Modern-day humans are a species of primate, a large group of mammals that includes the lemurs of Madagascar, monkeys and apes. All these species have eyes that face forwards, and most species are agile and at home in the branches of trees. Primate hands are anatomically similar to their feet, and are highly dextrous. These hands and feet evolved to allow them to climb trees, handle and manipulate fruit and leaves in the canopy, and to catch insects, yet dexterous hands enabled our ancestors to develop the technologies described in this chapter. Good fossil evidence reveals the earliest primates lived in forests 55 million years ago, although some palaeontologists have made claims based on less convincing evidence that they date back to the Cretaceous, the geological epoch that started 145 million years ago and ended with the Chicxulub impact event that drove the dinosaurs extinct 66 million years ago.
Timeline of Human History
Our closest living ancestors are chimpanzees and their close relatives, bonobos. The most recent common ancestor of chimpanzees and humans lived 5–10 million years ago, and may have survived until about 4.2 million years in the past. Fossil evidence of the common ancestor that everyone can agree on has not been found, with some paleontologists arguing that species of the European genus Graecopithecus is the most likely candidate while others favour the African species Sahelanthropus tchadensis. The disagreements will continue until new fossil remains of cousins of our ancestors are found.
What is clear is that our ancestors began to spend less time in trees and more time on the ground, living a lifestyle that was perhaps similar to that of modern-day baboons. The move to a more ground-based existence may have been due to a decline in forest cover in parts of Africa leading to a need to travel between trees as forests became savannas. As forests began to disappear as the climate changed, our ancestors’ diets also changed, expanding to become more diverse. A more ground-based existence favoured upright walking, and this freed up dextrous hands for other tasks. Evolution took some time to deliver these changes, with them not happening all at once.
There is an emerging consensus that Australopithecus anamensis is the first species on our branch of the tree of life that is not on the chimpanzee branch. It lived in what is now Kenya and Ethiopia between 4.2 and 3.8 million years ago, by which point the human and chimpanzee branches had split. Male Australopithecus anamensis were about 1.5 metres tall, with females 20 centimetres shorter. The numerous fossils that have been discovered reveal that the species walked upright in a similar way to us, but with long arms that would have made them good tree climbers. They lived in a woodland habitat, still primarily eating fruit and vegetation, but they might have supplemented this diet with insects and small mammals and birds they were able to catch or scavenge. The species lived in small family groups, but although they were quite social there is no evidence of any tool use or culture, although some scientists have speculated that they may have used twigs and sticks to help forage in the way that modern-day chimpanzees do. The size of the skull suggests that species in the Australopithecine genus, including Australopithecus anamensis, were not as smart as their descendants, but they were possibly at least as smart as the non-human great apes alive today: the Bornean and Sumatran orang-utans, the Eastern and Western gorillas, chimpanzees and bonobos.
Several other Australopithecine species in addition to A. anamensis have been discovered, some from only single fossils. These fossils reveal that the Australopithecine lifestyle was successful, with fossils of descendant species found in central, eastern and southern Africa. The most famous fossil specimen comes from Ethiopia. It is a near-complete skeleton of a female, named Lucy, from the species called Australopithecus afarensis. The Beatles song ‘Lucy in the Sky with Diamonds’ was repeatedly played during the expedition that found her remains, and this was the cause of an individual of a long-dead species acquiring a modern-day name. Lucy lived at the peak of the Australopithecines’ success, 3.2 million years ago, nearly 1 million years after the genus evolved and 1.8 million years before it disappeared. What caused their extinction is not known, but palaeontologists have speculated that either climate change or competition from a new species on the landscape that had evolved from an Australopithecine ancestor played a role.
When evolution discovers a successful new way of life, body shape or behaviour, individuals exhibiting these attributes increase in number, and the range of the species can increase as offspring move away from their parents in search of pastures new. The world is not homogeneous, however, and some individuals may end up living in environments that differ from those of their ancestors. Over time, lineages in different environments can start to diverge as they adapt to local conditions, particularly if there is no, or only very occasional, mating between individuals living in different environments. Evolution happens as populations in different areas adapt in different ways to the different environments in which they live. Australopithecines spread across much of Africa and in different environments they adapted to either more forest- or more savannah-dominated habitats. Their body shapes, diets and behaviours evolved to be different, and in time, separate species emerged. A mosaic of Australopithecine species living in a range of environments evolved, and somewhere, a species emerged that thrived in a drier, cooler, less forested environment than other species in the lineage. At the time, this species might have appeared a little peculiar, being the odd cousin in a family of otherwise largely similar-looking species, living in the margins that the other Australopithecines avoided.
As we have seen, eccentricities in the Earth’s orbit, the gradual shifting of tectonic plates and volcanic activities can lead to changes in the climate. The Earth may appear stable and unchanging for centuries or millennia, but on longer timescales change is inevitable as the climate warms or cools and nature adapts. As Africa started to cool, prime Australopithecine real estate started to disappear, and as it did some species started to go extinct. But one species’ loss can be another’s gain, and the cooling world and the spread of savannahs favoured the odd cousin. In the fossil record we see the demise of the Australopithecines and the rise of the first species of Homo, the genus of closely related species to which we belong. If the fossil record were more complete, we might be able to link the first Homo species to the peculiar Australopithecine cousin, but currently scientists have not been able to do this. What we do know is that 2.3 million years ago a new species starts to appear in the fossil record, Homo habilis, which translates from the Latin to handyman. Nine hundred thousand years later the Australopithecines disappear from the fossil record, and the species of Homo that evolved from them were the dominant apes to walk the planet.
The gradual nature of evolution has meant that some authors have attempted to classify Homo habilis as an Australopithecine species, but currently it is considered to be the first species in the genus Homo, the species group that includes Homo sapiens. Homo habilis evolved from its Australopithecine ancestor to consume more meat, spend more time on the ground and less in the trees, to thrive in savannah environments, and in the use of stone tools. These tools were simply hewn rocks, formed by crashing rocks together to create smaller, sharper objects that were then used to butcher animals. The first human technology that became widely used was hand axes simply made from these knapped stones. These tools heralded the start of human-like species developing technologies that 2.3 million years later resulted in the LHC, a machine capable of probing the dynamics of the fundamental building blocks of matter.