Graphic by Stephanie Fox, http://i9.com.
The incredible story of our evolution from ape ancestors spans 6 million years or more, and features the acquirement of traits from bipedal walking, large brains, hairlessness, tool-making, hunting and harnessing fire, to the more recent development of language, art, culture and civilisation.
Darwin's The Origin of Species, published in 1859, suggested that humans were descended from African apes. However, no fossils of our ancestors were discovered in Africa until 1924, when Raymond Dart dug up the "Taung child" - a 3-million to 4 million-year-old Australopithecine.
Over the last century, many spectacular discoveries have shed light on the history of the human family. Somewhere between 12 and 19 different species of early humans are recognised, though palaeoanthropologists bitterly dispute how they are related. Famous fossils include the remarkably complete "Lucy", dug up in Ethiopia in 1974, and the astonishing "hobbit" species, Homo floresiensis, found on an Indonesian island in 2004.
Humans are really just a peculiar African ape - we share about 98% of our DNA with chimpanzees, our closest living relatives. Genetics and fossil evidence hint that we last shared a common ancestor 7 to 10 million years ago - even if we continued hybridising long after.
At around 6 million years ago, the first apes to walk on two legs appear in the fossil records. Despite the fact that many of these Australopithecines and other early humans were no bigger than chimps and had similar-sized brains, the shift to bipedalism was highly significant. Aside from our large brain, bipedalism is perhaps the most important difference between humans and apes, as it freed our hands to use tools.
Bipedalism may have evolved when drier conditions shrank dense African forests. It must have allowed our ancestors to spot predators from further away, reach hanging fruit from the ground, and reduce exposure to sunlight. Evidence that Australopithecines walked upright includes analysis of the shape of their bones and fossilised footprints.
One famous member of the species Australopithecus afarensis is the remarkably complete fossil found by palaeaoanthropologist Donald Johanson in Hadar, Ethiopia in 1974. The 3.2-million-year-old fossil was named Lucy, after the Beatles' song Lucy in the Sky with Diamonds.
She stood around 1.1 metres (3.5 feet) tall and although she walked on two legs, she probably had a less graceful gait than us, since she walked with them bent.
Scientist's have modelled her gait using computers. Their characteristic long arms and curved fingers suggest that at least some Australopithecines were still good climbers.
Hundreds of other fossils of Australopithecus afarensis have now also been discovered. Other related early human species include Australopithecus africanus - such as the Taung child - 3.5-million-year-old Kenyanthropus platyops, 5.8-million to 4.4-million-year-old Ardipithecus, 5.8-million-year-old Orrorin tugenensis and 6 million year old Sahelanthropus tchadensis.
Australopithecines are thought to be the ancestors of Homo, the group to which our own species, Homo sapiens, belongs.
However, Australopithecines may also have given rise to another branch of hominid evolution - the vegetarian Paranthropus species. Around 2.7 million years ago, species such as Paranthropus bosei in east Africa evolved to take advantage of the dry grasslands. This included the development of enormous jaws and chewing muscles for grinding up tough roots and tubers.
By 2.4 million years ago, Homo habilis had appeared - the first recognisably human-like hominid to appear in the fossil record - which lived alongside P. bosei. Their bodies were around two-thirds the size of ours, but their brains were significantly larger than Australopithecines with a volume of about 600 cubic centimetres.
H. habilis had much smaller teeth and jaws than Paranthropus and was probably the first human to eat large quantities of meat. This meaty diet, acquired through scavenging, may have provided energy required to kick-start an increasing brain size. A mutation that weakened jaw muscles and gave our brains more space to grow may also lie behind the big brains we have today.
H. habilis - which means "handy man" - was also the first early human to habitually create tools and use them to break bones and extract marrow. This tool-making tradition, known as Oldowan, lasted virtually unchanged for a million years. Oldowan tools were made by breaking an angular rock with a "hammerstone" to give simple, sharp-edged stone flakes for chopping and slicing.
Despite their own increases in brain size, the Paranthropus group of species had become extinct by 1.2 million years ago. Some experts speculate that it was learning to work as a team against predators that gave Homo the edge.
At around 1.65 million years ago, another early human, Homo ergaster, started to create tools in a slightly different fashion. This so-called Acheulean tradition was the tool-making technology used for nearly the entire Stone Age, and practiced until 100,000 years ago. Acheulean tools, such as handaxes and cleavers, were larger and more sophisticated than their predecessors'. They may have been status symbols as well as tools.
Homo ergaster first appeared in Africa around 2 million years ago, and in many ways resembled us. Though they had brow ridges, they had lost the stoop and long arms of their ancestors. They may have been even more slender than us and were probably well-adapted to running long distances. Some experts believe that they were the first to sport largely hairless bodies, and to sweat, though another theory puts our hairlessness down to an aquatic phase.
One famous example of a more modern looking early human is the Turkana boy, a teenager when he died, 1.6 million years ago in Kenya. The shape of this fossil showed that the human pelvis had reached today's narrow proportions. Combined with the growing size of the human head and brain, this had far-reaching implications: human women now need help for a successful birth; and human babies are born earlier, and need a longer period of childhood care, than those of apes.
Meat-eating, however, may have allowed us to become early weaners.
