sites where footprints are preserved, and one of the sets of footprints was made by a group of adult males. Were they on their way to communally hunt? Some scientists think so, and they have interpreted this as evidence that different demographic groups may have carried out different tasks. Homo erectus may have been the first hominin where individuals, or groups of individuals, regularly performed specialist tasks beyond defending the group from predators.
As we saw with the Australopithecines and Homo habilis, success leads to diversification, and the same was true for Homo erectus. The species produced many descendant forms beyond Homo floresiensis, including the Neanderthals, Denisovans and Homo sapiens. Homo erectus conquered Africa, Europe and Asia, and evolution sculpted each population to the environments it encountered. The Neanderthals evolved to live in the forested and often cold climes of Europe, the Denisovans inhabited Asia, while modern-day humans arose in Africa. Fossil remnants from Denisovans are sparse, so we know little about their lives, but Neanderthals and modern-day humans shared many similarities. They both developed art, language, complex societies and they both buried their dead. Both species pushed the cognitive abilities of Homo erectus to new levels.
In the text above, I have simplified the story. There is a line of descent from Australopithecus anamensis through Homo habilis and Homo erectus to Homo sapiens, but there were many offshoots. Populations would have diverged from one another, with their descendants sometimes crossing paths and successfully reproducing. We know this happened between modern-day humans and Neanderthals when Homo sapiens left Africa, and more on that later. Evolution is rarely the straightforward progression the tree of life suggests. Populations can diverge, and then hybridize when they come back together, or individuals from different populations can mate, and if their offspring then preferentially mate with each other, new species can evolve. Both the Australopithecines and Homo erectus would have done all these things, and our genome is consequently a mix of genes that evolved in different places. The fossil record is a testament to this, with a wide variety of hominin forms found across the globe. Palaeontologists have described several species or subspecies of Homo, including H. rudolfensis, H. ergaster, H. georgicus, H. antecessor, H. cepranensis, H. rhodesiensis, H. neanderthalensis, H. floresiensis and H. heidelbergensis. Palaeontologists cannot agree on which of these should be classified as species, subspecies, or diverged populations. I tend to think of them as subspecies. Regardless, the number of forms shows how remarkably successful Homo erectus was at adapting to new environments.
One intriguing form is Homo naledi, which lived in South Africa between 335,000 and 235,000 years ago. The dating of fifteen individuals to the relatively recent past is surprising given the anatomy of the species. Individuals were small, averaging about 140 centimetres in height and weighing 40 kilos. They had relatively small brains for their body size, but the shape of their skulls suggests a modern brain structure. The bones that have been recovered were from a single site in the Rising Star cave system in Gauteng province. What is remarkable is that the bones of no other animals were found in the chamber where the Homo naledi fossils were discovered, and access to the cave would not have been straightforward a quarter of a million years ago. The bones were not washed into the cave, but instead the corpses of the dead may have been placed there, suggesting deliberate burial. Homo naledi exhibits a mix of Australopithecine and Homo erectus characteristics, yet they were alive at the same time as modern humans. Exactly how this unusual species is related to other early hominins remains to be discovered. What is clear is that many different types of hominins lived across Africa. Biologists refer to cases when many different closely related subspecies live close by as a radiation. Hominins appear to have quite easily adapted to different habitats within Africa and beyond, with species like Homo habilis and Homo erectus being particularly adaptable. As adaptation occurred, over the course of about 4 million years, our ancestors evolved many of the key characteristics that make us human and allow us to live in complex societies.
The above narrative briefly summarizes the development of early hominins, but it does not explain why they evolved the characteristics that led to us. We are unlikely to ever know for certain, but as already mentioned, some of the Australopithecines began to spend less time in the trees and more time on the ground as their forest homes disappeared as the climate became increasingly dry. The species would have evaded ground-based predators by taking to the trees, but some began to spend more time in more open savannah habitats. The more successful lineages developed strategies and behaviours to protect their groups from the big cats, bears and wolves that shared these environments. These strategies may have involved throwing stones, wielding sticks, and working together to scare off or even kill predators. Bigger individuals may have been favoured as their strength and speed may well have given them a survival advantage. As anti-predation behaviours evolved, an unexpected benefit was the ability to steal kills from other predators and eventually hunt them. This was made easier by the development of more complex tools and through social cooperation in joint hunting. Those individuals that were most able to effectively work together and to build the most useful tools were able to outcompete groups that were less well equipped or organized. The adoption of fire and the construction of shelters would have been adaptations that further aided survival.
These adaptations required a larger brain, and brain size has trended to become larger over evolutionary time in hominins, with the odd exception of species such as Homo naledi that appear to have briefly thrived with a relatively small brain. A large brain has many advantages, enabling improved communication, problem-solving abilities and abstract thought, but big brains are expensive to run, and difficult to give birth to. If you are an average-sized human, your brain will account for about 2 per cent of your body weight but will use 20 per cent of your energy. Despite the cost, having a large brain was clearly worthwhile for our ancestors.