H.ergaster may have been the first early human to leave Africa. Bones dated to around 1.75 million years ago have been found in Dmanisi in Georgia.
Shortly afterwards, Homo erectus appeared - the first early human whose fossils have been seen in large numbers outside of Africa. The first specimen discovered, a single cranium, was unearthed in Indonesia in 1891. H.erectus was highly successful, spreading to much of Asia between 1.8 and 1.5 million years ago, and surviving as recently as 27,000 years ago.
This species, with a brain volume of around 1000 cm3 would have interacted with modern humans. They may have been the first people to take to the seas and habitually hunt prey such as mammoths and wild horses, although there is some debate about this. They may also have harnessed the use of fire and built the first shelters.
In 2004, the remains of a tiny and mysterious human species, that may have lived as recently as 13,000 years ago, was discovered on an Indonesian island. More bones of the "hobbit", or Homo floresiensis, were uncovered in 2005. Some studies suggest it had an advanced brain and was unequivocally a separate species - but others argue that these people were modern humans suffering from a genetic disorder.
Early human fossil evidence from Spain, dating to around 780,000 years ago, points to the first known Europeans. Stone tools have also been found in England from around 700,000 years ago, attributed to Homo antecessor or Homo heidelbergensis.
More recently, 325,000-year-old H. heidelbergensis tracks were discovered preserved on an Italian volcano. Some of the biggest collections of hominid remains ever found are from Boxgrove in England and Atapuerca in Spain. Experts believe that these humans may have had ears equipped to detect nuances of human speech, whether or not they had simple language.
Some palaeoanthropologists believe that H. heidelbergensis evolved into our own species in Africa, whilst in Europe, the Neanderthals emerged as a separate species.
The Neanderthals were found across Europe, between 200,000 and 28,000 years ago. Though they still possessed pronounced brow ridges and were more thick-set, these people largely resembled us. They were as nimble-fingered, and matured at a similar age to us. Their brains were even slightly larger. It is not known if the Neanderthals had developed simple language. But they did possess some aspects of our culture, such as ritual burying of the dead; creating art; using tools to attack each other; and complex hunting methods - as evidenced by a remarkable butchery site in the UK.
Experts disagree about whether the Neanderthals hybridised with humans or not, or if our arrival killed them. Plunging temperatures, free trade and poor memory may all have contributed towards their extinction.
Out of Africa
There are several competing theories about how all these early humans are related to us today.
Most widely accepted is the "Out of Africa" hypothesis. This holds that ancient humans evolved exclusively in Africa, then spread across the world in two migration waves. The migration of H. erectus across Eurasia made up the first wave. Later, our own species evolved in Africa and fanned out in a second wave 200,000 years ago. These new people totally replaced H. erectus in Asia and the Neanderthals in Europe.
Advocates of the multiregional hypothesis instead believe that early humans started to leave Africa around 2 million years ago, and were never totally replaced by recent migrants. They believe these far-flung hominids exchanged genes and interbred, slowly evolving into modern humans - in many places, simultaneously. Through gene flow, modern characteristics such as large brains gradually spread, it is suggested. Some fossils seem to support the multiregional hypothesis. H. erectus skulls in Asia, for example, have similarly flat cheek and nasal regions as people there today do.
Most - but not all - genetic evidence appears to back the Out of Africa hypothesis. There is surprisingly little variation in the mitochondrial DNA (mDNA) of different people today, which suggest that humans evolved recently from a small ancestral population. In addition, the variation of mDNA in Africans is greater than elsewhere, suggesting that people have been evolving there for longer.
We may all be descended from a single African woman - dubbed Mitochondrial Eve - within the last 200,000 years. Male Y-chromosome DNA hints at a single male progenitor, too. Fewer than 50 people could have given rise to the entire population of Europe, experts believe.
The earliest anatomically modern humans are thought to have arrived around 200,000 years ago. These fossils show a rounded braincase and flatter face. Their brains had reached modern proportions of about 1350 cm3. Two skulls found in Ethiopia make up the oldest modern human remains known, at 195,000 years old.
Modern humans had made it to Asia by 90,000 years ago, Australia by 60,000 years ago, Europe and the Arctic by 40,000 years ago, and the Americas by 12,000 years ago.
Throughout history, tool use appears to have progressed slowly - once innovations were made, they lasted millions of years barely altering. But around 50,000 years ago something changed, and culture started to develop at a much more rapid rate.
Modern humans habitually began innovating new tools types, burying their dead, creating jewellery, developing sophisticated hunting techniques such as pitfall traps, using animal skins for clothing, decorating their bodies, and creating art and cave paintings. Although some of these traits appeared earlier, they seem to have only have been used sporadically until this time.
These changes may have been linked to increasing brain size or the way we thought - or could also be due to free trade, and the evolution of language and communication. The dawn of human civilisation has been dated to around 30,000 years ago. The earliest agriculture and domestication of species is known only as recently as 10,000 years ago. The first human cities appeared in Mesopotamia around 4,000 years ago.
Are we still evolving today? If so, how will we evolve in the future? Some argue that humans have evolved little in the last 50,000 years - but other studies suggests that thousands of genes have changed since then.
We may even be on the verge of the next step of human evolution - the human global "superorganism".
- by John Pickrell NewScientist, 2006.